Colorado Flora - Eastern Slope - Third Edition by William A. Weber and Ronald C. Wittmann have Japanese Knotweed as Reynoutria japonica. They call it Japanese Buckwheat, and give Polygonum cuspidatum in parentheses. Polygonum is represented in the key, as is Fallopia. Fallopia is keyed out immediately as herbaceous vines (Black Bindweed). Polygonum is separated later from Reynoutria.