Phoenix, AZ

Phoenix's defining characteristic is year-round warmth with an essentially frost-free calendar (last spring frost January 5, first fall frost January 3, spanning 365 days). But the real constraint is extreme summer heat. From May through September, temperatures regularly exceed 110°F, shutting down flowering and fruit set for most vegetables and many ornamental plants. The effective growing season for traditional warm-season crops, tomatoes, peppers, eggplant, runs October through April. Summer is dormancy time or reserved for heat-loving species only. This inverts the typical US gardening calendar entirely.

What thrives in Phoenix: pomegranate (loves extreme heat), fig (heat-tolerant despite minimal winter chill), desert-adapted citrus, goji berry (cold-hardy and heat-loving), and properly-timed tomatoes and peppers planted for fall-to-spring production. Soil is typically alkaline (pH 7.5 to 8.5 or higher), requiring amendment for acid-preferring crops. Water availability, not frost, is the genuine scarcity constraint in this Sonoran Desert climate. Compared to milder parts of zone 10a like coastal California or southern Texas, Phoenix's extreme summer heat is the trade-off for a frost-free year. Success here means embracing seasonal inversion and choosing varieties bred for desert heat.

Summer heat dormancy. Most vegetables stop setting fruit when night temperatures stay above 75°F and days exceed 105°F, typically May through September. Tomatoes planted in spring will be non-productive by June; peppers decline sharply in quality. Many gardeners mistakenly plant in spring, expecting a traditional season, then watch yields collapse. The solution is counter-intuitive: shift all warm-season crop planting to late summer.

Alkaline soil nutrient lock-up. Sonoran soils run pH 7.5 to 8.5 or higher. At this pH, iron, manganese, and zinc become unavailable even when present in soil, causing yellowing and stunted growth. Vegetables and fruits prefer pH 6.0 to 7.0. Without amendment, supplemental fertilizer won't correct the deficiency.

Water stress and irrigation demands. Low desert humidity and intense sun cause rapid evaporation. Summer watering restrictions coincide with highest plant water demand. Mulching and drip irrigation are not optional; they're essential survival tactics.

Plant warm-season crops in fall, not spring. Sow tomato and pepper seeds in late July through early September; transplant by mid-September. These establish roots through mild winter and produce heavily October through April, avoiding the May to September heat wall. Spring-planted tomatoes wilt and stop producing before they justify the space.

Amend soil pH aggressively before planting. Add 2 to 3 pounds elemental sulfur or 3 to 4 inches peat moss per 100 sq ft and work into the top 8 to 12 inches. Retest in 6 weeks; pH shift takes time in alkaline soils. Mulch heavily (3 to 4 inches) afterward. This is not a one-time fix; high-pH soils revert, requiring periodic maintenance.

Use 30 to 50% shade cloth May through September. Even heat-loving crops like eggplant and pepper benefit from afternoon shade when temperatures exceed 105°F. Shade cloth improves fruit set, reduces sunscald, and cuts water needs by 20 to 30%. Install in late April; remove by early October.