Impatiens glanduliferaCC BY-SA 4.0

Invasive plants in the EU Part 6: Annual herbaceous plants

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Union list of invasive species

There are around 12,000 alien species in the EU. A small number of them require special attention as they can endanger the populations of native species.

The EU regulation on the prevention and management of the introduction and spread of invasive alien species is intended to prevent these species from spreading and to enable a rapid response when the first signs of spread appear. The exact species concerned are listed in the “Union list”. Of the 88 invasive species listed, 40 are vascular plants. In this series’ previous articles, we have presented shrubs and trees, aquatic plants, grasses, vines and perennial herbaceous plants; today we are looking at annuals.

Impatiens glandulifera, the Indian balsam

The Indian balsam (Impatiens glandulifera) is widespread in large parts of Europe, where it mainly colonizes moist forests and alluvial and riparian landscapes. The plants can quickly grow to over 2.5 meters tall and form a dense, shallow root network. Due to their large leaves and dense stands, they shade their location so much that no other plants can establish themselves during the course of the year.  If the Indian balsam dies back in the fall, the ground is left bare and susceptible to erosion. This is particularly problematic along watercourses. As it is an annual, it is still possible to manage the first colonizations by cutting them low before the seeds mature. Pulling them out is also effective if all parts of the plant are quickly composted. If parts of the shoot remain in the soil, they will quickly take root again.

Parthenium hysterophorus, the Congress weed

Parthenium hysterophorus blooms from March to November. There are already populations in Belgium and Poland, which are currently being controlled. Due to climate change, experts assume that other regions will be potential areas for establishment.  Just a few plants produce a large number of seeds, which are dispersed by the wind and remain viable for a long time. Congress weed is considered particularly problematic in pasture and arable land as well as in horticulture, as the species has allelopathic properties – it releases substances into its environment that suppress the growth of other plants. But the species is also harmful to humans: the substances it contains can trigger hay fever and dermatitis.

Documenting the spread

If you identify plants with Flora Incognita and allow access to your location, your find will become part of a scientific data collection that enables our scientists to research the distribution of species in time and space. You can read about our research work here. This data can also be used to plan and implement nature conservation management measures quickly and effectively. Thank you for your help!

Attention! The species on the Union list may not be intentionally introduced into the territory of the EU, kept, bred, traded, used, exchanged, used for reproduction or released into the environment!

Featured image: Indian balsam. (MHNT) Impatiens glandulifera.jpg by Didier Descouens. CC BY-SA 4.0.

Heraclum persicumPublic Domain

Invasive plants in the EU Part 5: Perennial herbaceous plants

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Union list of invasive species

There are around 12,000 alien species in the EU. A small number of them require special attention as they can endanger the populations of native species.

The EU regulation on the prevention and management of the introduction and spread of invasive alien species is intended to prevent these species from spreading and to enable a rapid response when the first signs of spread appear. The exact species concerned are listed in the “Union list”. Of the 88 invasive species listed, 40 are vascular plants. In this series’ previous articles, we have presented shrubs and trees, aquatic plants, grasses and vines; today we are looking at perennials. Make sure to also check out our article about invasive annual herbaceous plants.

Gunnera tinctoria, the giant rhubarb

Gunnera tinctoria was introduced to England in 1849, from where wild populations have been known since 1908 and from Ireland since the 1930s. There are already isolated and established populations in other European countries. The giant rhubarb prefers the banks of running and still waters, but also roadsides and quarries. It forms dominant stands that shade out all original vegetation and thus displace it. Its dense rhizome system also influences the nutrient balance of the soil.

Heracleum persicum, the Persian hogweed

Heracleum persicum was introduced to Europe from Central Asia as an ornamental plant for botanical gardens. It currently only occurs wild in northern Europe, where it spreads rapidly along the seashore and riverbanks, displacing other species through dominance. The exact differentiation from the other two Heracleum species on the Union list often poses a challenge even for experts:

Heracleum mantegazzianum, the giant hogweed

Originating from the Western Caucasus, giant hogweed (Heracleum mantegazzianum) was considered a good bee pasture and was also popular for planting as cover for wild animals. The species was quickly spread by building rubble and garden waste. With a height of up to five meters, a stem diameter of around 10 cm and leaves that grow up to two meters long, they are impressive plants, but have a considerable impact on human health. Their sap contains phototoxic substances that can cause severe skin inflammation and burns when touched and exposed to sunlight. Dense populations of this species have a negative impact on plant composition and the seed bank in the soil. The management of giant hogweed in Germany is estimated at 10 million euros per year.

