Knotweed Control and Eradication

In 2004 the Upper Nehalem Watershed Council in cooperation with the Oregon Department of Agriculture (ODA) initiated our first projects to control the nasty and invasive species called knotweed. Knotweed is to Oregon what Kudzu is to the Florida Everglades. If not caught early and repeatedly suppressed, Knotweed can completely dominate stream sides, suppressing tree growth and undermining the integrity of the stream bank. This leads to less shading of the stream and thus increased stream temperatures, a decrease in large woody debris, increased erosion and sedimentation and a general reduction of the diversity of plant life along the stream or river to just one -- knotweed.

See pictures of our knotweed eradication projects here

What Does Knotweed Look Like?

Knotweed grows in dense stands up to 12 feet tall. The stems resemble those of bamboo with leaves that are bright green but vary from as little as 1" across (like a bay leaf) to 8 inches wide. The leaf edges are smooth, not saw-toothed. It starts growing in spring, but is dormant in winter, losing it's leaves.

Why is it So Hard to Control?

Knot weed is superbly adapted to live in the most meager of soils. It can regenerate itself from even the smallest root fragment. Because of this, and the easy transport that rivers and streams give it, knotweed can spread rapidly and effectively down stream. Even the effort to remove it, if not done carefully, can help to spread it. Knotweed quickly establishes a dense labyrinth of a root system. For this reason it can appear to be completely controlled only to have the root system produce new shoots even years later.

How is it Controlled?

Unfortunately today, the most effective means for controlling knotweed is chemical. Repeatedly mowing, cultivating and/or pulling small patches may be effective if the effort is sustained. But for large patches, spraying or direct injection of herbicide into the plant is the only 100% effective means of killing it. Even so, repeated spraying over a period of years is usually necessary.

Since knotweed is generally found along fish bearing streams, herbicide application by professionals may be required so as to limit the contamination of soil and streams. Check with your local soil and water conservation district or Oregon State University (OSU) extension for the herbicide application requirements specific to your area.

I Think I Might Have Knotweed on my Property. What Should I Do?

Contact us by phone or email. For areas within the Nehalem watershed we can either come out and survey the site ourselves or arrange for a local agency or other organization to do so. If you are outside of our jurisdiction, we can usually point you in the right direction.

How can I learn More?

Download a 15 page brochure about knotweed produced by the Nature Conservancy (2.3MB PDF)

See KNOTWEED! the movie. Movie? Yes, this 30 second, horror-short conveys the urgency of getting a handle on knotweed ...before it's too late.