Japanese Habitats

The dwarf variety of F. japonica growing in a volcanic ash fieldIn Japan, although sometimes regarded as a weed, F. japonica, is not nearly as problematic as it is in its introduced range, nor is it usually such a conspicuous part of the ecosystems. The dwarf variety var. compacta is often the earliest coloniser of volcanic lava and ash fields, where its extensive rhizome system and tolerance of sulphur dioxide enable it to thrive.  In such habitats, var. compacta is sometimes the only higher plant to be seen, recruitment from seed is commonplace and there is much genetic diversity present. Some of these var. compacta plants are really small < 10cm tall, and can also be found as a minor constituent of high altitude acid heathland vegetation, a habitat I have never observed F. japonica growing in its adventive range.

On volcanoes in Japan F. japonica seedlings are often found Female plants of var. compacta tend to have distinctive red fruits, this picture shows some of the diversity found in these populations

Another threat to Japanese Knotweed in Japan is this giant parasitic CuscutaAlthough in Europe Japanese Knotweed (F. japonica var. japonica) is something of an ecological bully, easily outgrowing the native Pueraria lobata easily outcompete F. japonica in Japanherbaceous communities with its prodigeous growth rate, the situation in its native regions is rather different.  In Japan it is just one member of the giant herb community, all struggling for survival and space along river banks and the edges and clearings in the streamside.  In addition to competition with other plants such as Miscanthus, Wisteria, Pueraria and Cuscuta, they are also subject to predation from a whole range of invertebrates and fungi, some of which are currently being screened for their suitability as Biological Control agents.

Japanese Knotweed  in its native habitat in Japan

Bailey (submitted)

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