(Polygonum cuspidatum) is a perennial species with spreading rhizomes and numerous reddish-brown freely branched stems. The plant can reach four to eight feet in height and is often shrubby. The petioled leaves are four to six inches long and generally ovate with an abrupt point. The whitish flowers are borne in open, drooping panicles. The plant is dioecious, so male and female versions of the inconspicuous flowers are produced on separate plants. The approximately 1/8 inch long fruits are brown, shiny, triangular achenes, (Hitchcock and Cronquist 1964; Hickman 1993).
Japanese knotweed was first introduced to Europe and North America in the late 19th century for ornamental use, for planting to prevent soil erosion, and sometimes as a forage crop for grazing animals. It is typically considered an invasive plant or weed where it has been introduced, and is a frequent colonizer of temperate riparian ecosystems, roadsides and waste places. It can be found in 39 of the 50 United States (PUSDA) and in six provinces in Canada. Japanese knotweed is a very aggressive species that is capable of crowding out all other vegetation.
Like other invasive herbaceous plants, such as Phragmites (Phragmites australis, giant reed), Japanese knotweed forms dense stands of stems that become impenetrable by other plants once well-established. The rapid growth of new shoots and leaves in the spring shades out any vegetation below, suppressing the growth of other plants, including established native species. The monocultures that often form following Japanese knotweed invasions contribute to reductions in native biodiversity.
Control has proven to be difficult. Glyphosate has been shown to be effective in controlling Japanese knotweed under certain conditions. 2-4,D top burns the plant, but it's strong root system sends up new shoots before the above ground growth dies back. Frequent cultivation to grub out rhizomes may be a more effective control method. Covering with heavy plastic or rubber matting of one kind or another has also shown good results in open areas.
All the traits of this invader are not entirely bad. However it's few uses do not mitigate the negative impacts. Japanese knotweed flowers are valued by some beekeepers as an important source of nectar for honeybees, at a time of year when little else is flowering. Japanese knotweed yields a nice monofloral honey, usually called "bamboo" honey by northeastern U.S. beekeepers, that is like a mild-flavored version of buckwheat honey (a related plant also in the Polygonaceae). The young stems can be used as a spring vegetable similarly to asparagus. Both Japanese knotweed and Giant knotweed are sources of Resveratrol.
Other English names for Japanese knotweed include Fleeceflower, Huzhang, Hancock's curse, Donkey rhubarb (although it is not a rhubarb), Japanese bamboo, American bamboo, and Mexican bamboo (though it is not actaully a bamboo).
Rationale for concern: Japanese knotweed is an escaped ornamental that is becoming increasingly common along stream sides and rights-of-way in Northern Utah. The species forms dense stands that crowd out all other vegetation, degrading native plant and animal habitat. In addition, Japanese knotweed can create a fire hazard in the dormant season. This perennial plant is difficult to control because it has extremely vigorous rhizomes that form a deep, dense mat. Also, the plant can resprout from fragments; along streams, plant parts may fall into the water to create new infestations downstream.