When you're looking at a seedling and trying to figure out if it might be
Japanese knotweed, compare it to the photo here:
Do you see the little red line around the stem at each leaf, with the
papery pair of bract-looking things sort of wrapping around the stem?
Those are modified stipules, and the structure, called an ocrea and it's pretty
diagnostic for the knotweed/buckwheat family, Polygonaceae. No ocrea,
and you can forget about the possibility that you're dealing with
P. cuspidatum. This family also typically has swollen nodes, so the stems
tend to look like knobby knees.
Good weed site: http://www.invasive.org
Page on P. cuspidatum: http://www.invasive.org/weedcd/species/3414.htm