Some peony poppies reseed themselves, and sometimes California poppies do also. Is it better to wait until March in Northern climates?

I have done both. We get LOTS of volunteers from the peony poppies, but I sometimes start particular ones indoors in March with equal success. I haven't had the same luck with California poppies. Since they're native to a warmer climate, they may not like those long cold winters!

I also was hoping that the Calif would reseed themselves and come back. Never happened. I would have to save seed and resow in the spring. Now the peony poppies reseed too well. I also have bottles of their seeds by color. I pitched last year's seeds just yesterday as I have a good group this year. I purchase new colors as I lose them over time. White is a hard one. But I was fortunate to have a healthy plant this year and have seed from that one. Will plant in flats or pots next spring and then just move the clumps out side.

I get better results that way too.

I think that's probably best. I read somewhere in my travels that the California poppy seeds don't like extremely cold winters. Fall sowing was only recommended where the winters were mild.

Mild is relative, and while we are 'mild' compared with my home in Fairbanks, it is definitely not mild relative to planting tender seeds.

My only experience with Calif poppies were some stray ones that came along with plants from a friend. And there were no volunteers the next year. So it does sound like for us cold climate folk, we need to gather seeds and store them till Spring. Darn!

Annual poppies are the only type I try to grow, and with our weather conditions I'd say that any and all of them would be best sown in fall. Still, I think that the luck of the draw, snow-wise, plays a part in how many survive till spring; bitter cold temps before the snow cover appears may thin the ranks...though this is just a guess on my part.

The standard deep-yellow/gold CA poppies have self sown here for probably 5 or 6 years now, but last year's spring (soil) warmup was so delayed that I didn't see blooms until fall. I've never had much luck with direct sowing poppies in spring. I usually get better results if I transplant them out as early as possible. It seems to me that poppy seeds like the reverse of what we have to offer in our typical spring - cool soil, and cool air. We seem to shoot almost directly to very warm air and hot sun, while the ground soil is still very cold and wet.
We do get self sown seedlings of other annual poppies that do well, so my best guess is that those seeds have germinated the fall prior. I have a newer raised bed that's in full sun now, and planted rather thickly with spring bulbs, so I may try direct sowing poppies in that bed this spring...perhaps its soil will warm up sooner than other beds here.

I am saving the annual poppy seed until March this year.
Some years there are self sown ones, but not all that many.
The orange California poppies occasionally self sow, but not reliably here.
Oriental poppies (perennial) will be started indoors, or by winter sowing.

I have two orientals I have started indoors from roots store-bought. One is failing but the other looks good. Also have WS lots of somni--- whatever they are They are on the back porch covered in snow as of today. Drat. Late snow storm. Probably good for them. Not so good for the iris in pots.