The plant is a vigorous growing herbaceous perennial with annual tubular, glabrous stems that ascend from an erect base. These stems are light green often with reddish flecks, branched and reach up to 3 m in height (Beerling et al., 1994). Where introduced, F. japonica is generally taller than in its native range in Japan (Holzner and Numata, 1982), where it is recorded as being 0.3-1.5 m tall (Makino, 1997). Stems arise from strong rhizomes to form a dense thicket. Rhizomes are thick and woody when old, and have been recorded as spreading 5-7 m laterally (Pridham et al., 1966). The rhizome has ring-like structures at about 2 to 4 cm intervals which are reduced leaf scales, whilst on the underside are adventitious roots travelling into the soil. The rhizome snaps like a carrot when fresh to reveal a yellow/orange colour. The main aerial shoots emerge from the large bulbous rhizome crown about 30 cm x 30 cm across. This acts as a carbohydrate store in the winter months when it represents the complete live biomass of the plant. Spreading out from this central region are a number of radial penetrating rhizomes that twist together to form a sizeable and considerable penetrating force. The leaves are 5-12 cm x 5-8 cm, broadly ovate, cuspidate at the tip and truncate at the base. At the base of each leaf petiole is located a small gland that functions as an extra-floral nectary. The flowers are off-white and borne in ochreate clusters of 3 to 6 on terminal and axillary panicles, with the main axis up to 10 cm long and with slender branches 5-9 cm long (Lousley and Kent, 1981). Sepals 5, the outer 3-keeled;stamens 8, included within a perianth in male-sterile plants, filaments 0.4 mm, anthers small, flat, empty 0.3 mm, styles 3, distinct, stigma fimbriate, exceeding the perianth;perianth greatly enlarged in fruit and conspicuously winged, completely enclosing the trigonous achene. Achenes (or nuts) 2-4 mm long, 2 mm wide, dark brown and glossy, mean weight 1.6 mg. Inflorescences initially erect but drooping at maturity. Male fertile plants are not known from the introduced range.
The UK Environment Agency have produced a Code of Practice, and the Cornwall and Devon Knotweed Forum have produced an excellent guide which has advice on identifying the plant in the field at various stages of the season. as have the British Columbia Ministry of Forest and Range.
Related invasive species
- Fallopia japonica
Related Farm Practice
- Soil
- Light
F. japonica is an extremely invasive weed despite its lack of extensive sexual reproduction in most of its introduced range. It is included on various lists of invasive weeds and is one of the 100 worst invasive species as identified by the IUCN. It is a potential contaminant of soil, and its ability to tolerate a remarkable range of soil types and climates means that it has the potential to spread much further than it has to date. It has gained a fearsome reputation for breaking through hard structures in the built environment and being almost impossible to eradicate once it has taken hold and is often recognized as one of the most pernicious weeds in any recipient country.
Amphibians have been shown to have reduced foraging success in knotweed patches (Maerz et al., 2005) and any native species forced to compete with knotweed, i.e riparian plants, are likely to suffer consequences, as demonstrated by Gerber et al. (2008).