Cosmos is strictly a warm weather plant and is highly sensitive to frost.

Cool weather flowers:
larkspur (plant fall, blooms spring),
shirley and/or california poppies (plant fall, blooms spring),
snapdragons,
pansies,
calendula (like cool weather. might make it through a mild winter for you).
ornamental kale and cabbages
dianthus (technically a biennial. some are perennial)

There are others, but these are the ones most commonly grown.

I planted them in the spring and here it is fall and they are blooming like crazy! I had them in the summer and they did not like something in the soil I guess, because the whole bottom of them looked like they were dying, yet, when it turned cooler they bloomed. Answer that for me, if they are a warm weather flower, why are mine blooming so good now, it is cold and rainy.

I've grown cosmos for years, and it seems that the shorter varieties bloom earlier, and then there are certain very tall plants that take several months to start blooming. I don't know if it's simply that the plants need to reach a certain size before blooming, or if it's related to temperature or length of day or what. I've been collecting seeds from mine for years, and I don't think to start collecting seeds until late in the season. So, I think I'm actually selecting for even later blooming varieties. To combat that tendency, I've already purchased several different varieties of cosmos from reputable commercial dealers for next spring's planting. I'm going to make an effort to collect seeds from the earliest blooms and see if I can't select for some extra early blooming cosmos. I'm also planning to wintersow some of them to get at least a slight jump on the growing season. If I transplant them early, hopefully they won't suffer too much transplant shock.

Martha

I planted some cosmos seed inside in a pot about this time last year. They germinated and quickly gave me a couple of blossoms and then ceased to thrive and quickly died. I may try it again just for my amusement.

Cosmos here are still going strong. But the problem I have is they fall over. They really do better here in the cool weather of the fall. Here are some cheerful ones, not realizing they are living on borrowed time:
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In my zone 7, Cosmos self-sows from year to year, which means the seed can tolerate low temps and freeze/thaw cycles, and survive. That said, Cosmos are heat lovers, C. sulphureous more so than bipinnatus, in my experience anyway. The seeds won't germinate until the soil is warm enough. That said, I find that C. bipinnatus does better in the cooler temps of late spring-early summer, and fall. C. sulphureus wins the hot summer show. C. bipinnatus does not self-sow as prolifically as C. sulphureous either.

In your zone, a few may come up now, depending on the weather, and some in spring, assuming the birds and other critters don't find them first.

There is always a question, too, about whether self-sown seeds will grow "true to form". My 'Cosmic Orange' Cosmos have reverted to the species and grow to about 6 or 7' tall in some cases.

What kind did you plant?

Susan