Heracleum sosnowskyi, the Sosnowskyi hogweed

Heracleum sosnowskyi was also introduced to Europe as an ornamental plant and bee pasture. Like the other two Heracleum species, it spreads by shoots in soil and earth transports, as well as the numerous seeds that are produced each season. There are already stable populations in the Baltic states, the European part of Russia and in the Ukraine, while there are unstable findings in other countries. Like the other Heracleum species, it displaces existing diversity, alters soil chemistry and causes severe phototoxic burns on contact.

Lespedeza cuneata, the Japanese clover

The Japanese clover (Lespedeza cuneata) is considered a good forage plant and may have been introduced to Europe via hay imports. Release into the wild from horticultural facilities is also considered a possible import vector. It is likely to colonize regions in northern Europe with drier winters and wetter summers. It prefers habitats such as meadows, pastures, semi-natural grasslands, and heathlands. Studies from other regions indicate that this species displaces others through dense populations, chemically alters the soil through nitrogen enrichment and attracts more pollinators than native species flowering at the same time.

Koenigia polystachya, the Himalayan knotweed

In its natural range, the Himalayan knotweed (Koenigia polystachya) colonizes forests and valleys at high altitudes between 2,200 and 4,500 meters. It reproduces through both seeds and vegetative means. Stem segments as short as one centimeter are enough to create a new colony. Once established, this species forms dense populations that crowd out native alpine and subalpine plants. The Himalayan knotweed can also be found outside the mountains, along roadsides, transportation routes like railroad tracks, forest edges, meadows, water banks, wetlands, and in towns and villages

Lysichiton americanus, the western skunk cabbage

Beautiful to look at, but a threat to regional biodiversity: the western skunk cabbage (Lysichiton americanus) found its way into Europe as an ornamental plant, and was even deliberately introduced into the wild. On near-natural, shaded banks of watercourses, springs, swamps and swamp forests, it spreads mainly vegetatively, but also via the many seeds it produces (300 to 650 per cob). These plants have a long lifespan of up to 80 years and grow slowly, which sets them apart from other invasive species that spread rapidly. It has already been detected in Finland, Sweden, Denmark, Ireland, Great Britain, the Netherlands, Belgium, France, Germany and Switzerland. The robust plants, which are around 1 meter tall, displace sensitive wetland species by competing for resources.

Documenting the spread

If you identify plants with Flora Incognita and allow access to your location, your find will become part of a scientific data collection that enables our scientists to research the distribution of species in time and space. You can read about our research work here. This data can also be used to plan and implement nature conservation management measures quickly and effectively. Thank you for your help!

Attention! The species on the Union list may not be intentionally introduced into the territory of the EU, kept, bred, traded, used, exchanged, used for reproduction or released into the environment!

Featured image: Persian hogweed (Tromsopalme-topdown.jpg) by Krister Brandser. Public Domain.

Persicaria perfoliataCC BY 2.0

Invasive plants in the EU Part 4: Climbing plants (vines)

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Union list of invasive species

There are around 12,000 alien species in the EU. A small number of them require special attention as they can endanger the populations of native species.

The EU regulation on the prevention and management of the introduction and spread of invasive alien species is intended to prevent these species from spreading and to enable a rapid response when the first signs of spread appear. The exact species concerned are listed in the “Union list”. Of the 88 invasive species listed, 40 are vascular plants. In this series’ previous articles, we have presented shrubs and trees, aquatic plants and grasses; today we are looking at vines. Make sure to also check out our articles about invasive perennial and annual herbaceous plants.

Cardiospermum grandiflorum, the balloon vine

As an ornamental plant, Cardiospermum grandiflorum from Central and South America has spread almost worldwide. However, its small white flowers and attractive fruits should not obscure the fact that the woody shoots can form massive carpets that can completely overgrow trees 10-20 meters high. As a result, they obscure the habitat below and their weight can also lead to the collapse of overgrown structures. Furthermore, wild and grazing animals are prevented from moving or migrating freely.

Lygodium japonicum, the Japanese climbing fern

Lygodium japonicum is native to eastern Asia. The plant forms fronds on its creeping rhizomes, which can grow up to 30 meters long and display climbing traits. The species produces very small spore capsules that can easily be spread via clothing and luggage, but also in potting soil. The Japanese climbing fern is well adapted to the cold and its spores remain viable despite frost. Nevertheless, it prefers warm, humid locations to spread. In North America, the species is already changing the intensity and extent of fires, as it allows flames to climb quickly into the crowns of trees.

