White-tailed deer hind, jumping over a fence, Great Smoky Mountains National Park, Tennessee. (Photo copyright © by Kaj Halberg)
An invasive species is a species of animal, plant, or fungus, which is not native to a specific area, and which tends to spread beyond control, causing damage to the environment and often expelling native species. Most of these harmful species have been introduced by man, deliberately or accidentally. The term invasive is sometimes applied to native species, whose numbers increase to an extent that they become harmful to their environment. Three such examples are white-tailed deer (Odocoileus virginianus) in North America, and common broom (Cytisus scoparius) and bracken (Pteridium aquilinum) in Europe.
On this page, the species are divided into three categories: animals, plants, and fungi, each of these arranged alphabetically according to scientific names (sometimes the family name).
Achatina fulica Giant African snail
This large land snail, which belongs to the family Achatinidae, has become a pest in many parts of the world, feeding voraciously and causing severe damage to agricultural crops and native plants, also through transmitting plant diseases. It competes with native snail taxa, and it possibly also spreads human diseases. (Source: Global Invasive Species Database)
Giant African snail, Nanhua Ecological Park, near Yujing, southern Taiwan. (Photos copyright © by Kaj Halberg)
Acridotheres javanicus Javan myna
As a native, this species, which is also called white-vented myna, is found only on the Indonesian islands of Java and Bali. However, it has been introduced to numerous other countries as a cage bird and has escaped in many places to form wild populations, including in Taiwan, Japan, Southeast Asia, and Puerto Rico. Today, the Javan myna is very common in Singapore and Taiwan, where it out-competes local bird species, in Taiwan especially the native crested myna (Acridotheres cristatellus), which has become very rare.
Other pictures depicting Javan myna, and one depicting crested myna, may be seen on the page Animals: Birds in Taiwan.
Javan myna, feeding in a flower of a silk-cotton tree (Bombax ceiba), Tunghai University Park, Taichung, Taiwan. (Photo copyright © by Kaj Halberg)
Javan myna, sitting in front of a water tower, Taichung. (Photo copyright © by Kaj Halberg)
Alopochen aegyptiacus Egyptian goose
This bird, which is a native of Africa south of the Sahara, was introduced to Britain in the 1700s, and later to other European countries. It has become naturalized in many areas, where it competes with indigenous geese and ducks. It was officially declared a pest in the U.K. in 2009, and later in several other countries. In many European countries, it may be hunted year-round, but few birds are bagged, and the species is still expanding.
Egyptian geese often become very confiding and bold. This picture was taken in a campground in Namibia. (Photo copyright © by Kaj Halberg)
Many species of cockroaches have been introduced by Man during his expansion to inhabit the entire planet. In countless homes, they are a true menace, consuming food, making a mess, and possibly spreading diseases to humans.
The usage of cockroaches in Chinese folk medicine is described on the page Traditional medicine.
Cockroaches are partial to humid places and are often encountered in bathrooms, as in this picture from Luzon, Philippines. (Photo copyright © by Kaj Halberg)
Branta canadensis Canada goose
The Canada goose is native to northern North America, but has been introduced to Britain, Sweden, New Zealand, Argentina, and other places. This species is very bold and has been able to establish populations in urban areas, where it has no natural predators. In many areas, it has been declared a pest because of its noise, droppings, and aggressive behaviour.
Canada geese, grazing on a golf course, Long Island, United States. (Photo copyright © by Kaj Halberg)
Capra hircus Domestic goat
As a close acolyte of Man for thousands of years, the domestic goat is now found in almost every corner of the planet. In countless places, the goat is doing serious damage to the environment by overgrazing, and it is often the culprit in the increasing desertification, which takes place across the world.
The taming and usage of the domestic goat is described on the page Animals: Animals as servants of Man.
This goat is standing on its hindlegs, feeding from a bush in the Thar Desert, Rajasthan, north-western India. (Photo copyright © by Kaj Halberg)
Erinaceus europaeus European hedgehog
Various species of hedgehog are often killed by cars, and they have declined drastically in many places. In New Zealand, however, where the European hedgehog was introduced at an early stage, this species is regarded as a pest, as it competes with kiwis, five species of flightless birds of the genus Apteryx, which are restricted to New Zealand. The hedgehog eats worms and other invertebrates, which is the preferred food of these birds.
European hedgehog, killed by a car, near Kai Iwi, New Zealand. (Photo copyright © by Kaj Halberg)
Felis silvestris ssp. catus Domestic cat
Feral domestic cats are a serious threat to the wild fauna in many parts of the world. For instance, a study from 2013 by S.R. Loss, T. Will and P. Marra. The impact of free-ranging domestic cats on wildlife in the United States, Nature Communications 4:1396, suggested that free-ranging domestic cats (mostly feral) are the top human-caused threat to wildlife in the U.S., killing an estimated 1.3-4 billion birds and 6.3-22.3 billion mammals annually.
The origin of the domestic cat is described in detail on the page Animals: Animals as servants of Man.
This domestic cat is resting on a tree stump. (Photo copyright © by Kaj Halberg)
Hemitragus jemlahicus Himalayan tahr
This species of wild goat, which lives in the temperate zone of the Himalaya, has been introduced as a hunting object to other parts of the world, including New Zealand and South Africa. However, in these countries numbers of tahrs have exploded, as they have no natural enemies here, and it has become a serious threat to the local environment through overgrazing.
This picture shows grazing Himalayan tahr bucks in the Khumbu area, eastern Nepal, where this species is common. (Photo copyright © by Kaj Halberg)
Herpestes javanicus Small Asian mongoose
To control populations of rats and other rodents, this species has been introduced to many parts of the world, mostly small islands. In most localities, however, the species has had negative impact on ground-nesting birds, and it is often able to out-compete smaller local carnivores. It is considered to be among the world’s one hundred worst invasive species. (Source: International Union for Conservation of Nature, 2000)
The small Asian mongoose is widespread in Asia. This one was encountered in Keoladeo National Park, Rajasthan, India. (Photo copyright © by Kaj Halberg)
Myocastor coypus Nutria, coypu
This animal is native to South America, but was introduced to North America and Europe for fur production. Over the years, numerous animals have escaped, and the species has become naturalized in many places. They are considered a pest, as they compete with – and sometimes expel – native species, erode river banks, destroy irrigation channels, and chew up house panels etc.
This coypu is sitting on its nest, Lac de Grand Lieu, Loire Atlantique, France. (Photo copyright © by Kaj Halberg)
Neovison vison American mink
The American mink is a semi-aquatic North American mustelid, which has been introduced in many parts of the world for fur production. In countless places it has escaped, and has become a serious threat to the native fauna, as it is a notorious predator of birds’ eggs. It is also blamed for the serious decline in populations of European mink (Mustela lutreola), Pyrenean desman (Galemys pyrenaicus), and European water vole (Arvicola amphibius).
This picture is from Iceland, where teams of professional ‘mink hunters’ control mink numbers by locating as many dens as possible, removing the young and, if possible, kill the mother. (Photo copyright © by Kaj Halberg)
Odocoileus virginianus White-tailed deer
This deer, which is a native of North and South America, has been introduced to several European countries, and in Finland it is now considered to be a pest, which expels native deer species.
