Invasive plants: Asian knotweeds

Among Asian knotweeds, the Japanese knotweed, whose scientific names are Fallopia japonica, Polygonum japonicum or Reynoutria japonica, is the most widespread. It belongs to the Polygonaceae family and originates from the Far East (eastern China, Japan, Korea). Introduced to Europe in the 19th century for its ornamental and forage properties, it proved to be invasive, spreading rapidly across large areas, particularly in moist environments.

Another knotweed species, Reynoutria sachalinensis, is also present in Switzerland, although more sporadically. The different Fallopia species can hybridise with one another, increasing their genetic diversity and further promoting their invasive behaviour. In Valais, they are recorded throughout the Rhône plain, particularly in the Pfyn forest area, where efforts to eradicate them are very active.

Japanese knotweed is a perennial herbaceous plant with white flowers arranged in upright clusters. Its apicultural interest lies in the abundant nectar production during the flowering period (July to September), while the pollen is less sought after. However, like all invasive plants, it tends to reduce the diversity of native melliferous species. Its rapid growth leads to the formation of such dense foliage that, due to lack of light, no other plants can grow at its base. In addition, knotweed produces metabolites that inhibit the growth of other plants in the surrounding area.

Its powerful rhizomes (underground stems) are responsible for its invasive character, as each fragment constitutes a potential cutting. For this reason, efforts should be made to uproot the plant with all its roots, and it is essential to avoid composting the waste or leaving it lying around.

In addition to their melliferous qualities, Asian knotweeds are also recognised for other beneficial effects: roots used as macerations or infusions have anti-inflammatory, digestive, expectorant, diuretic and hypotensive properties. In their region of origin, the plant is even appreciated in cooking, its stems having a flavour similar to that of rhubarb.

Here, however, it is preferable to forgo these advantages and devote all efforts to the eradication of this true biological scourge!

Sources

Infoflora.ch

J. Piquée, Melliferous plants month by month, 2014

T. Silberfeld, C. Reeb, Melliferous plants, 2016

2302_18 Cantonal concept for the control of neophytes.pdf

pfyn-finges.ch