What's this plant?

[Polyethyl]

Set about clearing my balcony boxes of weeds, so I can plant tomatoes soon. But what is this that's taken over every box?

That looks like corn salad/lamb's lettuce. Very edible, though I'd be hesitant to eat it without 100% solid ID.

Looks like lambs lettuce to me too. Did you maybe use a seed mix? If you let it go to seed, you get LOTS of new new plants.

Thank you. That makes sense!