Photo by Angus MacAskill

A knotty problem around the world

Bohemian knotweed grows along rivers and streams, wreaking havoc on salmon habitat

Name: Bohemian Knotweed (Polygonum bohemicum)

How it spreads: Bohemian Knotweed spreads through roots underground and by relocated broken stalks above ground

Difficulty to remove: 10 out of 10


Bohemian knotweed really likes it out here in the Pacific Northwest and happens to wreak havoc on our precious salmon habitat. It’s listed as a Class B Noxious Weed in Washington state—meaning the goal is to prevent the spread of the weed.

Knotweed grows extremely well along rivers and streams. Its shallow root system leads to increased erosion, making riverbanks unstable and water quality poor. The resulting lack of trees growing beside the river reduces the shade cover and leads to higher water temperatures: a fatal condition for salmon. While native trees and shrubs would drop their leaves into the water each fall providing food for bugs, our regional bugs don’t want to eat knotweed! And without more bugs to eat, our salmon are left hungry and unable to grow fast enough to survive.

Forterra’s riparian restoration team works with partners and our WCC crew to control knotweed infestations on some of our region’s most important rivers. It’s tough work! During the winter months the plants die back completely, leaving their tall canes to stand in an empty space. By summer the plants have grown to their full height again, forming a nearly impenetrable jungle. The canes look similar to bamboo because they are segmented and hollow, but don’t be fooled—this is way worse.

A single segment of knotweed or a small chunk of its root can break off and grow to become another full-sized plant. This is how knotweed spreads along rivers—going for a swim during floods and landing somewhere new. It can also spread when people unknowingly move it around, sometimes by moving contaminated soil to a new location, or by mowing down the weed and breaking each cane into hundreds of little chunks.

If you see this weed in your yard, make sure to not cut or mow it. King County Noxious Weed Control Program has great advice on how to control knotweed yourself. They also offer local workshops each summer to show you how it’s done.

Bohemian knotweed is a problem plant around the world. In some countries it has caused big headaches for home buyers and sellers, as the presence of knotweed on a property has led to banks refuse to provide loans—check out this video segment about it. We’re working to make sure it never gets that bad here in Washington state and to keep our rivers happy and healthy!