Say “No” to Knotweed
Japanese knotweed
https://dnr.wisconsin.gov/topic/Invasives/fact/JapaneseKnotweed
There are three species of knotweed that have invaded Wisconsin and spread across the state at lightning speed. Once thought to be an attractive landscaping plant, they are all regulated plants by the Wisconsin DNR due to their rapid growth and destructive ecological impact.
Japanese knotweed (Polygonum cuspidatum), Giant knotweed (P. sachalinense), and Bohemian knotweed (P. x bohemicum) all have been found in the state. They are all extremely quick growing plants and are a threat to the habitat. Japanese knotweed is an herbaceous perennial that grows up to ten feet tall with hollow stems and arching bamboo-like stems. Giant knotweed is similar, but grows up to thirteen feet tall with larger leaves. Bohemian knotweed is a hybrid of the two that grows even taller, up to fifteen feet, and has even more invasive characteristics. While it is convenient to distinguish between the species, the control of all of them is the same.
All three species have dark green leaves on the upper surface with paler green undersides. The leaves vary from spade to heart shaped. They bloom during August and September with sprays of greenish white flowers. The stems are smooth and hollow between swollen joints, resembling bamboo and giving rise to the common name, Japanese bamboo. The stems will persist during the winter, turning orange to rust colored, and making identification easy.
These plants reproduce through underground rhizomes. They are prolific invaders that can spread up to 65 feet from the parent plant. The fibrous roots go six feet deep, making eradication a monumental task. Although they do not all reproduce readily by seed, studies have shown that the hybrid Bohemian does, in fact, reproduce easily by seed. The plant can be spread into both disturbed and undisturbed sites by contaminated fill dirt, flooding, discarded cuttings, and careless handling of neglected gardens. Because the spread is so rapid and the plant tolerates drought, adverse growing conditions, heat, and high salinity, it is invading forests, stream banks, and roadsides. It chokes out all other vegetation and disrupts the ecosystem.
Control is difficult and is a multi-year process. Unfortunately mowing, mulching, controlled burning, and grazing are not effective means of control. At best, these methods will achieve a fifty percent control and only if repeated multiple years. The best method is to do a cut stump control. Cut the stump at or below the second joint from the ground. Apply either glyphosate or triclopyr while the stem is still rooted in the ground. This is best done when the plant is actively growing and will likely need to be repeated over several season. More information is available in the Renz Weed Science bulletin on Japanese Knotweed: https://cdn.shopify.com/s/files/1/0145/8808/4272/files/A3924-11.pdf
If you are tempted to ignore these cautions, be aware that this invasive plant has been in Europe since the 1800’s (in the US since the 1900’s). According to a DNR publication, one business in the United Kingdom spent over $600,000 to repair parking lot damage from the rhizomes. In Green Bay, a man has been mowing his lawn for 40 years in an attempt to control Japanese knotweed. If you are considering planting knotweed, just say “no”.
If you suspect you’ve found knotweed on your property, send photos and specific locations using GPS points, street addresses or road crossings to Invasive.Species@wisconsin.gov.
Carol Shirk
Certified Master Gardener