Phoenix, Arizona is in the U.S. Department of Agriculture's Plant Hardiness Zone 10, the second warmest in the classification system. Average annual minimum temperatures for this zone are between 30 and 40 degrees F. Frost does occur in the winter, but winter months are generally so mild that you can grow vegetables throughout the year. Vegetable gardeners take advantage of the abundant warm, sunny days by planting crops in three growing seasons--early spring, late spring/early summer and fall.
Having three growing seasons requires putting together a garden plan that shows the time to maturity of each vegetable so you make certain one crop can finish producing before you plant the next one. Vegetables require at least 6 hours of sunshine, preferably 8 hours or more. Don't locate the garden where a high wall or trees will shade it during the spring and fall when the hours of daylight are fewer. Placing vegetables that grow tall on the north side of the garden will prevent them from shading shorter plants.
Soil in Phoenix may not have enough organic matter to produce healthy vegetables. Other problems are poor drainage and heavily compacted soil. Tilling or turning the soil to a depth of at least 1 foot helps break up compacted areas. Then, add compost or other organic material, along with fertilizer, to improve soil quality.
Gardeners in Phoenix cannot depend on rainfall to water their crops. Successful vegetable gardening in an arid climate requires having an irrigation system, preferably on a timer, so you can carefully measure--and conserve--the amount of water going onto the plants. The best system is one that dispenses the water near the plant roots rather than spraying it in the air. Emitters connected to plastic tubes, and flexible tubing with perforations, called soaker hoses, are used extensively in Phoenix.
Early Spring Planting
Start early spring crops from seed and grow them indoors for transplanting just after the average date of the last frost in Phoenix, usually mid-February. Peas are often one of the first crops planted in early spring, as are leafy greens such as spinach, kale and chard. Should temperatures be unseasonably cool, placing clear plastic sheets in between the vegetable rows can help keep the soil warm. In mid to late March, the garden soil will be warm enough to grow tomatoes and corn.
Late Spring Planting
Traditional summer vegetables such as squash, melons and peppers are planted in April. As the daytime temperatures in Phoenix soar over 100 degrees F in June, vegetable plants begin to experience stress. If plants noticeably wilt in the afternoon, you can apply more water. Take care not to apply hot water right out of the hose, which can burn the plants. To keep moisture near the roots, spread mulch, straw or grass clippings.
The fall growing season presents the chance to have another crop of cool season vegetables, such as loose-leaf lettuce, mustard greens, broccoli and cauliflower. You can also grow root vegetables, such as carrots and beets. Plant the autumn vegetables in seed trays in August and transplant them in September when temperatures begin to drop. Move the trays into shade during the midday heat.