Japanese - Reynoutria japonica, Fallopia japonica, Polyganum cuspidatum
Giant - Reynoutria sachalinensis, Fallopia sachalinensis, Polyganum sachalinensis
Bohemian - Reynoutria x bohemica, Fallopia x bohemica, Polyganum x bohemica
What Problems Does It Cause?
Like other invasive plants in the top 12 for northwest Michigan, invasive knotweeds out-compete native plants and provides little food or other habitat for wildlife. Knotweeds are 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 and its hybrids/cultivars are some of the few prohibited plant species in Michigan. Although illegal to buy, sell, plant, or otherwise spread knotweeds, occasionally it is still sold under other names, like “Michigan bamboo” and “pink fleece flower.” Knotweeds are very hardy and can tolerate ash, sulfur, and toxic gases in its native range. In addition, knotweeds can tolerate high salinity, full shade, high temperatures, and drought.
What does knotweed look like?
Knotweeds are 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. Giant and Bohemian knotweeds are typically taller and have more spade-shaped leaves. All species are very invasive.
How do I manage knotweed?
Avoid planting or spreading knotweed (it’s illegal!), monitor natural areas for signs of knotweed, and report any plants found as soon as possible. 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--contact ISN with questions.
ISN hosted an online workshop for people affected by knotweed in June of 2020; you can view a recording here. Check our Events page for future knotweed workshops.