In many parts of North America, it has also spread beyond control. As far back as in 1949, in A Sand County Almanac, environmentalist Aldo Leopold (1887-1948) wrote: “I now suspect that just as a deer herd lives in mortal fear of its wolves, so does a mountain live in mortal fear of its deer.”
In August 2012, The Bloomberg View published a staff editorial, entitled: “Deer Infestation Calls for Radical Free-Market Solution.”
About two months later, The Wall Street Journal ran a story, entitled: “America Gone Wild,” noting the impact of overabundant deer.
The Nature Conservancy then noted: “No native vertebrate species in the eastern United States has a more direct effect on habitat integrity than the white-tailed deer. (…) In many areas of the country, deer have changed the composition and structure of forests by preferentially feeding on select plant species.”
White-tailed deer hind, eating grass, Cathlamet, Washington State. (Photo copyright © by Kaj Halberg)
Oryctolagus cuniculus European rabbit
This species has been introduced to many countries as a hunting object. It was first brought to Britain by the Romans, following their invasion in A.D. 43, and today it is extremely common – an estimate in 2004 suggested about 40 million.
In Australia, 24 European rabbits were introduced in Victoria in 1859. The vast farmlands were an ideal habitat for the rabbits, and the mild winters allowed them to breed year-round, so they quickly spread over most of the country. Australia’s equivalent to the rabbit, the greater bilby (Macrotis lagotis), could not compete with the fast-breeding rabbits, and today it is an endangered species. Other places of introduction include New Zealand, certain Hawaiian islands, and islands off the coast of South Africa. There are also many instances of pet rabbits, which have escaped, forming feral populations.
In countless cases, this species has done severe damage to the environment, partly through overgrazing, partly through its system of underground tunnels, and partly through competition with local wildlife. It is regarded as an invasive species in most countries.
In 1972, English author Richard Adams (1920-2016) wrote a novel, Watership Down, describing the life of a group of wild rabbits in southern England. Although the rabbits have been anthropomorphized, the story gives a fair account of the life of these animals.
The European rabbit was introduced to Britain almost 2,000 years ago, and today it is extremely common. These are sitting outside their den in the Spey Valley, Scotland. (Photo copyright © by Kaj Halberg)
This picture shows a dark European rabbit, probably with genes from escaped pets, nibbling at a flower of common broom (Cytisus scoparius), Bornholm, Denmark. (Photo copyright © by Kaj Halberg)
Passer domesticus House sparrow
The house sparrow was introduced to the United States several times between 1851 and 1875, and also to New Zealand, around 1865. In both countries, it was thought that the sparrows would be able to control populations of harmful insects in cereal crops. However, they only catch insects when feeding their young, whereas the rest of the year they eat the very crops they were supposed to protect. As early as the 1880s, the sparrow itself was regarded as a pest in New Zealand, and today it is considered a serious crop raider in large parts of North America. It also competes seriously with local species at bird feeders etc.
As early as the 1880s, the house sparrow was regarded as a pest in New Zealand, where this picture was taken. (Photo copyright © by Kaj Halberg)
The male house sparrow is a pretty bird, with chestnut nape and various subtle brownish colours on back and wings. This one was observed at Muriwai Beach, New Zealand. (Photo copyright © by Kaj Halberg)
This man is hand-feeding house sparrows in St. James’ Park, London. (Photo copyright © by Kaj Halberg)
Pomacea canaliculata Golden apple snail
This species is native to southern South America, but has been introduced elsewhere, either unintentionally, or, as in Taiwan, deliberately, with the intention of producing these snails as a substitute food source for native snails. However, their texture was too soft for consumers’ taste, and, consequently, rather than killing the snails, many were released into irrigation canals and rivers. Being highly adaptive, they thrived and soon spread beyond control. Rice shoot are a favourite food of these snails, and, having no natural predators in Taiwan, they have become a huge menace in rice fields. Bright pink masses of snails’ eggs are often seen on concrete surfaces along river and ditches, and also on the rice plants themselves. (Source: culture.teldap.tw)
By the Global Invasive Species Database, this species is considered to be among the worst 100 of the ‘World’s Worst Invasive Alien Species’. It is also regarded as being among the 40 worst alien species in Europe.
Pink egg masses of the golden apple snail adorn a concrete embankment along a rainwater canal in the city of Taichung, Taiwan. (Photos copyright © by Kaj Halberg)
Procyon lotor Raccoon
The American raccoon has become naturalized and invasive in large parts of Europe, and in Japan. In many places, it is regarded as a pest, plundering birds’ nests and competing with local mammals. In America, it is a nuisance in urban areas, as it will turn over garbage cans, looking for food, and spread the garbage over large areas. It also consumes food at bird feeders, if the food is not being placed out of reach.
This raccoon has come to a house at night, eating from a dog’s feeding bowl, Long Island, United States. (Photo copyright © by Kaj Halberg)
Pycnonotus cafer Red-vented bulbul
The red-vented bulbul is a very common breeding bird in Pakistan, India, Nepal, Bangladesh, Sri Lanka, and Myanmar. It has been introduced to many other parts of the world and has become invasive in several Pacific countries, including Fiji, Samoa, Tonga, and Hawaii. It has also established itself in parts of southern Arabia, New Zealand, the United States, and Argentina. It is considered to be among the world’s one hundred worst invasive species. (Source: Lowe et al., 2000: 100 of the World’s Worst Invasive Species: A Selection from the Global Invasive Species Database. The Invasive Species Specialist Group, IUCN)
Other species of bulbul are described on the page Animals: Birds in Taiwan.
Resting in a tree-like spurge of the species Euphorbia antiquorum, which grows in front of a ruined pagoda in Bagan, Myanmar, this red-vented bulbul clearly shows, why it got this name. (Photo copyright © by Kaj Halberg)
Pycnonotus sinensis Light-vented bulbul, Chinese bulbul
In many parts of eastern Taiwan, large numbers of this species have been mass-released during Buddhist festivals. They have become naturalized in many areas, where they compete with – and also hybridize with – the local, endemic Taiwan bulbul (Pycnonotus taivanus), causing the latter to have declined drastically, and already gone extinct in the Yilan County.
Other pictures, depicting this species, may be seen on the page Animals: Birds in Taiwan.
The light-vented bulbul is one of the most common birds in Taiwan. This one is sitting on a balcony rail in a skyscraper in the city of Taichung. (Photo copyright © by Kaj Halberg)
Rattus norvegicus Brown rat
The brown rat originated somewhere in Asia, but has spread with humans to almost all parts of the world. It is a serious pest, as it competes with native mammalian species, raids birds’ nests, and consumes huge amounts of cereals and other food items. Furthermore, it spreads various diseases among humans. During the Middle Ages, fleas of the brown rat were carrying bubonic plague, which in some places reduced the human population by 50 to 75%. This much feared disease was aptly named The Black Death.
Brown rat, caught in a trap, Nature Reserve Vorsø, Denmark. (Photo copyright © by Kaj Halberg)
Sciurus carolinensis Eastern grey squirrel
This is the most widespread squirrel in North America. It has also been successfully introduced to South Africa, Britain, and other places, and in Britain it has become a pest, as it competes with the native European red squirrel (Sciurus vulgaris), which is now scarce south of Scotland.
Eastern grey squirrel and many other North American squirrels are described on the page Animals: Squirrels of North America.
