It is VERY difficult to do plant identification by description alone, but it sounds like you may have Japanese Knotweed (Polygonum cuspidatum) It is Extremely (!) hard to control this plant. Rototilling it will make hundreds of new plants because it grows from the tiniest root fragment, so do not do that! There is a lot of good information on it on the Internet, but here are two good links:

King County, which lists it as a Class B noxious weed (control recommended but not required by law)

Season All Season
Date 2006-11-07
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Here is a list of helpful resources:

Washington State Noxious Weed List from the USDA

State noxious weed list and schedule of monetary penalties from the WSL

Class A, Class B, and Class C

Washington Department of Ecology (aquatic plants)

Washington Invasive Species Coalition

King County Noxious Weed Lists

National Invasive Species Lists

Plant Conservation Alliance Alien Plant Invaders list

Invasives in British Columbia

The lists which are national in scope are useful too, as some plants not yet officially listed as invasive here may still be plants to watch out for.

There are a great many books on this subject. A recent one, co-authored by a faculty member here, is Invasive species in the Pacific Northwest edited by P.D. Boersma, S.H. Reichard, and A.N. Van Buren; Rebecca L. Gamboa, photo editor. University of Washington Press, c2006.

Season All Season
Date 2007-10-10
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I'm sorry to hear you are struggling with knotweed. Manual methods include cutting, mowing, pulling, digging, or covering. Dig surface roots in loose soil. Check frequently for new shoots and dig them up as soon as you notice them. Cut stems close to the ground every two weeks throughout the growing season. After cutting completely, you could instead decide to cover the area of the knotweed patch and the immediate area around it with black plastic or other impermeable material. This is a long-term process, and it may take several years to eradicate the Polygonum. Here are some helpful links on knotweed:

Invasive Knotweeds from the King County Noxious Weed Control Program

Knotweed Biology and Control from the King County Noxious Weed Control Program

Controlling Knotweed in the Pacific Northwest (a large file!)

Japanese Knotweed from the PCA Alien Plant Working Group

There is even a film about this weed, from the Whatcom County Noxious Weed Board!

This link is to a list of slide presentations from the 2007 Western Society of Weed Science Knotweed Symposium.

This presentation describes non-chemical controls, from Seattle Public Utilities.

Season All Season
Date 2007-11-01
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