Japanese knotweed is an invasive plant that must be controlled.
Japanese knotweed (Fallopia japonica) originates from East Asia. It was introduced in the 19th century for ornamental and fodder purposes, after which it spread throughout Europe and North America. Japanese knotweed has colonised every type of environment, with a preference for disturbed and altered areas, silted-up land and riverbanks, open environments, embankments and even private gardens. The plant is sometimes deliberately planted because of its impressive and spectacular appearance.
Japanese knotweed is now one of the most destructive species for biodiversity. Without local predators and competitors, it can wipe out the local flora within a few years. Insects, amphibians, reptiles and birds are declining because these animals are losing their habitat.
The plant spreads so quickly that a small colony can rapidly colonise an area of several dozen square metres. It thus takes precedence over low local vegetation, even if it is well established.
More info about the fight against invasive species:
Japanese knotweed is a large perennial plant (up to 3.5 m high) with deep roots, consisting of a rhizome and branched, hollow stems (2 to 4 cm in diameter) with multiple sections. The stem is green with small red spots and the leaves grow to between 5 and 12 cm in length.
In winter, the above-ground part dies off and dries out. The plant flowers in autumn, producing clusters of numerous small white, sometimes pink flowers. In spring, the stems emerge rapidly from the ground among the remains of the previous year's withered stems and produce light green leaves at regular intervals along the stem.
It develops enormous rhizomes in the ground: these can grow to 10 m or even longer and reach depths of up to 15 m. It is through these rhizomes that the plant can spread and build up enormous food reserves.
Japanese knotweed spreads exclusively by vegetative means, so via cuttings, rhizomes and stem pieces. The movement of contaminated soil and the transport of plant pieces by watercourses are the main causes of the spread of this species.
Japanese knotweed likes rich, moist soils and grows in colonies. It occurs in all types of environments, with a preference for the banks of watercourses and disturbed areas (fallow land, near infrastructure and transport routes). It prefers sunny locations, although it also tolerates shade, and grows poorly in densely wooded areas.
For humans: no risks to public health.
For biodiversity: the plant can form very dense populations that can quickly displace or even eliminate other plants. This also causes the habitats of certain animals to disappear. Due to its mode of dispersal, Japanese knotweed can colonise new locations, where it reproduces rapidly.
- Make an inventory of the places where the plant occurs.
- Do not transport soil that may be contaminated (rhizomes, roots and stems).
- Never plant this species in gardens or green spaces.
- Pull up the plants and let them dry completely on a concrete surface or transport them in suitable packaging to prevent parts of the plant from spreading.
Take action as soon as possible if an infestation is discovered. Pull the plants out by hand, preferably in spring when the soil is moist.
During the establishment phase (population present for less than 2 years), knotweed has poorly developed rhizomes and stems that do not yet form dense thickets. The plant can then be removed by pulling out the above-ground and underground parts completely. In the young plant stage (first year), early eradication of knotweed is most effective, as the limited root system does not hinder uprooting. The risk of rhizome spread increases as the plant ages.
- Remove all rhizomes and roots by digging around the stem with a spade, fork or small pickaxe. Make sure that no parts are left behind.
- Do not pull on the stems without digging the soil (risk of the rhizome breaking apart).
- Repeat the uprooting as long as new shoots are observed at the location.
If Japanese knotweed is not uprooted soon after it appears, it is virtually impossible to control this plant. The root system and rhizome penetrate very deeply into the soil. Even if the plant is well rooted, it must be pulled out regularly and carefully to prevent spread and to gradually reduce its volume.
