Mulch your boxwood hedge as a final step. Boxwoods have shallow roots that mind the heat, so mulching will shade the plants' roots from the blazing sun.
Avoid the pitfall of thinking, "More is better" when it comes to mulch. Apply only 2-3 inches of mulch, and leave an inch or so around the base of the shrub completely open, to promote air circulation.
If you make the mistake of applying more than 2 or 3 inches of mulch, you'll exacerbate the natural tendency of boxwoods to produce shallow roots, instead of encouraging them to strike down deeper into the soil. This is undesirable, as the shrub becomes even more susceptible to damage during periods of drought.
I explain some of the other benefits of applying an appropriate layer of landscape mulch in an FAQ.