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A knotweed success story on the Necanicum

Japanese knotweed is one of the toughest invasive plants to eradicate. But with persistence, creativity, and a lot of hard work, NCLC has managed to knock knotweed back—for now—at a property where it had gained a substantial foothold: Necanicum Forest, east of Seaside.

This 28-acre habitat reserve lies at the confluence of the Necanicum River and Klootchy Creek. Its understory is dominated by native plants such as elderberry and salmonberry—along with a dense area of knotweed growing right at the confluence of the creek and river. Knotweed tends to grow along waterways. It grows fast and tall—6 to 12 feet—quickly shading out native trees that keep streams cool and stabilize stream banks. Its roots run deep and spready widely. Even a tiny chunk of knotweed—such as a root or stem, cut by people or even beavers—can float downstream, take root, and form a whole new colony. That explains both how the knotweed invaded this site (from sources upstream on the Necanicum) and why NCLC was keen to keep it from spreading further.

Hand-pulling knotweed is considered far less effective than using herbicides for knotweed, but volunteer site stewards Vince Huntington and Bill Hutmacher were eager to give it a try. Research indicates that for it to be effective, hand-pulling of knotweed must be done at least monthly throughout the growing season. So that’s what Bill and Vince did in 2017 and 2018, aided by additional volunteers and stewardship interns, in a 1200-square-foot test plot. Meanwhile NCLC staff arranged to test a modified approach with herbicides on the rest of the knotweed at Necanicum Forest: rather than spray full-grown plants, they cut it to the ground in spring and, in fall 2018, treated only the low regrowth with chemicals, working with an experienced applicator to apply the smallest effective amount. This approach also limited damage to healthy native plants. Then rather than transporting the cut knotweed off the property (and risk dropping small pieces of the plant, possibly spreading it further), it was enclosed in layers of landscape fabric and buried it on site—an innovative but untested technique. Finally, in February of this year, volunteers helped Land Steward Eric Owen plant hundreds of willow, western redcedar, big-leaf maple, ninebark, salmonberry,and other native trees and shrubs to fill in gaps in the forest left by removal of the knotweed.

The results: hand-pulling was effective, but only because it was done consistently throughout the growing season. The limited herbicide application technique worked even better, and better than expected; when NCLC staff returned in April 2019, they found just a few handfuls of knotweed growing at the site. (A follow-up visit in June by Bill and Vince yielded so little remaining knotweed, they were able to pull it all in an hour.) And despite worries that the buried knotweed might take root and sprout right through its envelope of landscape fabric, it didn’t. It decomposed, just as staff had hoped.

“It’s an on-going effort,” said NCLC Project Manager Amy Hutmacher. “There remain other sources of knotweed in the Necanicum basin. We want to keep that area in check. We don’t want it to be a source. That’s also why we followed the treatment with a lot of planting of native shrubs and trees, to keep protecting that dynamic area from again becoming a deposition zone for knotweed.”

NCLC partnered with Necanicum Watershed Council on the project, with funding support from the Oregon Watershed Enhancement Board.


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