This hollow-stemmed wild plant grows amid salmonberries and horsetails. It grows up to nine feet high and twelve feet wide, and dies back each winter. It has multiple stems growing out of the ground. The heart-shaped leaves can grow to eight or nine inches long. I live on southern Puget Sound in Washington State. 2 photos attached
I think you're on to something! I can't find that specific polygonum in my plant identification book, but drawings of the giant knotweed, polygonum sachalinense, look much like my plant. And to think it's related to the buckwheat! Thanks again! Judy Watt judithw@centurytel.net
