A few days ago, the State Office for the Environment (LfU) warned against the spread of invasive needleweed in Schleswig-Holstein. But not only the needlewort (Crassula helmsii), other non-native plant species are also spreading in the north. An overview:
What are invasive species anyway?
Not every so-called alien species, i.e. plants and animals that were not originally found here, is invasive. According to the Ministry of the Environment, a species is classified as invasive if it is able to establish and reproduce outside of its natural range and negative effects are noticeable.
According to the LfU, they can affect native species and valuable habitats, cause damage to infrastructure or pose a threat to human health.
Which plant species are particularly problematic?
According to the LfU, from a nature conservation perspective, invasive aquatic plants such as needleweed (Crassula helmsii) are particularly problematic. They can easily be spread unintentionally and can often only be eliminated with extensive measures.
Coniferous cabbage is originally from Australia and New Zealand. Since it also grows on sand, it endangers the particularly rare and legally strictly protected habitat types of moist dune valleys.
The Schleswig-Holstein Nature Conservation Foundation also points out that coniferous plants can develop enormous masses of plants in small bodies of water, so that bodies of water quickly become overgrown and are therefore impaired as a habitat for amphibians, among other things.
According to experts, species that accumulate nutrients in the soil, such as the multi-leaved lupine (Lupinus polyphyllus), are also problematic. “Not only do they compete directly with native species, but they can also change valuable, nutrient-poor habitats in the long term,” says an LfU spokeswoman.
Invasive knotweeds (Reynoutria japonica, R. sachalinensis, R. x bohemica and Koenigia polystachya) can affect the native flora by forming large-scale monospecies stands, the LfU spokeswoman continues. At the same time, they can cause considerable damage to structures, for example by growing through paths or terraces: “They are very difficult to remove because of the roots that reach several meters deep. In addition, the species can spread through even the smallest parts of plants.”
Are there other problematic species in the north?
Yes. The giant hogweed (Heracleum mantegazzianum) is also not welcome. It spreads widely across bodies of water in Schleswig-Holstein. “In addition to the negative impact on nature conservation, this species also poses a threat to human health. When making contact, it is therefore important to ensure adequate protection,” says the LfU.
For example, the Schleswig-Holstein Nature Conservation Foundation and the Nordfriesland district are also concerned, among other things, with the potato rose (Rosa rugosa), which can primarily overgrow protected dune and dry grassland habitats on the coast.
One challenge is that small initial stocks are often inconspicuous, as experts emphasize. “An occurrence is often only noticed when larger and conspicuous populations of one species have already formed,” explains the LfU spokeswoman.
How are invasive species combated?
This depends on the respective plant species, the habitat affected, the size of the population and the local conditions. According to the LfU, this can involve the complete elimination, control or containment of a population.
“There is no one-size-fits-all solution,” says the spokeswoman for the Nature Conservation Foundation. For knotweed, for example - depending on the location - grazing, digging, covering or repeated mowing are possible. According to the Lower Saxony State Forests, they are testing the use of electricity to combat the plant.
In the case of giant hogweed, year-round grazing can keep populations in check; In particular, larger populations in rivers generally require manual control.
On the island of Sylt, potato roses are first removed and the cleared areas are covered with geotextile for two to three years in order to sustainably prevent new shoots from sprouting, as the North Friesland district reports. This process is also used elsewhere on the North and Baltic Sea coasts.
Basically: “The earlier problematic populations are identified, the better,” says the spokeswoman for the Nature Conservation Foundation. “Once invasive plants have formed large dominant populations, pushing them back can be very difficult.”
What can gardeners do if these plants grow in their area?
“We recommend not planting new invasive plants in the garden and removing existing ones as professionally as possible,” says the spokeswoman for the Nature Conservation Foundation. This also applies if a plant blooms attractively or is visited by insects. The reason: “The fact that insects consume nectar from the flowers of a non-native plant does not automatically mean that the species has a high ecological value.”
“In any case, the plants should be prevented from spreading further,” says the LfU spokeswoman. One possibility of containment is, for example, to remove inflorescences after they have faded, but before the seeds ripen.
For species that reproduce via root runners, such as the tree of heaven, these runners should also be removed regularly. As a rule, follow-up inspections lasting several years are also necessary to ensure that a species has actually been successfully eliminated.
Important: Some of these neophytes are listed on an EU list. They are subject to a ban on trading and possession. This also means: the stocks from your own garden must be removed.
What is the best way to dispose of plant residues?
Plant remains from invasive species should under no circumstances be disposed of in the open countryside or as garden waste in nature. Germinating seeds or regenerative root and shoot parts can also be problematic in your own compost. For reasons of caution, all components of invasive plants should be disposed of with residual waste, advises the LfU.
