I would say Nipplewort, Lapsana communis. Not something you would have planted. The seed is like dandelion with parachutes. They can blow in from miles away.

And I'd confirm that! They are all over the fringes of my garden at the moment and more pop up daily!

Thanks! There’s nothing like double confirmation. I actually remember a day when there was so much airborne fluff it seemed like it was snowing. My open pots were covered by screens which might explain why those seeds didn’t land among them. Thousands of spirea seeds slipped through though.

I cannot overstate how much admiration I have for those of you who can identify plants at various stages of development. I pulled the nipplewort seedlings out of the pot and a couple other of places where volunteers of any kind weren’t welcome and moved on to other stuff. Over the last week or so, the nipplwort in the back 40 (actually the side 2000 sq ft) bloomed. I’d totally forgotten that I asked for an ID of the plant in bloom a few years ago. There’s a lot more of it now. I like it where it is and it seems happy, but unfortunately so does the stilt weed. I have a load of wood chips and lots of cardboard so this patch nipplewort’s going to be collateral damage, but so it goes. Thanks again!