Common Name: Japanese Knotweed
Alternate Names: Japanese bamboo, Mexican bamboo, Japanese fleece flower, crimson beauty, Reynoutria, Hancock’s curse
Scientific Names: Polygonum cuspidatum Siebold & Zucc., synonym Fallopia japonica (Houtt.) Ronse Decr.
Related Species: Giant knotweed (Polygonum sachalinense F. Schmidt ex Maxim.) and hybrids
Legal Status: Specially Regulated
Any person, corporation, business or other retail entity distributing Japanese and/or giant knotweeds for sale within the state must have information directly affixed to the plant or container packaging that it is being sold with, indicating that it is inadvisable to plant this species within 100 feet of a water body or its designated flood plain, as defined by Minnesota Statute 103F.111, Subdivision 4. View Minnesota Noxious Weed Law for more information.
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.
- 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.
Japanese knotweed flowers, photo by Minnesota Department of Agriculture (MDA).
Flowers, photo by MDA.
Leaves, photo by MDA.
Leaves and stem, photo by MDA
Stalks, photo by MDA.
Infestation, photo by MDA.
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 intentionally planted 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 and therefore cannot 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 US and reported in 41 states. There are multiple, isolated infestations in Minnesota.
Japanese knotweed forms tall, dense thickets that shade out and displace native vegetation, degrade habitat for fish and wildlife, 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 eradicate any existing plants from your property. Do not move soil that may contain knotweed rhizome fragments to uninfested areas. A sound management plan is necessary to manage this species and will take a commitment of several years to ensure that the population has been eliminated.
- Monthly cutting or weekly mowing throughout the growing season may reduce knotweed stands. However, because knotweeds reproduce primarily by abundant root rhizomes, mowing is not recommended as a single management option. Mowing in conjunction with selective herbicide applications will produce better results than mowing alone. Always clean and inspect equipment after working in an infested area to reduce transport of seed and plant fragments to new areas.
- Foliar, stem injection, and cut-stem application of herbicides that translocate their active ingredients into the root system can be very effective. Treatments will need to be repeated for several years to eradicate a population. If using herbicide treatments, check with your local University of Minnesota Extension agent, co-op, or certified landscape care expert for assistance and recommendations. There are several businesses throughout the state with certified herbicide applicators that can be hired to perform chemical applications. Japanese knotweed also grows commonly in riparian and wetland habitats. If treating plants near water with herbicide, please be aware of the state pesticide laws and use only products labeled for aquatic use.
- 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. Visit the Japanese Knotweed Alliance for more information.