Gardening in Arizona: zones, frost dates and when to plant

Elevation sets the calendar

University of Arizona Cooperative Extension’s Ten Steps to a Successful Vegetable Garden (AZ1435) treats elevation as the best guide to climate. It identifies two problem periods: the hot summer at lower elevations and the cold winter at higher ones.

• Low desert, up to about 3,000 feet (Yuma, Phoenix, Tucson and Safford, by AZ1435’s elevation table): two planting periods, early spring for warm-season vegetables and late summer through winter for cool-season crops.
• Highlands, about 3,000 to 7,000 feet (Sierra Vista, Prescott, Payson, Show Low and Flagstaff): one main season, planted in spring and early summer. AZ1435 adds that an early fall planting of cool-season vegetables usually does well at these elevations in central and southern Arizona.

Yavapai County Extension’s planting-date bulletin shows how fast the calendar shifts. It gives separate schedules for Black Canyon City (2,000 to 3,000 feet), the Verde Valley and Sedona (3,000 to 4,500 feet), and Prescott and Chino Valley (4,500 to 6,000 feet). A companion bulletin puts the growing season at about 221 days in the Verde Valley and 153 in Prescott. Look up the frost dates for your city above, then use the county calendar that matches your elevation.

The low desert: a fall-to-spring garden

Extension’s Vegetable Planting Calendar for Maricopa County (AZ1005) explains why the low desert works backward from most of the country. Long stretches of high day and night temperatures, low humidity and intense sun put heavy stress on plants, and an occasional hard frost can still kill tender crops. Few annuals do well in both the cold, short days of winter and the long, hot days of summer, so the calendar favors quick-maturing varieties that finish within one season. It recommends checking soil with a thermometer and planting when it reads 65°F to 85°F, the range many common vegetables need.

The Yuma County calendar (AZ1615) lays out the two seasons plainly. Cool-season crops such as lettuce, spinach, broccoli, cauliflower and cabbage grow from September through February, when the sun is low. Fruiting crops such as tomatoes, squash, melons and peppers go in for the March-through-August season. Although some cool-season crops can be sown in September, the Yuma calendar suggests most gardeners wait until October unless they can provide shade and extra water for seedlings.

Pest timing matters too. AZ1005 advises delaying fall planting until whitefly numbers drop with cooler weather, and waiting in spring until soil is warm and dry to reduce fungal and bacterial disease.

The highlands: short season, cold soil

In the Prescott area, the Yavapai bulletin recommends starting tomato and pepper seed indoors because the weather is too cold and the season too short to germinate them in the ground. Set pepper plants out two to three weeks after the last frost, once soil reaches at least 65°F. The bulletin warns that many seeds germinate poorly while soil stays below 45°F and will need row cover if they do come up. It also notes that peppers and tomatoes will not set fruit above 90°F, so even the highlands have a midsummer slowdown. Bulletin #30 suggests warming spring soil with black plastic or row cover. AZ1435 cautions that organic mulch keeps soil cool and can slow growth at higher elevations.

Alkaline soil, salt and caliche

Arizona soils work against eastern habits. The Yuma calendar points out that local soils typically run pH 8 or 9, so adding lime, a routine step in acid-soil states, is normally a mistake. Bulletin #30 notes that most vegetables prefer pH 6.0 to 7.5 while Southwest soils tend to be more alkaline, and suggests soil sulfur to bring pH down.

Salt builds up as irrigation water evaporates, and AZ1615 notes that drip systems concentrate it in the root zone. The fix is calcium from gypsum followed by a long, slow soak to push sodium below the roots. AZ1435 also cautions against overhead sprinklers when water is salty.

Caliche, a layer cemented by calcium carbonate, is common in southern Arizona. Extension’s Managing Caliche in the Home Yard (AZ1281) explains that it can stop roots, trap water and salts, and raise pH enough to cause iron deficiency, which shows as yellowing between the veins of the newest leaves. Dig through and remove it where you plant, or build raised beds with at least a foot of good soil mix, as AZ1435 suggests.

Water and testing

Because rainfall is limited and unreliable, AZ1435 says every garden crop in Arizona needs irrigation. Water when soil crumbles as you squeeze it. Shallow-rooted crops such as lettuce, onions and corn need water more often than deep-rooted tomatoes, asparagus and watermelon, and sandy soils need it more often than clay. Bulletin #30 notes that your local Cooperative Extension office may offer free soil pH tests, a sensible first step before buying amendments.