Answer: I grew the flowers for my own wedding last year--it was fun!
I'd caution you not to "put all your eggs in one basket". I'd grow lots of different things so you can be sure some things are in bloom, and have a backup plan. All sorts of things can create problems, and it would be awful if the flowers didn't turn out as you wanted! Perhaps you can consult with a florist about a contingency plan to add some flowers if you don't have enough.
My wedding was very informal, in the back yard, so I used lots of sunflowers, zinnias, and cosmos. I think different types of cosmos might be nice for you. Be warned, however, that the tall varieties tend to fall over, creating crooked stems. You can stake or cage them, or grow the shorter varieties. Asters would also be nice, and should be in bloom in mid-September. Purple salvias and Sweet William would be nice too. Stocks and snapdragons are beautiful, but I'm afraid your summers might be too hot for them--they did fine up here in northern Vermont though. Heliotrope is pretty, and has a nice scent, though it's a little bushy to use in arrangements. Bachelor's button, scabiosa, and blue lace flowers are all pretty. And baby's breath is a classic.
You could also grow small potted flowers, and use these to decorate. The heliotrope might be nice for this, if it doesn't get too big. A friend of mine used small clay pots, and created a topiary out of wire and English ivy. She formed a heart-shape out of the wire, then trained the ivy vines up and over the top--it was really nice!
And be sure to prepare the beds before planting. Unless you have perfect, loamy soil I suggest you add some compost or composted manure. A soil test will tell you if you need to add limestone to adjust the pH, and will clue you in on the soil's nutrient levels.
Hope this helps. Congratulations!
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