Ted Shoemaker said:

I would like to plant some annuals that will reseed themselves. That
sounds okay; it's how flowers reproduce naturally. BUT I live in Zone
4, so some seeds will not survive the winter.

What about marigolds? lobelia? Will these seeds survive
temps below zero F?

I'm looking for something that will be a ground cover. There are plenty
of leggy or weedy-looking wildflowers; let's try for something full and low.

I have a short variety of Nigella (Love in a Mist) that has been a very
reliable reseeder. Bonus points for an easily recognizable seedlings!
I think it was 'Dwarf Moody Blue' but it was years ago that I planted
it, so I'm not 100% sure on the variety.

I used to have dwarf annual candytuft (Iberis umbellata) reseeding in
one garden, until it got too shady for it to bloom reliably. Probably
'Fairy' mix.

California poppies come back reliably for me.

Sweet alyssum reseeded for me, but rapidly reverted to something
wild-looking (not showy at all).
--
Pat in Plymouth MI ('someplace.net' is comcast)

Any technology distinguishable from magic is insufficiently advanced.
(attributed to Don Marti)