Japanese knotweed stems with fruits still intact, mid-January. Each fruit is a papery, winged achene containing a single seed. By the spring many of the achenes still attached to the plant will be empty, eaten by birds or more likely knocked away by the wind.
A lot of people are under the mistaken impression that Japanese knotweed doesn't spread by seed. While it has been quite successful at invading new sites via pieces of rhizome or stem, its seed is viable and is actually easy to germinate (don't try this at home folks :-)).