It looks like a smartweed, Polygonum.

Here is one for example.

http://www.plantsinusa.com/show/plant/Pennsylvania-Smartweed/69161

Opinion: a vicious weed :) I would like to eliminate it from where it's growing in my yard (started last year). It seems to be opportunistic with high moisture years.

Thank you Carol! I saw Smartweed on another website but it didn't look like the flower in the link you posted.

Dbarron, the link said they do like areas around water and this specific area where I found the plant had a big dip in the ground and after heavy rains it would get a LOT of water in it. There's a pond south of that area and I bet some is growing there and a seed blew to my yard. Or a birdie flew over. :)

I do like them though, and may try to grow a patch since the plant is beneficial to a lot of birds.

There are other, perennial kinds of knot-weeds you can find in nurseries.