Here's the fern-like volunteer, as described above.

Funny! Though I suppose not at the time. I help manage an herb garden at the community garden where I have my veggie plot. Last year, I thought I'd put my basil over by the fence. First year I'd been there (or anyone else who was around), and it looked like a nice spot. Digging to prep/compost, I kept finding all these funny looking bulbs. Close to 6-7 dozen of them. Just starting out, I hated the thought of throwing them, but hated to have something toxic in the herb garden. Went to two different greenhouses and still couldn't identify. So, I threw them in a couple clumps at either end. Turns out.....sunchokes. Pretty darn good at choking out my basil! But, fun to play with. This year, I'll put the basil in a different spot where it doesn't have such competition for sun & water. And, thankfully I figured out my mislabeling before planting cold season broccoli--tomatillos really wouldn't make it here right now.

Interesting. I assumed this was the same weed that I had at our old house, but I remember the old weed having nice, red fall color. The weed pictured seems to be ID correctly as Parietaria (I checked the leaf margins.) The fall color on the old weed must make it Acalypha. They really do resemble each other.

Three-seed Mercury is not all that rare.. im surprised you never noticed it before... ken