Polygonum virginianum, aka Persicaria virginiana and several others; Polygonaceae (buckwheat family)
Most of the LWFs* I wrote about earlier in the season were tiny little flowers on tiny little plants. This one is a tiny little flower on a very large inflorescence on a medium-sized plant. The plant grows about three feet tall, and usually sports only a single foot (or more) long flower spike with dozens of buds; only a few open at a time, and each flower is less than 1/8″ across.
Jumpseed is found throughout woodlands in North America, from northern Quebec south through Florida and west to Texas.
*little white flowers