I'm in South Texas. In my humble experience I find that heavy mulching, deep watering, and covering plants with old bed sheets or shade cloth can keep plants alive just enough to survive the 'dead of summer'. Then when the temperature starts to cool off, the plants will produce again. This only works with plants that were healthy to begin with and not heavily burdened by pests or disease. In mid summer, I expect to harvest very little except for certain heat loving plants like okra.
But in the meantime, I will be starting with with seeds in pots in the shade. As soon as the Temps cool back, I'll slowly start to plant them into the ground and keep well watered. I find it best to get a head start your garden early in the spring, expect a drop off during the hottest months of the summer, and continue for another round in the fall and sometimes into the winter. Hope that helps.
