Contrary to what many gardeners think, unlike other vegetables, tomatoes are a perennial, not an annual. In warm climates like Phoenix, tomatoes may grow for an entire year or more, and gardeners have two seasons for setting tomatoes: April through May and late August through November. In warm years, it's possible to harvest red ripe tomatoes for Christmas.
Select an early maturing tomato variety. Because summers are long and hot in Phoenix, it would seem that a long maturing variety would do well; however, the months of June through August are too hot for tomatoes to thrive. Plant an early tomato in March and harvest will be completed by June.
Choose a location with rich, loamy soil that gets a minimum of eight hours of sun a day. Afternoon shade during the summer will help the tomato plants live through the heat and be able to start producing again in early fall.
Dig a hole one foot deep and wide. Add compost and organic matter as well as 2 cups (4 handfuls) of gypsum. Mix well. Desert soil doesn't have a lot of organic matter, and is alkaline. The gypsum gets rid of the alkalinity.
Back fill the hole with the soil mixture. Take the tomato transplant out of its pot and remove all the leaves except the top four. Bury the root ball and stem all the way up to the first set of leaves. New roots will grow along the buried stem, and this strong root system will help the plant during Phoenix's hot summers.
Water deeply so the first 6 inches of soil are soaked. Wait until the top 3 inches of soil dry out before watering again. Add water-soluble fertilizer to every other watering while the plant is actively growing and setting fruit.
Provide afternoon shade during the summer with a shade cloth, by planting the tomatoes by a fence or wall where they get afternoon shade or by throwing a sheet over the top of the tomato cage. Don't prune until the end of August, as the leaves provide shade that the plants need.
Cut the plant back to about 24 inches at the end of August when the temperatures are relatively cooler and the weather more humid. The plant will revive, put out new growth and start blossoming and fruiting again.