Pueraria montana, Kudzu

Pueraria montana, also known as Kudzu, belongs to the legume family and originates from East Asia. According to the IUCN, it is one of the 100 most aggressive invasive neophytes, as it can completely cover and destroy existing vegetation in just a few years. In addition to the perennial, strong lianas, which can grow up to 20 meters per year, it forms root tubers as survival organs. These can reach lengths of 2 meters, diameters of 18 to 45 centimeters and a weight of 180 kg. It colonizes gardens, road embankments and lakeshores. In Switzerland and Italy, Kudzu can already be found in warm habitats.

Celastrus orbiculatus, the round-leaved tree shrike

Once introduced as an ornamental plant, the round-leaved tree shrike (Celastrus orbiculatus) is still available here and there despite a trade ban. It is spread by improper disposal of plant parts and berry-eating birds. If the liana can climb up trees, it grows up to 12 meters high. Otherwise it forms a very dense, ground-covering tangle. When still young, it provides massive shading of the vegetation and increases the risk of young trees breaking. As it grows thicker, the tree strangler exerts increasing pressure on its supporting trees and can “strangle” them.

Humulopsis scandens, Japanese hop

Japanese hop (Humulopsis scandens) was a popular ornamental plant in many countries for greening trellises and fences. It naturally colonizes river courses quite quickly, but its new dispersal routes are determined by human activity. Once it has gained a foothold, it is able to change habitats structurally and functionally by overgrowing and shading, thus reducing species diversity. In particular, reedbeds and willow fringes are threatened by Japanese hops. The species is an annual, which consequently leaves the ground open at the end of the growing season, making it susceptible to erosion by water.

Persicaria perfoliata, the knotweed

Persicaria perfoliata is an annual to perennial liana and can climb up to 8 meters high. The species is not yet established in Europe, but would find good living conditions in forests and disturbed areas. Its seeds are readily eaten by animals, which enables it to spread quickly. In the USA, it forms dense mats that cover the vegetation and cause the underlying species to die. The forestry industry there is already reporting economic damage, as are fruit growers, tree nurseries and the tourism industry.

Documenting the spread

If you identify plants with Flora Incognita and allow access to your location, your find will become part of a scientific data collection that enables our scientists to research the distribution of species in time and space. You can read about our research work here. This data can also be used to plan and implement nature conservation management measures quickly and effectively. Thank you for your help!

Attention! The species on the Union list may not be intentionally introduced into the territory of the EU, kept, bred, traded, used, exchanged, used for reproduction or released into the environment!

Featured image: knotweed (Mile-a-minute-weed (28816287065).jpg) by Katja Schulz. CC BY 2.0.

Ehrharta calycinaCC BY 2.0

Invasive plants in the EU Part 3: Grasses

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Union list of invasive species

There are around 12,000 alien species in the EU. A small number of them require special attention as they can endanger the populations of native species.

The EU regulation on the prevention and management of the introduction and spread of invasive alien species is intended to prevent these species from spreading and to enable a rapid response when the first signs of spread appear. The exact species concerned are listed in the “Union list”. Of the 88 invasive species listed, 40 are vascular plants. In this series’ previous articles, we have presented shrubs and trees and aquatic plants; today we are looking at grasses. Make sure to also check out our articles about invasive vines, perennial and annual herbaceous plants.

Andropogon virginicus, the blue-stemmed sedge

Seeds of the American bluegrass (Andropogon virginicus) were probably brought to Europe via horticultural seed mixtures, contaminated machinery, clothing and hay imports, and contaminated NATO munitions on a military training area in France probably led to its introduction there.
The plant forms persistent tufts that can grow to over 2 meters tall and contain large amounts of dead material, which is considered to be fire-inducing. They quickly develop a dense growth that takes away the space for existing species to develop. In addition, when they decompose, certain chemical substances are released that reduce the fertility of the surrounding soil.

Cortaderia jubata, the purple pampas grass

Pampas grasses are popular ornamental plants, but not all members of this genus have invasive tendencies. In addition, it is difficult to determine the exact species – some taxonomists assume that Cortaderia jubata is only a subspecies of Cortaderia selloana, which also tends to run wild. The seeds need open, sunny and moist conditions to germinate, which makes them particularly problematic in coastal regions and dune landscapes. Even though there are currently only a few places in Europe where Cortaderia jubata is established, model calculations assume that there is a considerable risk of invasion for large parts of southern and western Europe as a result of climate change.

