Japanese knotweed (Polygonum cuspidatum)
Introduced to North America from Japan in the late 1800s as an ornamental garden plant.
What Problems Does It Cause?
Like other invasive plants in the top 20 for northwest Michigan, Japanese knotweed out-competes native plants and provides little food or other habitat for wildlife. Japanese knotweed is problematic for infrastructure as well, as it can sprout up through concrete and asphalt, spread quickly, and is difficult to eradicate. In Great Britain, knotweed infestations drastically lower property values and may even prevent sale or insurance. For these reasons, Japanese knotweed is one of the few prohibited plant species in Michigan. Although illegal to buy, sell, plant, or otherwise spread Japanese knotweed, occasionally it is still sold under other names, like “Michigan bamboo” and “pink fleece flower.” Japanese knotweed is very hardy and can tolerate ash, sulfur, and toxic gases in its native range. In addition, Japanese knotweed can tolerate high salinity, full shade, high temperatures, and drought.
What does Japanese knotweed look like?
Japanese knotweed is a perennial plant that can grow up to 10 feet high. The leaves are heart-shaped, alternate, and can reach 6 inches long and 5 inches wide. In the late summer the spiky sprays of green-white may be confused with pokeweed, but lacks the black berries of the native plant. In winter, knotweed is easy to identify by its persisting hollow red-brown stalks.
Other knotweeds, such as giant and Bohemian knotweeds, look similar, but are taller or have more spade-shaped leaves. All species are very invasive.
How do I manage Japanese knotweed?
Avoid planting or spreading Japanese knotweed (it’s illegal!), monitor natural areas for signs of knotweed, and report any plants found as soon as possible. Japanese knotweed is extremely difficult to treat; its deep root system and fragmenting abilities (it is able to sprout into a new plant from a small piece broken off the plant) make it nearly impossible to dig out; and many herbicides have proven ineffective. Consult with a professional to control this invader.
Learn more about ISN's work on controlling invasive knotweeds on our Current Projects page.