March marks the transition into the warm-season gardening calendar for much of Arizona, making it a key month for planting. Success in this arid and geographically diverse state depends on understanding the region’s unique climate constraints. Gardeners in the lower elevations focus on establishing heat-loving crops quickly, while those at higher altitudes must still exercise caution against late frosts. This short window of moderate temperatures offers the best opportunity to prepare the soil and set plants for a successful harvest before the intense summer heat arrives.
Defining Arizona’s Spring Planting Zones
Arizona’s vast difference in elevation creates three distinct planting environments: the Low Desert, the Transition Zone, and the High Desert. The defining factor for March planting is the expected last frost date and consistent soil warmth. The Low Desert (Phoenix and Tucson) typically sees its last frost in mid-March or earlier, allowing warm-season planting to begin in earnest.
The Transition Zone (Payson and Kingman) experiences a later last frost, often in April or early May. This delay means March planting must prioritize frost protection or cool-season crops. The High Desert (Flagstaff) has a much shorter growing season, with the average last frost not occurring until early to mid-June. This necessitates starting almost all warm-season plants indoors during March.
Low Desert Planting Guide (Phoenix and Tucson)
March is the most active planting month in the Low Desert, often described as a “race against the heat” to ensure crops mature before June’s triple-digit temperatures. Warm-season vegetables should be transplanted now, as starting them from seed outdoors in March may not allow enough time to establish before the heat causes them to struggle.
Warm-season vegetables like tomatoes, peppers, and eggplant should be set out as transplants early in the month when the soil temperature consistently reaches 55°F. Direct sowing is appropriate for crops that germinate quickly in warm soil. Planting these directly avoids transplant shock and promotes robust root systems to handle the stress of the approaching summer.
Direct Sow Vegetables
- Bush beans
- Corn
- Cantaloupe
- Cucumbers
- All varieties of squash
Herbs and ornamentals also thrive with March planting. Basil, a heat-loving herb, can be planted from seed or transplant and will flourish in the rising temperatures. Other herbs to plant include chives, mint, and oregano. Annual flowers like zinnias, marigolds, and sunflowers can be direct-sown for color, attracting pollinators and providing shade to vegetable plants.
High Desert and Transition Zone Planting Guide
Gardening in the cooler, higher elevations requires a cautious approach in March, as frost is still a significant threat. Outdoor planting is limited to crops that can tolerate cold soil and light frost, but this is the ideal time to start warm-season varieties indoors.
Cool-season root vegetables and leafy greens can be direct-sown outdoors in March, as they are tolerant of freezing temperatures. These quick-maturing crops should be sown early to harvest before the soil warms too much, which can cause them to become bitter or bolt.
Cool-Season Crops for Direct Sowing
- Radishes
- Beets
- Carrots
- Spinach
Warm-season crops like tomatoes, peppers, melons, and squash should be started from seed indoors under grow lights or on a heat mat. This gives them a significant head start, allowing them to be transplanted outdoors only after the last expected frost date (as late as May or June in areas like Prescott or Flagstaff). Starting them indoors ensures they are mature enough to produce fruit during the short summer growing window.
March Soil and Watering Preparation
Garden beds require preparation before planting to handle the demands of the approaching heat. Soil health is paramount in Arizona’s low-organic-matter environment, making the addition of amendments a priority. Incorporating a two to three-inch layer of well-aged compost or worm castings into the top six inches of soil replenishes depleted nutrients and improves the soil structure.
Mulching is necessary this month to retain soil moisture and regulate temperature fluctuations. Applying a two to four-inch layer of organic mulch (wood chips or straw) around plants significantly reduces water evaporation. This practice helps to keep the roots cooler, which is crucial for warm-season vegetables like tomatoes that struggle in overheated soil.
The irrigation system needs transitioning from a winter to a spring schedule. Drip lines and emitters should be inspected for clogs, leaks, or damage before the system is needed daily. Watering frequency should increase as temperatures rise, but the focus must remain on deep, infrequent irrigation to encourage the development of strong, deep root systems. Deep watering helps flush accumulated salts away from the root zone, a common issue in arid climates.