Ehrharta calycina, the steppe grass

Ehrharta calycina, a grass native to Africa, is already established in Portugal and Spain. As its seeds are spread by the wind and germinate easily in open areas, there is also a risk that this species will successfully spread to other parts of Europe. Once established, it transforms diverse habitats such as dunes and bushes into grasslands that pose a major fire hazard. After a fire, it quickly sprouts again and thus successfully suppresses other species. Ehrharta calycina also has allelopathic properties: Its presence leads to a shift in the nutrient cycle by accumulating phosphorus from the biomass in the soil.

Microstegium vimineum, the Japanese stilt grass

Microstegium vimineum has already been documented to spread by various means: The adhesion of the seeds or fruit to travelers’ clothing and shoes, contaminated birdseed, equipment and vehicles used in agriculture, forestry, construction or waste disposal. So far, the species has been detected in Turkey, Georgia and the North Caucasus, along roads and railroads, ditches and forest roads, in riparian forests, wet meadows, commercial forests, along forest and river edges. Its numerous creeping shoots displace weaker species and shade the ground. This results in a variety of changes to the soil properties and the composition of the resident herbivore and arthropod communities.

Cenchrus setaceus, the African lamp grass

Once prized as an ornamental plant, Cenchrus setaceus is now spreading in Spain (Balearic and Canary Islands), France, Italy (Calabria, Sardinia, Sicily), the Portuguese Algarve, Cyprus and Malta. Climate change is making other regions of Europe attractive for colonization, especially dry and stony grasslands and coastal landscapes. As the species draws the remaining water from the soil and alters the nutrient cycle, its presence has a negative impact on local biodiversity.

Documenting the spread

If you identify plants with Flora Incognita and allow access to your location, your find will become part of a scientific data collection that enables our scientists to research the distribution of species in time and space. You can read about our research work here. This data can also be used to plan and implement nature conservation management measures quickly and effectively. Thank you for your help!

Attention! The species on the Union list may not be intentionally introduced into the territory of the EU, kept, bred, traded, used, exchanged, used for reproduction or released into the environment!

Featured image: steppe grass (Ehrharta calycina plant6) by Harry Rose, CC BY  2.0 via flickr.

Pontederia crassipes CC BY-SA 2.0

Invasive plants in the EU Part 2: Aquatic plants

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Union list of invasive species

There are around 12,000 alien species in the EU. A small number of them require special attention as they can endanger the populations of native species.

The EU regulation on the prevention and management of the introduction and spread of invasive alien species is intended to prevent these species from spreading and to enable a rapid response when the first signs of spread appear. The exact species concerned are listed in the “Union list”. Of the 88 invasive species listed, 40 are vascular plants. Today we are looking at aquatic plants. In this series you can also find articles about invasive shrubs and trees, grasses, vines, perennial and annual herbaceous plants.

Spread and damage

The main reason for the spread of invasive aquatic plants is the improper disposal of aquarium and (garden pond) ornamental plants in the wild. Despite the existing trade ban, some of the listed species are available in stores or through community-sharing events. Once in a suitable habitat, they spread rapidly through runners and pieces of the stem cut off by birds or boats. All of the species presented here can form massive populations on and underwater, resulting in serious disruptions to the food chain and nutrient dynamics in the water. In many countries, millions of dollars are already being spent on management measures to clean up water bodies.

Water hyacinth (Eichhornia crassipes)

The water hyacinth (Eichhornia crassipes) is one of the 100 most dangerous neobiota worldwide. It originates from the tropics of South America and has been commercialized worldwide as a pond plant. Without predators, it reproduces en masse, doubling its size in just 2 weeks. Other aquatic plants die under the thick carpet of plants due to a chemical change in the water, which also kills fish. The thick coverage obstructs shipping and fishing and reduces the water’s flow speed, which leads to siltation.

Invasive plants by the water and in shallow zones

Both the water primrose (Ludwigia grandiflora) and its relative, the floating primrose-willow (Ludwigia peploides) form dense carpets in a very short time, and pieces as small as 1 cm in length are enough to establish a new stand. Alligatorweed (Alternanthera philoxeroides) invades mainly natural and semi-natural forests, riparian areas and wetlands. With its stolons up to 10 meters long, it forms dense, interwoven clumps. At flowering time, these protrude above the water.

