Invasive plants in the EU Part 2: Aquatic 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. 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.