One of the Persicarias. Compare Persicaria pensylvanicum, Pennsylvania smartweed. [Formerly Polygonum pensylvanicum, and often found under that name.]

http://www.illinoiswildflowers.info/prairie/plantx/pa_smartweedx.htm

The horribly invasive Japanese knotweed is a relative. The docks (Curly dock, etc.) are also kin.

There are cultivated varieties, more likely to be found in Europe, though this nursery is American:
http://www.diggingdog.com/pages2/persicaria.php

This post was edited by missingtheobvious on Mon, Sep 22, 14 at 21:06

i would go with very aggravating weed ... and suggest you get rid of it.. before it seeds ...

ken

Thanks for the links and sending me in the right direction. This is what I found....

"Smartweed ( Polygonum pensylvanicum ) and closely related lady's thumb ( P. persicaria ) are a shade gardener's worst nightmare, because they grow best in moist partial shade.

Found throughout North America, these native annuals produce thousands of hard-coated seeds. Plants occasionally grow to 4 feet tall, but even when mowed repeatedly smartweed often manages to produce flowers."

I'll yank it out of there while I still can. I sure don't need any more aggravating weeds than I already have.