Japanese knotweed - flower
Japanese knotweed - leaf and flower
Japanese knotweed - stems
Japanese knotweed - shoots
Common names: Japanese knotweed, Japanese bamboo, Mexican bamboo, Japanese fleece flower, crimson beauty, Reynoutria, Hancock’s curse
Scientific names: Polygonum cuspidatum Siebold & Zucc. synonyms Fallopia japonica (Houtt.) Ronse Decr., Pleuropterus cuspidatus (Siebold & Zucc.) Moldenke, Pleuropterus zuccarinii (Small) Small, Polygonum cuspidatum Siebold & Zucc. var. compactum (Hook. f.) L.H. Bailey, Polygonum zuccarinii Small, Reynoutria japonica Houtt.
Related species: Giant knotweed (Polygonum sachalinense F. Schmidt ex Maxim.) and hybrids
Japanese knotweed is native to eastern Asia and was imported to England in the mid 1800s as an ornamental. It was prized and planted in many famous gardens. In the late 1800s, it was brought to the United States and was planted in gardens and used for erosion control along roadways and embankments. Japanese knotweed escaped cultivation, overtook desirable vegetation and was recognized as a problem by the early 1900s and now inspires dramatic headlines such as “The largest female on earth could strangle Britain” in the London Daily Telegraph.
Description: Japanese knotweed is a shrub-like, semi-woody perennial. It is fast growing and has hollow, bamboo-like stems that form dense leafy thickets 6-9 feet tall. Stems become tough and woody with age. The shoots arise from coarse, spreading rhizomes that can attain lengths up to 50 feet. Leaves are alternate, simple, and broadly ovate with pointed tips. The stalked blade is about 6 inches long by 4 inches wide. Plants produce either male or female flowers in white clusters at the upper leaf axils in late August and September. Female flowers can produce small 3-angled black-brown fruit, but seed production is rare. Giant knotweed looks similar, but is larger and has heart shaped leaves.
Habitat: Japanese knotweed can be found in sunny areas along roadsides and in riparian areas such as river banks. Knotweeds thrive in a wide range of soil types. Some homeowners have Japanese knotweed in their landscapes.
Means of spread and distribution: Rhizomes allow Japanese knotweed to spread quickly and aggressively, although seed is sometimes produced. In North America, Japanese knotweed plants produce only female flowers therefore can not produce seed without a pollen source. However, both giant knotweed and bohemian knotweed (a fertile hybrid) can produce both female and male flowers and may provide a pollen source for Japanese knotweed plants where they coexist resulting in viable seed production. New colonies can form from very small rhizome or root fragments that are moved by natural means such as waterways as well as by human activities that move soil such as construction. Japanese knotweed is very persistent after establishment. It is widespread in the eastern U.S. and reported in 41 states. There are multiple, isolated infestations in Minnesota.
Impacts: Japanese knotweed forms tall, dense thickets that shade out and displace native vegetation, degrade habitat for fish and wildlife, and can alter waterways, and facilitate erosion and flooding. Knotweed growth through pavement cracks and along paved surface edges can result in damaged pavement.
Prevention and Management: Do not plant knotweed as an ornamental and remove existing plants from your property. Prevent soil movement containing knotweed rhizome fragments to uninfested areas. Monthly cutting or weekly mowing throughout the growing season may reduce knotweed stands. Foliar, stem injection, and cut stem application of herbicide treatments can be very effective. Treatments will need to be repeated for many years to eradicate a population.
Biological control is not a management option at this time. It is in development in Europe where host specificity and efficacy testing of a sap-sucking psyllid and a leaf spot fungus is promising. Experimental releases of the psyllid began in Britain in 2010. Go to Japanese Knotweed Alliance for more information.
Additional identification information and management recommendations:
Legal status: Japanese knotweed is not regulated in Minnesota.
Noxious & Invasive Weed Unit
Plant Protection Division
Minnesota Department of Agriculture (MDA), 625 Robert Street N, St. Paul, MN 55155-2538, firstname.lastname@example.org