They reproduce and disperse without drawing attention to themselves and end up invading and colonising natural spaces, sometimes over considerable areas, to the detriment of indigenous species. Little known about, invasive plants constitute a recent environmental issue and represent a threat to biodiversity. The AlterIAS project, co-ordinated by the Biodiversity & Landscape Unit at Gembloux Agro-Bio Tech and jointly funded by the European Commission as well as the regional and federal authorities in charge of the environment in Belgium, aims at drawing attention to these tenacious plant invaders, which come from elsewhere but have been imported into our regions for ornamental uses, through prevention and awareness raising information campaigns.

We are between 1825 and 1850. Philip von Siebold, a Dutch officer for the East India Company posted to Nagasaki, imports new specimens of plant to his nursery, situated at Leiden, in the Netherlands. Japanese knotweed (Fallopia japonica), an exotic plant from East Asia, has just been discovered and introduced into Europe for horticultural use (hence the author name which follows one of the plant’s synonyms: Polygonum cuspidatum Siebold & Zucc). Trace of it would also be found in Siebold’s sales catalogue. It was cultivated there and subsequently won favour in other countries for its remarkable ornamental qualities. Imported from outside of its initial dispersal range, the plant progressively adapted to its new environment and ended up escaping from the gardens to establish itself in the natural environment. The follow-up to the story – without wishing to rehearse the disaster scripts of science fiction films – was that of an invasion: little by little, the knotweed dispersed and proliferated in the environment and established itself to the detriment of indigenous species. And it did so so well that it became a threat to the local biodiversity. 

In Belgium the date of its initial introduction is unknown, but it is highly probable that the plant also set up home following human importation. The herbarium series of the Meise National Botanical Garden show individual specimens archived in 1890, witnesses of the first observations in nature. Today the knotweed can be found almost everywhere, above all alongside water courses. ‘The Japanese knotweed is a rhizomatic ornamental species, formerly highly appreciated in gardens for its abundant flowering and its great covering capacity, which proves practical for garden management,’ explains Mathieu Halford, coordinator at Gembloux Agro-Bio Tech of the AlterIAS project, an information project devoted to invasive plants and prevention in the green sector (ornamental horticulture). Indeed most of these species are still available today on the market, for sale in nurseries, bought by consumers and then planted in parks and both public or private gardens. ‘The problem with invasive plants such as the knotweed is that they have the special characteristic of having a high population dynamics: they can escape and colonise semi-natural environments in forming dense, mono-specific populations, where very few other species can flourish. They thus cause losses of biodiversity in terms of flora but also fauna in that modifications of the vegetation have repercussions on the animal species which live in the biotope.’ The problematic, ‘which springs from a global phenomenon linked to globalisation and an intensification of the transport of organisms across the planet,’ adds the agronomics engineer between two explanations, needs to be taken seriously. It has given birth, in the 1990s, to a fully fledged scienctific discipline called ‘invasion biology.’