annuals that overwinter in Wisconsin

I have had impatiens, coleus, alyssum, and torenia reseed. Cleome also is a reliable self-sower but it is tall - not anything close to being a ground cover. I have never had marigold or lobelia return in subsequent years. The complication in this case is that you want a ground cover. I read that as a dense, continuous carpet of vegetation. Even when things self-sow, they don't generally do so in a consistent manner. You can get a clump here and a few plants there. Sometimes they come up places you don't want them. They usually appear late and sometimes they just don't return. I would say that off all the plants I can think of, alyssum fits your requirement the best. It is very reliable, forms a dense carpet, and tends to come back close to the original plant's location. I don't know about how hearty the seeds are in zone 4, but you could always just get a few packets of seed and direct sow them yourself.

Cleome, marigold, pansy, datura, snapdragon, petunia & moss rose to name a few. A few snapdragons in well protected areas will survive the winter. I live in IA Zone 5 but we had some -5 to -10 weather this last winter. In some of the milder winters I have had whole beds of snapdragons survive.

Marv-Montezuma, IA

There was a guy who for years lived in Wisconsin on the Wisconsin-Illinois border. One day a survey crew came by, and discovered that his property was actually in Illinois. When interviewed by a TV station, he was asked what he thought of the situation. He replied: "Thank goodness I won't have to live through another hard Wisconsin winter!"

But seriously, when I lived in Wisconsin the only annual that my parents had that over-wintered, and self-seeded was the moss rose. And that's why I was so surprised that I can't get them to do the same now that I'm in zone 8 in Oregon. I never saw any other annuals come back, but then again, my parents weren't very adventurous in the gardening arena. They viewed the moss roses as an invasive weed, and eventually widened the driveway, paving over their patch. (Five months of the year that part of the driveway was still buried under snow pushed off the rest of the driveway, so they didn't gain all that much by doing so.)

Scotch Moss or Irish Moss is excellent ground cover. Scotch moss takes a better winter than Irish Moss. Have some out front and we get minus

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).

You know what they say about winter in Wisconsin . . . . . . . . it's the best 8 months of the year!

Ted Shoemaker