striate knotweed —  Polygonum achoreum {buckwheat family — Polygonaceae}
Flora of the Yukon Territory, Cody, p. 245a
 { 1/30 sec, f10.0} – {Hidden lakes, Whitehorse, Yukon} – {16/08/08 Saturday, August 16, 2008; 2:26:35 PM}