My garden is holding up fairly well to the heat because of water conservation strategies I have employed through the years. I work every year to improve my soil because soil that contains lots of organic matter holds more moisture than sand or clay.
Mulch is my ally against drought and heat. Plant roots appreciate shade from temperatures over 100˚F as much as people do, and a thick layer of mulch insulates and cools the roots, and holds in available soil moisture. I choose cheap and plentiful sources of mulch over expensive and hard to get, and so I use partially rotten hay, leaves, grass clippings, newspapers, cardboard, and pine straw for mulch. Mulch also keeps the weeds from growing so I do not need to go out in this heat to weed the garden.
When I water my lawn and garden, I water them deeply and infrequently, and I water them only when rain does not fall. I do not sprinkle them every day because shallow, frequent watering encourages the plants to develop shallow roots that cannot tolerate drought. These lazy roots love their life of leisure without having to search for water. Like lazy humans, however, they are helpless when their water is not given to them. Deep, infrequent watering, which mimics natural rainfall, makes the roots of the plant search deep in the soil for moisture. When rain does not fall, the deep roots can find water in the soil for much longer than roots used to easy provision of water.
Drip irrigation is the best way to get irrigation water to the roots of plants. Sprinklers put most of the water on the leaves of the plants, where it dries up before doing much good. Sprinklers also promote disease by wetting foliage. I do use a sprinkler in the garden, but I try to keep it off my tomatoes and other plants prone to disease.
When it rains, either turn off your automatic lawn sprinklers, or buy a rain sensor to automatically stop the sprinklers if rain falls. I turn on my sprinklers manually when the grass looks shriveled. And please, please, adjust the sprinkler heads so you water plants instead of asphalt. I do hate driving by commercial landscaping and having my car bathed by water that’s supposed to be going on the plants.
Ideally, gardens thrive best on an inch of rain per week, so if an inch of rain falls from the sky there is no need to waste precious fresh water by watering the lawn that week. I turn on the drip irrigation to my tomatoes and run it for an hour or two once or twice a week, if it does not rain, which gives the tomatoes the consistent soil moisture they love.