(Mostly) submerged species in slow-flowing waters

The Nuttall‘s waterweed (Elodea nuttallii) thrives down to a depth of 3 meters and forms dense stands that displace other sensitive species such as the endangered water-soldier (Stratiotes aloides). The curly waterweed (Lagarosiphon major) even forms shoots up to 5 meters long. Two other underwater plants are the carolina fanwort (Cabomba caroliniana), which can survive freezing winters despite its subtropical origin, and the American water-milfoil (Myriophyllum heterophyllum), which can take root at depths of up to 10 meters and still completely cover water surfaces.

Species growing (mostly) on the surface in slow-flowing waters

The floating pennywort (Hydrocotyle ranunculoides) spreads along watercourses, where its up to 6 cm wide, roundish leaves form closed blankets above the water. Controlling the species costs around 10,000 euros per kilometer of canal in the Netherlands. The water lettuce (Pistia stratiotes) is often commercially available despite being banned. Its seeds can survive drought and frost. Large populations impair water management and reduce the availability of light in the water.

Climate change is driving spread

In some thermal waters in Hungary and Italy, the Senegal tea (Gymnocoronis spilanthoides) is already considered established and is changing water structures with its long shoots. However, due to climate change, numerous rivers, canals, lakes and ponds in large parts of Europe could be considered future habitats. The same applies to the free-floating water fern Kariba-weed (Salvinia molesta), which can cover slow-moving watercourses with mats up to 1 m thick under favorable conditions. But the Parrot’s feather (Myriophyllum aquaticum) also benefits from higher water temperatures.

Documenting the spread

If you identify plants with Flora Incognita and allow access to your location, your find will become part of a scientific data collection that enables our scientists to research the distribution of species in time and space. You can read about our research work here. This data can also be used to plan and implement nature conservation management measures quickly and effectively. Thank you for your help!

Attention! The species on the Union list may not be intentionally introduced into the territory of the EU, kept, bred, traded, used, exchanged, used for reproduction or released into the environment!

Featured image: Water hyacinth (Common Water Hyacinth (405615687).jpg) by Dinesh Valke from Thane. CC BY-SA 2.0 via Wikimedia Commons.

Ailanthus altissimaCC BY-SA 3.0

Invasive plants in the EU Part 1: Shrubs and trees

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Union list of invasive species

There are around 12,000 alien species in the EU. A small number of them require special attention as they can endanger the populations of native species.

The EU regulation on the prevention and management of the introduction and spread of invasive alien species is intended to prevent these species from spreading and to enable a rapid response when the first signs of spread appear. The exact species concerned are listed in the “Union list”. Of the 88 invasive species listed, 40 are vascular plants. Today we are looking at shrubs and trees. In this series you can also find articles about aquatic plants, grasses, vines, perennial and annual herbaceous plants.

Shrubs and small trees

The 2-6 meter tall willow-leaf acacia (Acacia saligna) originates from Australia and reproduces rapidly, allowing dominant stands to form. As it also accumulates nitrogen in the soil, it displaces species that are dependent on nutrient-poor soils. The crossbush (Baccharis halimifolia) tolerates high salt levels in the soil, displaces reeds and rushes and thus alters sensitive habitats such as salt marshes. The needle-leaved pincushion (Hakea sericea) colonises disturbed areas in dense stands, e.g. roadsides, forest edges, dry grassland and pine forests. The common milkweed (Asclepias syriaca) was once introduced to Europe as a bee pasture and is now displacing native plant species on dry grassland sites due to its rapid spread and large populations.

Larger trees

There are still no stable populations of the mesquite tree (Prosopis juliflora) in Europe except on Gran Canaria. It is one of the 100 most invasive woody plants in the world, which is why it was included on the list as a precautionary measure. The Chinese tallow tree (Triadica sebifera) is also under observation. Its potential distribution area is the Mediterranean region, the south-west of the Iberian Peninsula and the southern and eastern parts of the Black Sea coast. The tree of heaven (Ailanthus altissima) is already widespread in large parts of Europe, and climate change is further favoring its spread. A single individual can produce up to 325,000 seeds per year. It also reproduces vegetatively by stolons. Its metabolism inhibits the growth of plants in its neighborhood (allelopathy).

Documenting the spread

If you identify plants with Flora Incognita and allow access to your location, your find will become part of a scientific data collection that enables our scientists to research the distribution of species in time and space. You can read about our research work here. This data can also be used to plan and implement nature conservation management measures quickly and effectively. Thank you for your help!

Attention! The species on the Union list may not be intentionally introduced into the territory of the EU, kept, bred, traded, used, exchanged, used for reproduction or released into the environment!

Featured image: Tree of heaven (Ailanthus altissima 004.JPG) by H. Zell. CC BY-SA 3.0.