Eastern grey squirrel, Kew Botanical Gardens, London. (Photo copyright © by Kaj Halberg)
Sturnus vulgaris European starling
In an article, European Starlings: A Review of an Invasive Species with Far-Reaching Impacts, published in Managing Vertebrate Invasive Species: Proceedings of an International Symposium, National Wildlife Research Center, Fort Collins, Colorado, 2007, George M. Linz and others state the following:
“The introduction of European starlings (Sturnus vulgaris) in New York City in 1890 and 1891 resulted in their permanent establishment in North America. The successful occupation of North America (and most other continents as well) has earned the starling a nomination in the Top 100 list of ‘Worlds Worst’ invaders. Pimentel et al. (2000) estimated that starling damage to agriculture crops in the United States was $800 million yearly, based on $5/ha damage. Starlings may spread infectious diseases that sicken humans and livestock, costing nearly $800 million in health treatment costs. Lastly, starlings perhaps have contributed to the decline of native cavity-nesting birds by usurping their nesting sites.”
As opposed to the situation in the United States, the starling is declining in large parts of Europe, mainly because of persecution in its wintering areas. In later years, the species has begun to spend the night in cities, where it is considered a nuisance because of its droppings. It also does some damage on fruit trees and other crops.
This picture of a singing starling is from Denmark, where this species is a native. (Photo copyright © by Kaj Halberg)
Threskiornis aethiopicus Sacred ibis
In Ancient Egypt, the sacred ibis was venerated as a symbol of Thoth, the god of knowledge, who is often depicted with the head of an ibis. Today, the sacred ibis is a common and popular species in zoological gardens worldwide. It often escapes, and populations in the wild have been established various places, including France, Italy, Spain, the Canary Islands, Florida, and Taiwan. These populations are a great threat to many other birds, which breed in colonies. Ibises are predators, which can ravage colonies of terns and other birds by eating their eggs and young, and they also compete for nest sites with cattle egrets (Bubulcus), little egret (Egretta garzetta) and others.
The sacred ibis is very common in East Africa. This picture is from the Ngorongoro Crater, Tanzania. (Photo copyright © by Kaj Halberg)
Trichosurus vulpecula Brushtail possum
This animal is a cat-sized marsupial, which is protected in its native Australia. However, in New Zealand, where it was introduced in 1837 for the fur trade, it has become the country’s most damaging animal pest, wreaking havoc on native forests, where they eat the foliage.
Not only do possums destroy forests, they also do damage on pasture lands, causing a fall in farm production at an estimated 35 million NZ$ annually, and they also eat planted seedlings of pines and other trees. Besides, they can infect cattle with bovine tuberculosis, threatening the country’s valuable dairy industry.
Possums are found virtually everywhere on mainland New Zealand and Stewart Island, although they have been eradicated from major offshore islands since the early 1990s. Their number is difficult to accurately assess, but in the early 2000s, estimates ranged from 50 to 70 million. Their cost to the economy is considerable. In 2006, government agencies spent $111 million on possum control. (Source: teara.govt.nz/en/possums)
Numerous possums are killed by cars, as this one near Waipoua Forest, North Island. (Photo copyright © by Kaj Halberg)
Abrus precatorius Crab’s eye
The name crab’s eye stems from the pattern on the colourful red-and-black seeds of this plant, which has many other popular names, most of which also refer to the seeds. It is possibly a native of India, but at an early stage, it was introduced to many other countries, and today it has a pan-tropical and -subtropical distribution. In many areas, including Belize, West Indies, United States, Hawaii, and Polynesia, it is proclaimed an invasive weed.
Crab’s eye is described in detail on the page Plants: Plants in folklore and poetry.
Crab’s eye, Dehra Dun, Uttarakhand, northern India. (Photo copyright © by Kaj Halberg)
Acer pseudoplatanus Sycamore maple
This species, which is a native of Central Europe, was introduced to Britain around 1500. It has also become naturalized in other parts of Europe, and in Australia, New Zealand, and North America. In many places, it has become invasive, easily spreading by its winged seeds, which are produced in the tens of thousands on a single large tree.
An example of the effective spreading of sycamore maple is seen in nature reserve Vorsø, Horsens Fjord, Denmark, where former fields were abandoned in two steps, in 1928 and in 1978. In both cases, thousands of maple seeds, stemming from a few trees in plantations at the edge of the fields, were spread by the wind. The succession of these maple forests is described in detail on the page Nature Reserve Vorsø: Expanding wilderness.
This 25-year-old growth of sycamore maples has sprung up on an abandoned field in nature reserve Vorsø. (Photo copyright © by Kaj Halberg)
In this picture, a sycamore maple has taken root in a crack in a quarry with Nexø sandstone on the island of Bornholm, Denmark. (Photo copyright © by Kaj Halberg)
Ageratum Goat-weed, blueweed
Goat-weed (Ageratum conyzoides) and its close relative, Mexican blueweed (A. houstonianum), are both native to Central and South America, but have become naturalized worldwide in tropical and subtropical areas. Both are regarded as invasive weeds in numerous countries around the world, in Africa, Southeast Asia, China, Japan, Australia, New Zealand, and the United States.
You may read about the medical usage of these species on the page Traditional medicine.
Goat-weed, Marsyangdi Valley, central Nepal. (Photo copyright © by Kaj Halberg)
Mexican blueweed is very common in Taiwan, where this picture was taken. (Photo copyright © by Kaj Halberg)
Alliaria petiolata Garlic mustard
This plant, which is native to Eurasia and North Africa, was introduced to North America as a spice herb around 1860. Since then it has spread to most of the states of the U.S., and to Canada as well. Its natural enemies of the Old World, such as fungi and insects, are not present in North America, which leads to a much higher seed production. The garlic mustard has invaded numerous forests, where it is able to dominate the understorey, hereby expelling native plants. It is listed as a noxious species in at least nine American states.
This picture is from Denmark, where garlic mustard is a native. Nevertheless, it has also been expanding in this country during the last fifty years. The plant with the red flowers is red campion (Silene dioica). (Photo copyright © by Kaj Halberg)
Allium triquetrum Three-cornered leek
This species is also called three-cornered garlic or onion weed, the specific name and two of the English names referring to its three-edged flower stalks. It is native to coastal areas of north-western Africa and south-western Europe, from the Iberian Peninsula and southern France eastwards to Corsica, Sardinia, and Sicily, and also Madeira and the Canary Islands, growing in grasslands, on river banks, and along roads. It has been introduced to a number of other countries, including England, Ireland, Turkey, New Zealand, and Australia, and is regarded as an invasive in several countries.
Three-cornered leek is very common along roads in New Zealand, where these pictures were taken. (Photos copyright © by Kaj Halberg)
Allium ursinum Ramsons
Ramsons is hardly a native of northern Europe, but was probably introduced by monks during the Middle Ages as a nourishing herb. Today, it is very common in forests and groves here, often becoming an invasive, which dispels other plant species. As early as 1741, the famous Swedish botanist Carolus Linnaeus (1707-1778), also called Carl von Linné, noticed this phenomenon on his journey to the Swedish islands Öland and Gotland. He notes about ramsons: ”The farmers told me that where this plant is growing, it dispels other species. We had clear evidence of this fact, because where ramsons was growing there were no other plants.” (Source: C. Linnæus, 1745. Carl Linnæi Öländska och Gotländska Resa förrättad År 1741, new edition by Wahlström & Widstrand, 1975)
This picture is from the island of Bornholm, Denmark, where ramsons is very common and has become the dominating plant in several smaller forests. (Photo copyright © by Kaj Halberg)
Alternanthera philoxeroides Alligator weed
Members of the genus Alternanthera, of the amaranth family (Amaranthaceae), are characterized by their whitish, papery flowers. These plants, also known as joyweed or Joseph’s coat, mainly stem from South America, with some species native to Asia, Africa, and Australia. However, many species have been accidentally introduced elsewhere, and several are regarded as noxious weeds. The number of species in this genus varies enormously, from 80 to 200, depending on authority.
