If you can give it some extra shade through the hot weather, that will probably help the most of anything you can do. Any way to rig up a piece of shade cloth above it? It will keep the side of your house a little cooler, too.
Increase the water as the weather gets warmer. Water them deeply in the morning before the heat gets going, and make sure the whole root area gets moisture. Have you put down some mulch over the roots? That will help preserve moisture in the soil and keep the roots cooler too.
