Well I have never gardened in your region, so I am not sure, but what popped into my head is Annual Cosmos! Mine are blooming their heads off right now, I don't hardly water them in summer (we get less rain than you in summer). They start blooming here midsummer. They also reseed themselves. The colors available are mainly white and pink. I don't know how they would do in your summer heat and humidity. Cosmos are slow to get going in the spring, so they would not interfere with your big spring show.
I had 'Rubenza' one year, just loved the dark rosy pink.
http://parkseed.com/cosmos/c/c...