One of the worst invasive species is the alligator weed (Alternanthera philoxeroides), which is native to temperate areas of South America. This plant, which thrives in both dry and aquatic conditions, has been spread to more than 30 countries, notably the United States, New Zealand, and China. In many places, it has become a big problem in water courses, as it is most prolific, often blocking the flow of water.
Alligator weed is very common in Taiwan. These pictures show a large growth on the shore of the Agongdian River, south-western Taiwan. (Photos copyright © by Kaj Halberg)
Arundo donax Giant reed
This South European grass species has been widely planted in western United States as a means to control soil erosion, and as a biomass plant. It often escapes and has become highly invasive in numerous places, especially along rivers, where it will often displace the natural vegetation.
In this picture, giant reed grows on a bluff in Julia Pfeiffer Burns State Park, Big Sur, California. (Photo copyright © by Kaj Halberg)
Avena fatua Wild oat
Wild oat is believed to have originated in Central Asia, and it has been associated with the cultivation of oats (Avena sativa) and other cereals since the early Iron Age. In reality, wild oat also has edible seeds, but is unsuitable for cultivation due to the fact that as soon as the kernels are ripe they fall to the ground, as opposed to cultivated oat.
In former days, wild oat was so numerous that in certain areas it would lower the yield of crops by 40%. In Denmark, it was declared an outlaw in 1956, to be “removed from all cultivated and un-cultivated areas by the owner or the user.” Before the introduction of chemical herbicides, the plants had to be removed by hand, which was a very time-consuming task. An advantage was that wild is often taller than the surrounding crop, making it relatively easy to spot.
Today, wild oat is still is a serious pest in many countries, and as it is highly adaptable to various environments, it has become naturalized in numerous places. Wild oat is considered to be among the world’s worst agricultural weeds. (Sources: D.P. Jones (ed.), 1976. Wild oats in world agriculture. An interpretative review of world literature. Agricultural Research Council. London; and L.G. Holm et al., 1977. The World’s Worst Weeds. Distribution and Biology. University Press of Hawaii)
Wild oat, Crystal Cove State Park, California. (Photo copyright © by Kaj Halberg)
Bidens pilosa Downy bur-marigold
This plant is a pan-tropical and -subtropical weed of unknown origin, which has become a pest in many places, expelling native species. One isolated plant can produce over 30,000 hooked seeds, which are readily spread by sticking to animals’ furs, socks, trousers, etc. This way of seed dispersal has given it names like beggar-ticks, stickseed, farmer’s friend (ironic!), needle grass, Spanish needles, stick-tight, cobbler’s pegs, Devil’s needles, and Devil’s pitchfork. Other names include blackjack and hairy bidens.
A South African website, farmersweekly.co.za/animals/horses/beware-those-blackjacks, says: “The common blackjack is not only an irritant to horses, (but) can cause them injury. (…) There can be few of us who have not spent ages picking them off our clothes after walking through the veld to catch horses in the early winter. Blackjacks that become entangled in the forelock of a horse can be a great irritant, and the animal will toss its head, if you try to remove them. The spines can injure the eyes, so it’s better to clip the forelocks short. Blackjacks can also get caught up in the long hair behind the fetlocks and pasterns, causing chronic irritation and lameness.”
This species is reported to be a weed of 31 crops in more than 40 countries, Latin America and eastern Africa having the worst infestations. (Source: cabi.org/isc/datasheet/9148)
However, downy bur-marigold is not only a troublesome weed, it also has medicinal properties. In traditional Chinese medicine, it has been used for a large number of ailments, including influenza, colds, fever, sore throat, appendicitis, hepatitis, malaria, and haemorrhoids. Due to its high content of fiber, it is beneficial to the cardiovascular system, and it has been used with success in treatment of diabetes.
These pictures are from Taiwan, where downy bur-marigold is highly invasive, covering huge areas. In the bottom picture, a Taiwan small cabbage white (Pieris rapae ssp. crucivora) is feeding in a bur-marigold flower. (Photos copyright © by Kaj Halberg)
The hooked seeds of downy bur-marigold readily spread by sticking to animals’ furs, socks, trousers, etc. This picture was taken at Pokhara, central Nepal. (Photo copyright © by Kaj Halberg)
Brassica nigra Black mustard
This species is probably a native of southern Europe and possibly also the Middle East and the Indian Subcontinent, where it has been cultivated for thousands of years for its seeds, from which mustard is produced. It was introduced at an early stage to many other parts of the world and has become naturalized in numerous countries. It is classified as invasive in California, Hawaii, and some of the Great Lakes states in the U.S., in New Zealand, and on off-shore islands in Chile.
Black mustard, growing on a beach in Montaña de Oro State Park, California. (Photo copyright © by Kaj Halberg)
Broussonetia papyrifera Paper mulberry
This small tree of the fig family (Moraceae) is native to East and South Asia and possibly also to some Pacific islands. It thrives in a wide range of habitats and climates, readily growing in disturbed areas. It is dioecious, and when male and female plants grow together, and seeds are produced, it spreads rapidly. Birds and other animals eat the fruits and thus help dispersing the species. It can also form dense stands via its spreading root system.
As its name implies, fibres of paper mulberry were formerly utilized to produce paper, and in parts of the Pacific, cloth is still made from the bark. The wood is used for making furniture and utensils, and the roots can be used as rope. The orange fruit is fleshy and edible, and the leaves can also be eaten when cooked. Fruit, leaves, and bark were formerly used in traditional medicine.
In the United States, paper mulberry was introduced as a fast-growing shade tree, but due to its vigorous growth it soon began to displace native species, and today it is considered to be an invasive in the south-eastern states. In Pakistan, it is regarded as one of the worst weeds, and it is a highly significant invasive plant on the pampas of Argentina. It is also one of the most dominant invasive species in forests of Ghana and Uganda.
A dense growth of paper mulberry covers an abandoned plot outside Hanoi, Vietnam. (Photo copyright © by Kaj Halberg)
Paper mulberry is very common in Taiwan, even in cities, where it pops up in cracks everywhere. In this picture, it grows along a sidewalk in Taichung. (Photo copyright © by Kaj Halberg)
The leaf shape of paper mulberry varies from almost entire to deeply indented. – Taichung, Taiwan. (Photos copyright © by Kaj Halberg)
The orange fruit is fleshy and edible, and the leaves can also be eaten when cooked. – Taiwan. (Photo copyright © by Kaj Halberg)
Cardiospermum halicacabum Balloon vine
Balloon vine, which is also called heart pea or love-in-a-puff, is a climber, which often covers huge areas. It is widely distributed in tropical and subtropical Africa and Asia, and has also become naturalized in many other parts of the world. In New Zealand, where it is considered a pest, trading of its seeds is prohibited.
