1284 Fallopia japonicaCommon Names: Japanese knotweed,Asian knotweed,Mexican bamboo Family: Polygonaceae (buckwheat Family)
Japanese knotweed is an herbaceous perennial that gets so big (more than 10 ft (3 m) tall by the end of the summer), that it seems more like a shrub. It has thick hollow stems with swollen bamboo-like nodes and it spreads on vigorous rhizomes, forming dense stands. The leaves are around 5 in (12 cm) long and 3 in (8 cm) wide, more or less egg shaped, and tapering abruptly to a point. The individual greenish white flowers are small and inconspicuous. However, they are clustered in showy upright feathery racemes 3-6 in (8-16 cm) tall that originate in the leaf axils. The triangular shaped fruits are shiny black and very small.
‘Compactum’ is a smaller cultivar, to just 18 in (45 cm) tall and has pinkish flowers. ‘Spectabile’ has red leaves that become marbled with yellow in fall.
Fallopia japonica is native to Japan, Korea and eastern China. It was brought to the US as an ornamental in the late 1800s and has since become a bothersome weed throughout much of the nation, now recorded from all but the southernmost states. Japanese knotweed also has been introduced into Europe, Australia and other parts of Asia and is now listed as an invasive species in many countries. Japanese knotweed occurs in many habitat and soil types, from dry to wet. It is found along roadsides, ditches, utility rights-of-way, old home sites, wetland edges, fields and waste places. It takes over disturbed sites and (disturbingly) even natural communities.
Japanese knotweed tolerates a very wide range of soil types and pH, as well as high salinity, high temperatures, freezing, drought and flooding. Light: Japanese knotweed grows strongest in full sun but can tolerate even full shade. Moisture: Japanese knotweed can tolerate complete inundation for brief periods, and it survives extended periods of drought. Hardiness: USDA Zones 3 - 8 . Propagation: Japanese knotweed spreads rapidly from long, thick rhizomes. Severed pieces of rhizome and stem can start new plants. The tiny seeds are dispersed by water, transported with fill dirt and carried on the soles of shoes. Once established, Japanese knotweed is extremely persistent and nearly impossible to eradicate.
Japanese knotweed has been used as an ornamental and for erosion control and privacy screening. In parts of Asia the young stems of Japanese knotweed are eaten as a cooked vegetable, albeit a very sour one. Japanese knotweed flowers can be an important late summer source of nectar for honeybees, who produce a honey that beekeepers call bamboo honey.
Parts of Japanese knotweed have been used in traditional Asian medicines to treat various skin infections and cardiovascular diseases.
Many states (most states in the northeastern US) forbid the cultivation of Japanese knotweed. Where still legal, only the less invasive varieties ‘Compactum’ and ‘Spectabile’ should ever be planted, and these only in large landscapes where their spread can be absolutely restricted. Floridata recommends that it not be grown at all.
Truly the Weed from Hell, Japanese knotweed has established itself all over the world as it overtakes native species and dominates wetland communities. The World Conservation Union calls Japanese knotweed one of the world's worst invasive species.
Japanese knotweed is a threat to riparian and wetland habitats. It forms thick, dense colonies that completely crowd out other herbaceous species. The aggressive root system can damage concrete foundations, buildings, tarmac and roadways. The roots can run laterally for 20 ft (6 m) and go 10 ft (3 m) deep, making them near impossible to dig up. The plant resprouts vigorously when the above ground parts or the roots are cut. Every little piece is capable of producing a new plant.
A biological control for Japanese knotweed is being tested in the UK, but not yet available in the US.
Steve Christman 4/23/17