Knotweed is becoming increasingly widespread. Initially unnoticed, it is now hard to overlook how rapidly this invasive neophyte is spreading and displacing the native flora.
All knotweeds have a perennial underground rhizome network from which the two to four metre high shoots with hollow stems grow in spring. Depending on the species, the oval leaves are 20 to 40 cm long and slightly pointed towards the front. The many white inflorescences appear relatively late in July to August;
Small initial stands, for example after earthmoving or the creation of new areas, can still be controlled by digging up the rhizomes or consistently pulling out all the shoots. If these new stands are not controlled right at the beginning, the knotweed will spread rapidly within a few years. Regular mowing every 3 - 4 weeks is then necessary, whereby the cuttings are best removed. In any case, the cuttings should not be carried over to neighbouring areas, otherwise new plants may grow there again. In this way, the plants can be pushed back over a period of years. Even more time-consuming is a complete soil replacement at a depth of at least one metre - in some cases up to two metres for old stands. This soil, which is contaminated with rhizomes, must then be disposed of and must not be spread over other areas as topsoil.
Due to its widespread distribution, it has now become one of the worst weeds in Europe and is combated with great effort.
The plant reproduces via root runners.
A root fragment of less than 1 gram can grow into a new plant in moist soil within 6 days.
Such a plant manages to build up a permanent root system within a year, reaching a depth of 3 metres and a distance of over 6 metres. This poses a great danger to paths, water pipes, sewage pipes and much more.
The above-ground part of the plant dies when it freezes and the falling leaves decompose very slowly, preventing other plants from growing again.