Knotweed: a Weed or Not?
Indeed, the plant known to many as Japanese knotweed (Polygonum cuspidatum) is very much a weed. So much that it is now legally declared "noxious" in Washington State and Whatcom County, and is considered in the top ten most invasive species in the world by the World Conservation Union. It also has relatives: Giant knotweed (Polygonum sachalinense) and Himalayan knotweed (Polygonum polystachyum), are also present in Whatcom County. The Giant and Japanese species are hybridizing to create what is now known as the most common species of local knotweed: Bohemian knotweed (Polygonum x. bohemicum). This hybrid is often more robust and more difficult to manage than either of its parents. Is there any good news? Research has come up with some successful means for managing it.
The Knotweeds, introduced from Asia as garden ornamentals, are perennials that grow up to 8-12 feet tall in shrubby clumps. The reddish stems are hollow, smooth and jointed, similar to bamboo stems, with which it is sometimes confused. Knotweed develops a deep, matted root system, with rhizomes that can grow to 30 feet or more in length and 6 feet in depth. The leaves are between 4 and 6 inches long. In late summer, the white flowers grow in drooping clusters and attract many bees. The plant dies back in the winter and the tall, dead, brown stems remain standing. Knotweed has long since escaped cultivation and become a large problem in riparian ecosystems, preventing water access, crowding out native plants and interfering with the food web for salmon and other fish. Infestations along the Nooksack River are being picked up by flood events and spread downstream. Homeowners are mistakenly taking this plant home from “the wild” and planting it, creating problems in urban areas and home landscapes.
Since knotweed is so difficult to eradicate once it has become established, it is important to prevent new infestations and remove small patches before they spread.
Please don't mistake this plant for a good natural screen and transplant it. Knotweed has been observed growing up through asphalt and rock, between bricks, and into the interiors of homes! Importing soil is also a common means of spread, so be careful when importing soil onto your property. Digging the plants is NOT recommended unless it is a very small and isolated infestation (5-10 stems). It is extremely difficult to remove all the rhizomes, and fragments of roots, and these root parts may spread the infestation and make it worse. These plant parts should never be composted in a backyard composting system. Usually a combination of cutting and chemical treatments is what is needed to manage knotweed successfully.
Please see our “Control Options for Knotweeds” flier for more information, or call us if you would like a site-specific recommendation.
For information on Whatcom County's Nooksack Knotweed Management Project, please visit our "Special Projects" page.