This plant is growing in my yard. It sent out very long spikes of the flowers...
This is a native plant, Tovara virginiana or Persicaria virginiana. Common name is Virginia knotweed. It's a good native plant because deer don't seem to eat it, so it helps form the forest understory. Early in the growing season, the leaves are attractivee with chevron markings. Later, they get the flower stalks. What you're seeing now are the seeds forming on the stalks.
This is a perennial plant and can be a nuisance because it seeds itself around and is hard to pull up. We wouldn't recommend it in ornamental beds, but if you have a wooded or wild area, it is a good addition.