The medical usage of balloon vine is described on the page Traditional medicine.
Balloon vine, growing in a dried-out riverbed in Taichung, Taiwan. (Photo copyright © by Kaj Halberg)
Carduus nutans Musk thistle
This plant, also called nodding thistle, is native to parts of Eurasia and North Africa, but has spread to many other areas of the world. It grows in open areas, preferably on disturbed ground, such as fallow fields and road sides. In the early 19th Century, it was introduced to North America, where it quickly became a nuisance in agricultural areas. It is declared a noxious weed in the U.S., Canada, Australia, New Zealand, and South Africa.
Musk thistle, Devil’s Tower National Park, Wyoming, United States. (Photo copyright © by Kaj Halberg)
Carpobrotis edulis Hottentot fig
Several species of the iceplant family (Aizoaceae), which are native to Southern Africa, have become established on numerous beaches around the world, often covering huge areas and expelling native species.
Two such species are hottentot fig and red iceplant (Malephora crocea), which have been introduced as an ornamental in many countries around the world. Red iceplant was also widely planted along Californian highways in areas prone to fire, as its succulent leaves do not easily catch fire. However, it soon escaped, spreading to numerous beaches in California and Baja California, where it has become a noxious weed, covering large areas.
Hottentot fig is regarded as an invasive plant in numerous areas, especially around the Mediterranean, along the American Pacific coast, and in Australia. This picture is from Mac Kerricher State Park, California. (Photo copyright © by Kaj Halberg)
Red iceplant, Border State Park, south of San Diego, California. (Photo copyright © by Kaj Halberg)
Celastrus orbiculatus Oriental bittersweet
This climber, of the staff-tree family (Celastraceae), is native to eastern Asia. It was introduced to North America from China around 1860 as an ornamental, but has since escaped cultivation and become widely naturalized. In the eastern part of the continent, it is a menace, affecting the ecology in about 33 states. Birds and other animals relish its fruits, which has contributed to its spreading. It closely resembles the native American bittersweet (Celastrus scandens), with which it readily hybridizes, thus threatening the existence of this local species.
Flowering oriental bittersweet, Massachusetts. (Photo copyright © by Kaj Halberg)
Oriental bittersweet produces lovely fruits. – Long Island. (Photo copyright © by Kaj Halberg)
In this picture, oriental bittersweet is enveloping two conifers near Lenox, Massachusetts. In the background autumn foliage of sugar maple (Acer saccharum) and other trees. (Photo copyright © by Kaj Halberg)
Cirsium arvense Creeping thistle
This Eurasian plant, in America called Canada thistle, is a most troublesome weed in most cooler parts of the World. As far back as 1863, Danish research showed that a large thistle can produce about 80 flowerheads in one season, and as flowerheads on average contain 120 flowers, one single plant is able to produce about 10,000 seeds per year!
However, the success of this plant is not only due to its production of seeds. Its many spines cause many animals to avoid eating it, but the smartest ‘trick’ is its root system. The underground stem, which is situated at a depth of 20 to 40 cm, grows lively in all directions, and just a tiny bit of underground stem is able to produce a large colony of plants, sending up stems at regular intervals. Some shoots grow downwards to a depth of a couple of metres, where the ground is always humid. In this way, the plant always has a secure source of water, no matter how dry the weather may be. Also, an even deep-ploughing plough does not reach the deepest stems.
If you want to control creeping thistle in your garden, the best way is to exhaust it by constantly removing the green shoots, as soon as they appear, and keep on doing it. As Danish poet Thomas Kingo (1634-1703) expressed it: “Evil weed does not perish by itself.” And a proverb by Danish author Peder Syv (1631-1707) says: “Evil weeds grow fastest and perish latest.” (Source: H. Knudsen 2014. Fortællingen om Flora Danica. Lindhardt & Ringhof, in Danish)
In his delightful book All about Weeds, American botanist Edwin Spencer (1881-1964) writes: “(…) it is, perhaps, the worst weed of the entire United States. (…) Its seeds serve as food for goldfinches and sparrows, but even this good turn is offset by the fact that the birds, in getting their food, set free the winged seeds, and wherever those seeds fall trouble begins. The plant is outlawed in every northern State; thirty-seven States in all legislate against this rogue, but outlawing it has had very little effect upon it.”
Fallow field, full of creeping thistles, Jutland, Denmark. (Photo copyright © by Kaj Halberg)
One individual creeping thistle can spread up to 10,000 seeds per season. – Zealand, Denmark. (Photo copyright © by Kaj Halberg)
Clematis vitalba Traveller’s joy
Traveller’s joy is also known as old man’s beard, referring to its fuzzy fruits. This climber of the buttercup family (Ranunculaceae), which has pretty white flowers, is native to Europe and North Africa. It has been introduced to many other countries, where it has become naturalized in numerous areas. Due to its vitality, it is often able to restrict the growth of young trees in plantations, and it is now listed as an invasive in several countries, including New Zealand, where it has been declared an ‘unwanted organism’, and in the United States.
Young shoots of this species can be cooked as a vegetable, but must be cooked thoroughly, as it is slightly toxic. Formerly, its stems were used to make baskets.
In this picture, traveller’s joy is climbing over young noble firs (Abies procera) in a Danish plantation. (Photo copyright © by Kaj Halberg)
Convolvulus arvensis Field bindweed
This is a Eurasian species, which twines around other plants to compete for sunlight and nutrients. It was probably accidentally introduced into North America around 1740. Today it is regarded as one of the world’s worst weeds, which invades agricultural fields, reducing crop yields by 20-80%. In 1998 alone, it was estimated that crop losses due to field bindweed in the U.S. exceeded 377 million Dollars.
In America, popular names of this species include Creeping Jenny and many others, alluding to its invasive nature. In his delightful book All about Weeds, American botanist Edwin Spencer (1881-1964) writes: “Creeping Jenny is one of the meanest of weeds. That name aptly describes it. A whispering little hussy that creeps in and spoils everything. The weed needs no other name than this, but it has several others (…) hedge bells, corn-lily, withwind, bellbine, lap-love, sheep-bine, corn-bind, bear-bind, and green vine.”
Many other members of the morning-glory family are described on the page Plants: Morning-glories and bindweeds.
Field bindweed, growing on a beach, Bornholm, Denmark. Beaches are among the natural habitats of this species. (Photo copyright © by Kaj Halberg)
Conyza canadensis Canadian fleabane
See Erigeron canadensis.
Cortaderia selloana Pampas grass
This huge grass species, which may grow to a height of 3 m, is native to southern South America, named after the Pampas region. It has been introduced as an ornamental plant, and also as animal feed, to numerous other areas, including southern Europe, the United States, China, Australia, and New Zealand. When dried, the plumes are widely used in flower arrangements, and in China, the strong stems have been used in the construction of kites.
Pampas grass is very adaptable, and a single plant can produce over one million seeds during its lifetime. As a result, it has become invasive in many places, including New Zealand, Florida, California, Hawaii, South Africa, and Spain.
