It's milkweed. I let it grow in my garden for monarch butterfiles.

Thanks! I think I must have planted it intentionally, since I've been trying to add butterfly-friendly plants, but then I seem to have forgotten that I'd done that. :(

Diansan, do you let the seedpods mature and drop their seed? I'm wondering if I'll have way too many plants if I let that happen ...?

My first milkweed was a volunteer. I leave the pods on because my beds are mulched and I don't get too many new plants sprouting from the seeds. I think I have about 4 or 5 plants growing this year.