I live in the Austin area and I have St. Augustine grass in my yard. In my back yard under a Bradford pear I have Amerishade but I have some bare spots where the soil gets packed and it cracks so the grass is not spreading to those areas. Even if I water the soil does not seem to hold the moisture. Should I add sand in those areas to hold the moisture and hopefully get the grass to spread?
Try watering at a slower rate to get deeper penetration. Remember to specifically water the tree, once a month, under the shade line, along with watering your grass. If this does not help, yes you could add some sand to the soil. First, you will want to aerate the area, if possible, to allow the sand to enter the soil profile. After topping the aerated area with sand, rake the sand to work it into the profile. Then, you will want to sprinkle a little fertilizer on the aerated area, and water in.