I've visited greenhouses and talked to friends, but even though everyone's seen this weed, no one seems to know its name! Haven't found anything in these forums or online elsewhere. Anyone have any idea?
Looks like it may be garlic mint.
Update! The weed has now flowered. Nice yellow flowers that very much resemble evening primrose flowers. Is this possible? They're everywhere and very much act like weeds!
The whitish vein in the middle of the one leaf where I can see it better indicates that it certainly could be Common Evening Primrose. They are extremely prolific! I have been using it as a free "groundcover" in an area that I haven't had time or money to plant yet. Also makes a great "trap crop" for Japanese Beetles, I discovered! It is easy to get rid of when you want to-all but the biggest roots pull up fairly easily in moist soil.