Are your trees killing your lawn?
Apart from lack of water, trees are the number one killer of lawns.
How is that possible??? Easy...shade.
Raise. Your. Canopies!
Bermuda needs a minimum 7+ hours of direct sunlight per day. St. Augustine, depending on the variety, needs anywhere from 4 to 6+ hours of direct sunlight...MINIMUM.
Large trees will very quickly and easily rob your turf of the sunlight it needs. You need to be raising your canopies AT LEAST once per year (some trees may need more). This will allow your trees to get a good amount of morning and evening sun.
Taking it a step farther by thinning your canopies will allow for more filtered sunlight, which can be of benefit as well.
Your lawn will thank you!