insects & light

Invasive species: a threat to biodiversity

Animal and plant species, displaced by humans, lead to continuously increasing problems for biological diversity at a global scale. Impacts are strongest on oceanic islands, but invasive species also have increasingly negative impacts on continents. Introduced predators directly decimate other species, or transfer deseases to endemic species. Invasive plants claim increasingly much habitat space and outnumber native plants which serve as food for insects. In intensively used landscapes as in Central Europe, the last remainders of biological diversity may soon be lost to highly competitive and agressive invaders: Dry grasslands are invaded and fertilized by Black locust (Robinia pseudoacaciae), heathlands are overgrown with North American Black cherry (Prunus serotina), and grassland is colonized by vast amounts of Turkish wartycabbage (Bunias orientalis). Current EU and national legislation is completely ineffective against these increasing threats. It is often argued that some of these plants have positive functions (e.g. provision of nectar, positive for city climate), but as compared with native communities, their ecological balance is clearly negative (e.g. fewer species at local and eventually global scale, very few herbivores), not to mention additional costs of cutting and mowing fast growing and persistent plants such as Ailanthus.


Black Locust is among the most agressive invasive plants worldwide.
Dense stands of Echinops sphaerocephalus
Population of ca 5000 Bunias specimens under management in Jena (Beutenberg Campus)
Japanese Knotweed (Fallopia japonica) needs regular management.
Black Locust quickly invades open habitats along railroads.
Dense stand of knotweed at river Saale with Gunnar Seibt. This population was eradicated within four years by out-rooting four times per year.
Impatiens glandulifera is invasive along the Saale river and similar habitats.
Bunias can quickly dominate calcareaous grasslands (near Bucha, Jena).
Large population of Bunias in the nature reserve Pennickental, Jena. Bunias outcompetes nearly any other plant species in open grasslands.
Box elder (Acer negundo) is a widespread invasive tree species, particularly in eastern Germany and along rivers.
Ailanthus altissima is a quickly growing invasive tree species in urban areas. This planted tree has been removed in 2020.
Single specimen of Bunias orientalis
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Important invasive plants in Jena

At a regional scale around Jena, Turkish wartycabbage (Bunias orientalis) has proven an exponentially growing problem. As in many other temperate places, species of knotweed (Fallopia / Reynoutria) are persistent but many populations have successfully been managed by volunteers in the last years by regular out-rooting. North American Box elder (Acer negundo) is especially common along waters, and Black locust (Robinia pseudoacaciae) has established monocultures in dry sandstone habitats, including nature reserves. "Tree of heaven" (which actually is a "tree of hell") (Ailanthus altissima) is still represented by very small but fast growing populations. Thanks to recent political initiatives, the species was banned from being planted by the city of Jena. Globe thistles (Echinops sphaerocephalus) quickly colonize dry open grasslands and dominate plant communities.


Rosette of Bunias in August.
Seeds have a sphere like form with pinnacle.
Plants appear in February-March.
In July, regenerating after a first cut in April. A second cut might kill the individual.
Population of ca 5000 individuals at Ammerbach, under management since 2019.
Good management should avoide riping of seeds (July 2015).

Turkish wartycabbage (Bunias orientalis)

Turkish wartycabbage can become a massive problem in all kinds of open habitats on calcareous soils where it is able to establish populations of thousands of individuals within a few years, including valuable protected landscapes. The species originates from southern Russia and the Caucasus region and was already introduced in Central Europe more than 150 years ago. However, it only rather recently has become highly abundant since the 1990s and it obviously profits from ongoing land use changes. Since Bunias grows best on nutrient rich soils, it certainly profits from fertilization, including nitrogen deposition through the air from agriculture, industry and motors. Under optimal conditions, Bunias plants may reach up to 2 m, but it is remarkably plastic and can produce some seeds also under shadow or extremely dry conditions. It colonizes any kind of open and semi-open habitat on calcareous soil.

Bunias is often mistaken for rapeseed (same plant family, yellow flowers, similar size and phenology), but seeds are sphere-like with pinnacle; leaves are not round but lanceolate-pointed. Unlike rapeseed, Bunias comes to stay because it is perennial and does not disappear when it is mowed. It's major ecological feature are long tap roots that make it persistent against drought and mowing.

Group of volunteers cut Bunias in a nature reserve in Jena.
Plants can be detected and cut as early in March. Remainders quickly disappear.
Plants often spread along roads. Plants cut in April.
Slender spades are an ideal tool.
Small individuals, recently germinated, can be found and eradicated.
Tap root (lower part still in the ground) and spade.
Magaged site near Nennsdorf with a population of ca 3000 individuals.
Scheme for mowing management of Bunias

Management of Bunias

Many Bunias populations are managed in Jena and the region by volunteers including myself. We have gathered a good amount of experience how the plants can most effectively be managed. Most importantly, Bunias cannot be eradicated by simple mowing (because of the deep tap root). However, mowing at the right time can prevent the plants from seed production, and that is extremely important in terms of the prevention of further spreading. Primary colonization is often triggered by human activity, e.g. with the transport of contaminated soils. Plants should be mown ca 5 weeks after the plants have begun to flower (mostly at the end of May / beginning June). Only then it can be prevented that the plants start a new shoot with inflorescences (which happens when Bunias is mowed early).

For the permanent eradication of Bunias, one of the best methods is the use of a slender spade ("weed spade"). This allows to remove the root or a substantial part of it. Older plants with long roots need to be treated two or three times, ideally within the same year because the plants will otherwise regenerate from the remaining deeper part of the root. For larger populations and hard soils, the use of black PVC sheets appears to be promising method (gallery below). I am currently testing the most effective mangement. The sheet (2–24 m2) is regularly moved between two or three sites in order to allow plants to regenerate for a short period of time and seeds to germinate. Then the PVC sheet is moved back and plants are killed or weakened. In hot summers, sheets  kill all aboveground plant parts in a few days. After the management, native species will be allowed to recolonize the sites.

Managed site in June
Managed site in May
Managed site in April
Managed site in March
Managed site in May (PVC sheeting removed)
Managed site 2 in June (PVC sheeting partially removed)
Managed site 2 in June
Managed site 2 end of June (PVC sheeting moved further)
Managed site 2 in June

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