Looking for an ID on a particular species
I don't have any photos at the moment because I'm trying to weed it out (it's very aggressive, having appeared in one location last year but everywhere this year), but hopefully I can provide enough information.
Size: About 2-3ft after one growing season. Very healthy-looking lush growth when growing in shade on clay soil. Develops a sharp, narrow reddish-purplish-brown border at the leaf edge when exposed to more sun.
Growth pattern: Spreads upwards, forming an interesting woody base clump, and propagates itself by massive stolons beneath the soil surface.
Flowers: Purple, insignificant, borne on long thin spikes (one per stem, iirc). Once mature, the seeds have fluff on them like dandelion seeds do, and the fluff is fine enough for the individual seeds to fall in clumps or stay on the plant.
Looking at the topmost leaves alone, it bears more than a passing resemblance to basil. It has more leaves on the stem though.