Invasive plants in the EU Part 5: Perennial herbaceous plants

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.