A Weed or Not a Weed

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.

Knotweed Growth

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 pollinators.

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 or other creeks and streams 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.