Removing knotweed and other invasive plants and why it matters

The songbirds, salmon, and other wildlife once common in Eastlake would be more numerous if there were more of the native ground cover, shrubs, and trees which they had evolved to depend on for food and shelter. But ongoing development is paving green spaces, and in those that remain, the native plants are being overcome by Himalayan blackberry, English ivy, Scotch broom, Chinese clematis, field bindweed (wild morning glory) and Japanese knotweed.

These invasive plants fill the soil with roots, and their thick foliage makes it all but impossible for native plants to survive. Vines in a single tree can weigh more than a ton, blocking sunlight, views, and habitat, and catching the wind to bring it down. Fortunately, control is possible simply by cutting the stems; please help by doing so on public lands. Private landowners are usually unaware of how vines are damaging their trees, and may be happy to give you permission to cut the stems.

Unfortunately, normal control methods do not work with Japanese knotweed (Polygonum cuspidatum), which squeezes out all other shrubs and ground cover. Once deeply rooted, it should not be dug up or pulled out by the roots, as they will resprout more strongly. Repeatedly cutting off the stems at ground level will eventually kill or discourage eradicate knotweed, but don’t put the cuttings in with garden recycling or compost, as they will contaminate it with sprouts.

Neglect by the King County Noxious Weed Board is worsening the problem, as reflected in this defeatist philosophy: “Knotweed is so widespread that it is only regulated in the upstream reaches of watersheds because control is difficult and funding is limited. The issue is that if it became regulated on Lake Union, every property holder (public and private) would be financially responsible for dealing with it, which the county feels is an undue burden.”

Regardless of whether required to do so, public and private landowners should be aggressively controlling knotweed. Although most of Eastlake’s knotweed is on public land managed by SDOT, the Park Department, City Light, and the Washington State Department of Transportation, these agencies are doing almost nothing, allowing it to spread to private land and by Lake Union throughout the shorelines. Japanese knotweed is beginning to appear on private land as well, and should be eliminated quickly in order to avoid it becoming established.

For background on controlling invasive plants including Japanese knotweed, see the following links. The Eastlake Community Council seeks volunteers to work on the various aspects of this problem. Please contact ECC at info@eastlakeseattle.org.

Web links

Note: suggestions for additions or deletions from this list are welcome, to info@eastlakeseattle.org. The Eastlake Community Council does not necessarily recommend the methods suggested in these links.

Knotweed guide from government of British Columbia: Click here.

Knotweed guide from Michigan State University: Click here.

Knotweed guide from University of Wisconsin: Click here.

Knotweed guide from the City of Portland Oregon: Click here

Eradication advice from The Spruce. Click here.

Removal guide from Wisconsin Department of Natural Resources.
Click here.

Knotweed mechanical eradication advice from Engineers Ireland. Click here.

Scientific background from AAAS: Click here.

Knotweed article 2018 from scientific journal Biological Invasions. Click here

Bleak 2018 report from BBC News on recent science: Click here.

Article 2019 in Slate.com: Click here.

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