Pampas grass is highly invasive in New Zealand. These pictures were taken on the Karikari Peninsula (top) and near Trouson Kauri Park. (Photos copyright © by Kaj Halberg)
Cytisus scoparius Common broom
In northern Europe, and also in parts of North America, most common brooms are of South European origin, imported as ornamentals. They often escape, in many places becoming invasive, dispelling native vegetation. Genetically, they also pollute the original, low and creeping variety of broom, which is today very rare. In Denmark, for instance, it is only found in a few moors and dunes in western Jutland.
Common broom is described in detail on the page Plants: Plants in folklore and poetry.
In this picture from central Jutland, Denmark, common broom grows together with another alien invasive plant, the large-leaved lupine (Lupinus polyphyllus), which is a native of America. The latter is presented elsewhere on this page. (Photo copyright © by Kaj Halberg)
Here, a large growth of common broom has been established in the moorland Hammerknuden, northern Bornholm, Denmark. (Photo copyright © by Kaj Halberg)
Today, the original North European form of common broom, which is low and creeping, is only found in a few moors and dunes in western Denmark, here at Sønder Vosborg Hede, Jutland. (Photo copyright © by Kaj Halberg)
Eichhornia crassipes Water hyacinth
The water hyacinth has pretty flowers, and from its native South America it has been introduced as an ornamental to many tropical and subtropical countries. Almost everywhere it has escaped cultivation, invading countless streams and lakes. Through its massive vegetative growth, it blocks the free flow of water, thus being a serious threat to the natural ecology in lakes and rivers.
The water hyacinth has pretty flowers and has been introduced as an ornamental to many countries. (Photo copyright © by Kaj Halberg)
In this picture, water hyacinths cover the entire surface of a lake near Rajnagar, Odisha (Orissa), India. (Photo copyright © by Kaj Halberg)
In Keoladeo National Park, Rajasthan, India, these people are removing water hyacinths from a waterhole. (Photo copyright © by Kaj Halberg)
Elaeagnus umbellata Japanese silverberry
This species, also known as autumn olive, is indigenous to East Asia, from the Himalaya to Japan. As early as c. 1830, it was introduced into the United States. In the 1950s, it was widely promoted as a splendid species to control erosion in environmentally disturbed areas, at the same time providing food for wildlife. However, it soon became an invasive species, displacing native sun-loving plants by creating dense shade. One bush can produce up to 200,000 seeds a year, and its nitrogen-fixing root nodules allows it to grow in even the poorest soils. Attempts to remove bushes by cutting and/or burning are in vain, as it easily sprouts from the roots. Birds devour the seeds, thus aiding in the dispersal of the species.
Under the Alberta Weed Control Act of 2010, Japanese silverberry is characterized as a ‘prohibited noxious weed’.
Japanese silverberry, New Jersey, United States. (Photo copyright © by Kaj Halberg)
Elytrigia repens Couch grass
This grass, which is also called Elymus repens, is a native of Eurasia and north-western Africa. It is an extremely troublesome weed in fields and gardens, as just a tiny bit of underground stem is able to produce a large colony of plants, sending up stems at regular intervals. This species has become naturalized in North America, where it is regarded as an invasive.
The medical usage of couch grass is described on the page Traditional medicine.
Couch grass in evening light, Jutland, Denmark. In this picture, it grows in a natural habitat of this species: a sandy beach. (Photo copyright © by Kaj Halberg)
Erigeron canadensis Canadian fleabane
This plant, which is often called horsetail fleabane, and in Latin Conyza canadensis, is native to North America and parts of Central America. It has been accidentally introduced to large parts of the world, in many places becoming a serious pest, especially in Europe and Australia, but also in its native North America. It prefers to grow in undisturbed areas and is particularly troublesome in newly established plantations, where it is able to resist herbicides, growing to 3 m tall, thus depriving planted species of nutrients and sunlight.
This plant contains an oil with a turpentine-like smell, which, supposedly, should deter fleas, hence its common name. Another popular name is bloodstanch, given by herbalists, who claim that an extract from leaves and flowers arrests haemorrhages from the lungs and alimentary tract.
This horsetail fleabane, observed in a graveyard in the city of Taichung, Taiwan, is almost 2 m tall. (Photo copyright © by Kaj Halberg)
In Denmark, where this picture was taken, horsetail fleabane is a serious pest in young coniferous plantations. (Photo copyright © by Kaj Halberg)
Fallopia japonica Variegated Japanese knotweed
See Reynoutria japonica.
Heracleum mantegazzianum Giant hogweed
This large plant, often called giant cow-parsley, and in Latin Heracleum pubescens, is native to the Caucasus Mountains and Central Asia. In the 1800s, it was introduced as an ornamental to Europe, and later to the United States and Canada. It has become naturalized in many areas, especially wetlands, and due to its vigorous growth it often expels native species. Furthermore, its sap is toxic to the human skin, causing inflammation and blisters. In many countries, its spreading is inhibited by using herbicides – the only known efficient means to control this species.
Where undisturbed, giant hogweed grows to huge dimensions – note the person in the centre of this growth from northern Jutland, Denmark. (Photo copyright © by Kaj Halberg)
Ipomoea cairica Railroad creeper
The railroad creeper, also called coast morning-glory or mile-a-minute vine, is believed to be a native of Tropical Africa, but today it has a very wide distribution in tropical and subtropical areas around the world. This species is capable of very rapid growth, sometimes completely entwining trees and bushes, but it is also able to creep along the ground. It is regarded as an invasive in many areas, including eastern Australia, southern China, and Taiwan.
Other members of the morning-glory family are described on the page Plants: Morning-glories and bindweeds.
These pictures are from Taiwan, where railroad creeper is extremely common. (Photos copyright © by Kaj Halberg)
Leucaena leucocephala White leadtree
This species, also called white popinac, is a member of the Mimosoideae, a subfamily of the huge pea family (Fabaceae). It is native to southern Mexico and parts of Central America, but has become naturalized throughout the tropics and parts of the subtropics. White leadtree is a prolific species, forming dense growths, often dispelling native vegetation. It is regarded as an invasive in numerous countries, including Taiwan, Hong Kong, several Pacific islands, northern Australia, southern United States, Puerto Rico, and parts of Europe and South America. By the Invasive Species Specialist Group (IUCN) it is considered to be among the world’s one hundred worst invasive species.
White leadtree is extremely common in Taiwan, where it has taken over large tracts of fallow land. This picture, showing the reddish seed pods, is from Taichung. (Photo copyright © by Kaj Halberg)
A bee is collecting pollen in a flowerhead of white leadtree, Cigu Wetlands, Tainan, south-western Taiwan. A spider is lurking just to the right of the flower-head. (Photo copyright © by Kaj Halberg)
Lonicera japonica Japanese honeysuckle
This climber, which is native to eastern Asia, including China, Taiwan, Japan and Korea, has been imported into numerous countries as an ornamental and has become naturalized many places, including the United States, Mexico, Brazil, Argentina, Australia, New Zealand, as well as a number of Pacific and Caribbean islands. Due to its vigorous growth, and the production of numerous seeds, it is extremely difficult to control. In many U.S. states, it is classified as a noxious weed, and it is banned in New Hampshire. In New Zealand, the National Pest Plant Accord has declared Japanese honeysuckle ‘an unwanted organism’.
