We made the mistake of doing that one summer, and it burned areas of our yard (St Aug also). We have extreme heat and humidity, though. The LSU Ag Center has proved helpful for us: http://www.lsuagcenter.com/

Good luck! =)

Unless you can assure that ample soil moisture will be available to get the "fertilzier" into the soil applying that during the hot and normally dry summer is a waste of time, energy, and money. Grasses under stress, due to lack of ample soil moisture, are easily harmed by excess nutrient applications. If there is a need to apply a weed and feed wait until fall when the weather cools down and gets wetter.