Petunias really prefer cooler weather, but will "survive" our heat/humidity with some care. When blooming slows down, or when the plants become leggy, shear them back and fertilize them again. There are some petunias (the Wave series for instance) that are reputed to be bred to withstand our summer conditions better. I don't grow the Wave series so can't testify to whether this is true or not. Perhaps someone else can. When I grew Petunias in hanging baskets, I usually ended up pitching them about July/August. Even with the shearing/fertilizing routine, the older varieties of petunias were too unsightly by then to wait another couple of months for cooler weather.

Talked to my landscaper who said the key to petunias is watering in the morning. Not at night. Thought that was pretty interesting. My neighbors recently cut down two HUGE (probably 30+ year old trees) and suddenly everything was in full sun. Why people cut down trees I will never know?! So...I'm finding that I am paranoid of this Oklahoma sun killing everything! :)

Wave petunias do great for me - I plant them in an old wheel barrow just about every year. Don't remember having much of a wilting problem - but most of the time I think I did water earlier in the day. You can sheer them back like Maryl says if you can bear it - and they will bounce right back better than before.