What plant is this

[Chuck Uebele]

I had planted some milkweed that went to seed in the back of our house. This is what came up, but doesn't look like the milkweed that I planted. The leaves are tiny, and there is only a single small flower, rather than the normal clusters.

 

[Chuck Uebele · Accepted solution]

I think I found it: Tall willowherb.