Japanese honeysuckle, Great Smoky Mountains National Park, Tennessee. (Photo copyright © by Kaj Halberg)
Lupinus polyphyllus Large-leaved lupine
Lupines is a large genus of the pea family (Fabaceae), comprising more than 200 species, mainly distributed in the Americas, but also some species around the Mediterranean and in North Africa. According to Collins English Dictionary, the common, as well as the generic name of these plants, stems from the Latin lupinus (‘wolfish’), referring to an old belief that these plants would ravenously exhaust the soil.
A native of western North America, the large-leaved lupine is a pretty garden plant, which readily escapes cultivation, forming large growths. It is regarded as an invasive species in many countries, including New Zealand, Finland, Norway, Switzerland, Czech Republic, and Lithuania.
In this picture, large-leaved lupine has taken over a large clearing in a forest near Sankt Peter am Kammersberg, Austria. (Photo copyright © by Kaj Halberg)
Malephora crocea Red iceplant
See Carpobrotis edulis.
Melia azedarach Persian lilac
Persian lilac, or Chinaberry, is probably native to Iran and the Indian Subcontinent, but due to its beautiful flowers and fruits it has been widely planted elsewhere. It readily spreads and has become an invasive various places, including North America, East Africa, some Pacific Islands, New Zealand, and Australia.
The medical usage of this plant is dealt with on the page Traditional medicine.
Persian lilac, photographed in Taiwan, where this species is commonly planted as an ornamental. (Photos copyright © by Kaj Halberg)
Oxalis pes-caprae African wood-sorrel
This pretty plant is a native of South Africa, but has been accidentally introduced to many corners of the planet. It is regarded as an invasive in many places, including North America, Israel, Australia, and several European countries.
African wood-sorrel has displaced the natural vegetation in this littoral meadow near Cape San Martin, California. (Photos copyright © by Kaj Halberg)
Panicum repens Torpedograss
The native range of this tall grass species, which is also known by many other names, including panic rampant, wainaku grass, quack grass, dog-tooth grass, and bullet grass, is unknown. It has been introduced to many parts of the world and has become a noxious weed in numerous places.
J. Byrd & V. Maddox, Department of Plant and Soil Sciences, Mississippi State University, write that it “is considered one of the World’s worst weeds. (…) When fully matured, torpedograss can grow up to 3 feet tall and form dense monotypic stands that out compete native species and lead to a loss of diversity and overall ecological health.”
Torpedograss is extremely common in Taiwan, here photographed in the Huoyan Mountains (’99 Peaks’), Pinglin (top), and in a graveyard in Taichung. (Photos copyright © by Kaj Halberg)
Polygonum cuspidatum Variegated Japanese knotweed
See Reynoutria japonica.
Prunus serotina Black cherry
Black cherry, which is native to North America, has been widely planted in Europe, and also in South Africa. It readily grows in shady places, thus being able to spread easily. In many European countries the species has become an invasive, and in threatened habitats like moors and grasslands it is a threat to indigenous plant species. Leaves and bark of black cherry are toxic, and in South Africa it is regarded as a leading cause of livestock illness.
Black cherry, Cape May, New Jersey, United States. (Photo copyright © by Kaj Halberg)
Pteridium aquilinum Bracken
Bracken has an almost cosmopolitan distribution, found in most temperate and subtropical regions of the world. Areas, which have been utilized for farming or grazing, and then abandoned, are readily invaded by this species, which can cover huge areas, hindering growth of other species.
The forest on this mountain top north of Songea, Tanzania, has been felled, and the clearing has been completely taken over by bracken. (Photo copyright © by Kaj Halberg)
Despite having withered, these bracken still almost hide young trees in a coniferous plantation in Jutland, Denmark. (Photo copyright © by Kaj Halberg)
Pueraria montana ssp. lobata Kudzu
The kudzu vine, or Japanese arrowroot, is a native of eastern Asia, but was introduced to the United States in 1876 as a means to curb soil erosion. In the 1930s and 1940s, it was cultivated in an area covering more than a million acres. However, it soon escaped, spreading along roadsides to most of the eastern and south-eastern states, and also to southern Canada. It grows vigorously, often completely enveloping indigenous vegetation and over time suffocating it. Today, kudzu is regarded as a serious pest in several countries, including the United States and New Zealand. In the latter country, it has been declared an ‘unwanted organism’.
Kudzu is much utilized in Chinese traditional medicine, described on the page Traditional medicine.
These pictures of kudzu are from Taiwan, where it is a native. (Photos copyright © by Kaj Halberg)
Ranunculus acris Tall buttercup
Tall buttercup, which is native to temperate Eurasia, Greenland and Alaska, has been accidentally introduced by humans to many other parts of the world, and in North America (other than Alaska) it is regarded as an invasive. The specific name is from the Latin acer (’sharp’), referring to its bitter taste. All buttercup species are poisonous, for which reason they are avoided by grazing animals, as seen in the picture below.
This horse is grazing in a field, full of tall buttercups, Maryland, United States. (Photo copyright © by Kaj Halberg)
Ranunculus ficaria Lesser celandine
This species, which was formerly called Ficaria verna, is native to Europe and western Asia., but has been accidentally introduced to many areas in eastern and north-western United States, and to Canada. In these countries, it is regarded as an invasive, which has expelled native species in large tracts of forest.
Incidentally, the name lesser celandine is a misnomer. The greater celandine (Chelidonium majus) is a plant of the poppy family (Papaveraceae), presented in detail on the page Plants: Plants in folklore and poetry. In the early days of classification, certain botanists thought that the plant, which is today called Ranunculus ficaria, was related to greater celandine, naming it Chelidonium minus, and in English lesser celandine. For some reason or other, both English names of these entirely unrelated plants stuck.
Lesser celandine, Shu Swamp Preserve, Long Island, United States. Leaves of eastern skunk cabbage (Symplocarpus foetidus) are seen in the foreground. This species is described on the page Plants: Plants in folklore and poetry. (Photo copyright © by Kaj Halberg)
Reynoutria japonica Variegated Japanese knotweed
This plant, also known as Fallopia japonica and Polygonum cuspidatum, is native to the Far East, found in Japan, Korea, China, and Taiwan. It was introduced to Europe and North America in the 1800s and soon became a popular garden plant because of its bamboo-like appearance, vigorous growth, and pretty inflorescences. However, it quickly turned into a menace, as it spread beyond control, its strong root system being able to “damage concrete foundations, buildings, flood defences, roads, paving, retaining walls, and architectural sites.” (Source: gardenroots.co.uk)
Variegated Japanese knotweed is listed by the World Conservation Union as one of the world’s worst invasive species.
Variegated Japanese knotweed, growing on Hohuan Shan Mountain, central Taiwan, where this species is a native. (Photo copyright © by Kaj Halberg)
Seeds and autumn leaves of variegated Japanese knotweed, photographed in Denmark. (Photo copyright © by Kaj Halberg)
Rhododendron ponticum Pontic rhododendron
During his travels in the Middle East 1700-1702, French physician and botanist Joseph Pitton de Tournefort (1656-1708) encountered a species of rhododendron, growing on the Black Sea coast in the area of Pontus, in present-day north-eastern Turkey and Georgia. For this reason, Swedish naturalist Carolus Linnaeus (1707-1778), also called Carl von Linné, named it Rhododendron ponticum (‘Pontic rhododendron’). The nominate subspecies is native to the Caucasus, Turkey, Lebanon, and Bulgaria, whereas small populations of subspecies baeticum are distributed in south-western Spain and Portugal.
