Two specimens of this thing are growing in a very narrow little bed against the back wall of the house. They may have been planted by Kevin's ex-wife, who put some tiger lilies back there as well, or they may be a weed shrub/tree/thingy. They grow absurdly vigorously--we hacked them back to stumps last year, and they are about twelve feet tall now and have eaten the walkway, leaving only a beagle-sized tunnel underneath. They have flowerheads starting, but have not yet flowered. (Upon flowering, it may be abundantly obvious, but at the moment...no clue.)
The bit that puzzles me the most is that weird little leaf strip along the edges of the stem itself. Haven't seen that anywhere else, don't know what to call it. If it wasn't for that strip, I'd think it was tree-of-heaven and go screaming for the salt and hemlock, but none of the photos of ailanthus have that in it, so...stymied.
So ID welcome, so that I know whether to give it more water or go after it with fire and chainsaw.
UPDATE: Winged sumac! Thank you, guys! Darn, I would love to have that plant growing, but as people have pointed out--it's not a good one on the foundations. I'll let it flower and let the birds get a last meal in, then take it out.