I scanned the flower. It's not purple loosestrife, thank god, but it's definitely a vigorous spreader--it's all over the bed, and occasionally appears elsewhere in the yard. The young leaves have blunt, rounded edges, with an almost clover-like pale patch down the center. By the time it gets tall, they get pointier, and the pale patch fades. Japanese beetles eat it.
EDIT: Thanks to Starfallz and Winters_edge, we have an ID! It's a tick trefoil of some sort (and that's all I ask.) Editedit: Okay, I think it's "perplexed tick trefoil." What a great name!
It's a native wildflower. It's a host plant to the Southern Cloudywing, the seeds feed small rodents and gamebirds (not that I expect wild turkeys in the yard any time soon!) and is an important source of pollen for long-tongued bees. All sorts of grazers like the leaves.
I tore that shit out by the POUND. Now I feel guilty. I planted anise hyssop in its place, which isn't bad, but not the smorgasbord that the tick-trefoil would have been. That'll teach me to interfere with nature.
On the bright side, the stuff is VERY vigorous--it's a pea, after all--so it'll be back next year in spades. I'll probably leave the main bulk of it in the middle of the bed, and continue to rip out the volunteers elsewhere, lest this become an All Pea All The Time bed...
The flowers are bit squashed by the scanner, and when seen face on, strongly resemble the flowers of an Asiatic Dayflower, only pink and tiny and on a spike.
X-posted to gardening