In 1763, Pontic rhododendron was introduced to Britain as an ornamental, and it was also planted as cover for game birds. It quickly became naturalized, spreading by suckers on the tips of the branches. Today, in England, Wales, and Ireland, it is a widespread menace, which has colonized numerous hillsides, moorlands, and shady woodlands, often replacing local plant species.
Many pictures of other rhododendron species may be seen on the page Plants: Rhododendrons.
Pontic rhododendron, photographed near the town of Espiye, on the Black Sea coast, northern Turkey. (Photo copyright © by Kaj Halberg)
Rosa multiflora Many-flowered rose
This rose is a native of eastern China, Taiwan, Korea, and Japan, but has been introduced as an ornamental to many countries, including the United States, Canada, South Africa, and most European countries. It is also used as a rootstock for other ornamental rose species, and in the U.S. it has been planted to prevent soil erosion. However, it readily escapes cultivation, forming dense thickets along roads, in abandoned fields, on grazing grounds and prairies, and in open forests. In numerous places, it has become a serious pest, spreading beyond control and expelling native plant species. Today, it is regarded as an invasive plant in many countries.
Escaped many-flowered rose, Bornholm, Denmark. (Photo copyright © by Kaj Halberg)
In Taiwan, a variety of many-flowered rose, R. multiflora var. formosana, is native. (Photo copyright © by Kaj Halberg)
Rosa rugosa Beach rose
This beautiful species, which is also called rugosa rose, Japanese rose, or wrinkled rose, is native to south-eastern Siberia, north-eastern China, Korea, and Japan. It was first introduced as an ornamental to northern Europe in 1796, and to eastern North America in 1845. In Europe, it began spreading to coastal habitats in the 1850s, and in North America in 1899. Today, it is abundant on all North European and New England beaches, where it is considered a serious pest, which expels natural vegetation.
The beach rose is extremely common on Danish beaches, where it expels the natural vegetation, here on the island of Bornholm. (Photos copyright © by Kaj Halberg)
Rubus plicatus Blackberry
Blackberry, often named Rubus fruticosus, is a European native, but has been introduced to many other areas. It is highly invasive in some countries, forming dense thickets, which expel native vegetation and often threaten entire ecosystems. It is considered a noxious weed in many countries, including Australia, New Zealand, and the United States.
This picture of blackberry is from Denmark, where it is a very common native. (Photo copyright © by Kaj Halberg)
Solidago gigantea Giant goldenrod
Giant goldenrod is an American plant, which was introduced to Europe as an ornamental in the mid-1700s. The first naturalized populations were observed about a hundred years later, but it was not considered an invasive before the 1950s. Since then, it has spread rapidly, often expelling native vegetation over large areas, and today it is regarded as an invasive in all European countries.
The name Solidago is from the Latin solido, meaning ‘to join’, or ‘make whole’. Formerly, several species of goldenrod were reputed to be able to heal wounds.
Large growth of giant goldenrod, photographed on the island of Zealand, Denmark. (Photo copyright © by Kaj Halberg)
Spartina anglica Common cordgrass
Common cordgrass is a European species, which evolved as a fertile hybrid between Spartina maritima of western Europe and S. alterniflora of eastern North America. It has spread to numerous tidal flats, becoming a serious pest, which expels natural vegetation.
Common cordgrass, growing on a tidal flat, Fanø, Jutland, Denmark. (Photo copyright © by Kaj Halberg)
Tradescantia zebrina Wandering Jew
This plant, which is native to Mexico, Central America, and Colombia, was named after John Tradescant, a Royal British horticulturist in the 17th Century. It is a popular garden plant, which, however, has a tendency to spread, mostly in disturbed forests, but also in native forests of St. Lucia in the West Indies, and in Hawaii. Stems, which have broken off, are able to take root, and, over the years, create veritable carpets on the forest floor. Today, it is regarded as an invasive in many places, including South Africa, Brazil, Galapagos Islands, St. Lucia, Hawaii, and Taiwan. In South Africa, all trade in seeds and cuttings of the species is prohibited.
In Malabang National Forest, Taiwan, where this picture was taken, Wandering Jew covers large areas. (Photo copyright © by Kaj Halberg)
Ulex europaeus Common gorse
Due to its pretty flowers, this European species, of the pea family (Fabaceae), has been introduced to many other areas around the world, including North and South America, South Africa, Australia, and New Zealand. It readily escapes cultivation and has become a serious invasive in numerous areas, notably the western United States, New Zealand, Sri Lanka, and Chile. To some extent, its spreading can be controlled by the gorse spider mite (Tetranychus lintearius) and the gorse seed weevil (Exapion ulicis).
In California, common gorse is a very serious pest, here photographed at Tomales Bay. (Photo copyright © by Kaj Halberg)
Large growths of common gorse have taken over these bluffs near Kai Iwi, New Zealand. (Photo copyright © by Kaj Halberg)
Due to its pretty flowers, common gorse was introduced at an early stage to many places around the world. This one was observed in Jughandle State Park, California. (Photo copyright © by Kaj Halberg)
Vinca minor Lesser periwinkle
This creeping plant is probably native to central and southern Europe, from Portugal and France eastwards to the Caucasus, northwards to the Netherlands and the Baltic States. However, its original distribution is difficult to ascertain, as it was introduced widely in Europe at an early stage, cultivated as an ornamental. The word periwinkle, which was first used in English in 1922, was taken from the flower colour of this plant. It is a shade of lavender blue.
Lesser periwinkle was first introduced to North America in the 1700s. It has since escaped cultivation in many places throughout the eastern and north-western United States, often invading natural areas and displacing native species. It is also often an invasive in northern Europe and elsewhere.
Lesser periwinkle, Funen, Denmark. (Photo copyright © by Kaj Halberg)
In this picture, lesser periwinkle is covering the entire forest floor of a copse in Oyster Bay Cove, Long Island. (Photo copyright © by Kaj Halberg)
Large growth of lesser periwinkle in a forest, Funen, Denmark. (Photo copyright © by Kaj Halberg)
Zantedeschia aethiopica White calla
This spectacular plant of the arum family (Araceae), which is also called arum lily, is native to southern Africa. However, it is widely cultivated in warmer regions of the world, where it often escapes. It has become naturalized in a number of places, including other African countries, New Zealand, Madeira, coastal California, and Australia. In the latter country, it has been classified as a pest.
White calla often escapes cultivation in New Zealand, where these pictures were taken, in a ditch near Mokau River (top), on a beach near Omata (centre), and at Whanganui River. (Photos copyright © by Kaj Halberg)
Since 1975, the major part of larger elms (Ulmus) in Europe and North America have been killed by Dutch Elm disease, caused by the sac fungus Ophiostoma novo-ulmi (formerly Ceratocystis novo-ulmi), which is spread by elm bark beetles of the genera Scolytus and Hylurgopinus. It is generally believed that the disease originated in Asia, but was accidentally introduced to Europe and North America, where most native elm species had no resistance against the disease.
Dutch Elm disease is described in detail on the page Nature Reserve Vorsø: Dutch Elm disease on Vorsø.
These common elms (Ulmus glabra) were photographed on the island of Vorsø, Denmark, where all larger elms have succumbed to Dutch Elm disease. (Photo copyright © by Kaj Halberg)
(Uploaded March 2016)
(Latest update February 2020)