I'm not an expert in what you're trying to do, but I do know that seeds have temperature profiles that vary by species. Ever notice that crab grass germinates towards the height of summer? If its too warm, peas won't germinate. Of course this isn't what you're planting. There's usually a range of acceptable temps however, I've never seen a listing for flowers that shows the range for a species. I'd try the seed outdoors, but make sure you're keeping a close eye on moisture, because the higher temps are going to dry the soil out faster. You may want to (if you have the space) to try some in doors under lights where your temperatures may be more moderate, and then acclimate them to the outside.

One other thing. Some of the annuals (and some of our 'annuals' are actually perennials from warmer areas that we grow as annuals), may be sensitive to the daylength, and if the daylength is decreasing, they may not flower. Again, I don't know which annuals may or may not have this sensitivity.

Well, May is not midsummer, so a quick answer to your question would be yes. As Mauch says it depends a bit on the species but as late spring -summer in USA tends to be very warm and if the frosts stay off till late you could get a terrific display. Good luck - Ian.

Yes, you can sow seeds for annuals in midsummer if you keep them constantly watered which might be 3 times a day. They might not give as good a show as if they were sown in spring, but you can definitely try. Some seeds you think are annuals will actually winter over when seeded midsummer and bloom the following spring. It's all an experiment and you learn a lot as you go, even when experienced. Good luck!