Bill Johnson

Japanese Knotweed

Fallopia japonica (Hout.) R. Decr.
(previously Polygonum cuspidatum)
Buckwheat family (Lythraceae)

Origin: Eastern Asia

Background
Japanese knotweed was probably introduced into the United States in the late 1800s. First used as an ornamental plant, it has also been planted for erosion control and landscape screening. It is designated a noxious weed in the state of Washington.

Distribution and Habitat
Japanese knotweed occurs across the U.S. and has been reported to be invasive in natural areas throughout the northeast into Georgia and west to Missouri, with additional infestations in Oregon and Washington. It can tolerate a wide variety of challenging conditions, including deep shade, high salinity, high heat, and drought. Knotweed is commonly found near water sources, such as along streams and rivers, and in a variety of low-lying areas like ditches, waste places, utility rights-of-way and around old home sites.

Ecological Threat
It spreads quickly to form dense thickets and pushes out native plant species. Knotweed poses a significant threat to riparian areas where it can survive flooding events and rapidly colonize scoured shores and islands. Once established, populations are extremely persistent.

Description and Biology

• Plant: upright, shrubby, herbaceous perennial 4-10 ft. tall with stems that are smooth, stout and hollow; stem leaf junctures are swollen and surrounded by a membranous sheath called an “ocrea” which is typical of the family.
• Leaves: variable; about 6 in. long by 3-4 in. wide, broadly oval to somewhat triangular with a truncated base and a tapered tip.
• Flowers, fruits and seeds: tiny greenish to white colored flowers occur in attractive sprays in summer; fruits are winged on three sides; seeds are triangular, dark brown, shiny and about 1/10 in. long.
• Spreads: by seed and by vegetative means through growth of long, stout rhizomes; can be transported long distances by water as a contaminant in fill-dirt, or on the soles of shoes; escapes from gardens and landscaped areas through discarded cuttings.
• Look-alikes: the lovely native Virginia knotweed (Tovara virginiana), and two exotic invasive relatives - prince’s feather (Polygonum orientale) and giant knotweed (Polygonum sachalinense).

Prevention and Control
Japanese knotweed is an extremely difficult plant to control due to its ability to re-grow from vegetative pieces and from seeds. Mechanical and chemical methods are most commonly used to eliminate it. Single young plants can be pulled by hand when soil is moist and roots are small. Roots and runners must be removed to prevent re-sprouting. Glyphosate and triclopyr herbicides have been used effectively, applied to freshly cut stems or foliage (see Control Options). Although a biological control agent has been found and is being released in the United Kingdom in 2010, there are no agents available for release in the U.S. at this time.