When it comes to annual transplants, the month of September gives us a planting break. We're in a period during which it's too hot plant cool season annuals and won't be hot long enough to plant summer annuals.
The weather is much too humid and oppresive for cool season annuals to live for any period of time right now. Planting them now will yield you a plant that will hang on for a few weeks and essentially melt.
Summer annuals should have been planted months ago so they could be well established by this time of year. They would have had time to put out generous, healthy roots to help the survive the heat. Planting these plants now will not allow them to get properly established to perform as well as they could.
But we should start thinking now about which cool season annuals we want to plant next month. Start planning that fall seasonal color. Ordering the seed now and sowing the seed into transplants is a way to get a head start on the fall season.