Teucrium

I recognized the flowers but the leaves are different than our native one, so I looked it up. It appears to be Teucrium botrys, which is pretty sparsely introduced in the northeast. In PA it appears to only be recorded from Lehigh County.

Yeah, that's it. Thanks! I grew it in 2011 from seed obtained through NARGS; it bloomed nicely and didn't return the next year. No idea why it chose to re-emerge in such large numbers three years later...