September in Arizona is the most important period for local gardeners, signaling the start of the primary cool-season growing cycle. This month represents a fundamental shift away from the intense heat survival strategies of July and August toward active planting. As the sun’s angle lowers and extreme daytime temperatures begin to moderate, the soil cools sufficiently to support the germination and establishment of new crops. This transition allows Arizona gardeners to prepare for the abundant fall and winter harvests that follow the state’s mild climate.
Navigating Arizona’s Growing Zones in September
Arizona contains vast ecological diversity, creating distinctly different planting calendars across its major regions. The state is broadly divided into two primary zones: the Low Desert and the High Desert/Mountain areas. Understanding the differences between these zones is essential because a delay of just a few weeks can mean the difference between a successful harvest and a crop failure.
The Low Desert, encompassing major metropolitan areas like Phoenix and Tucson, operates primarily in USDA Hardiness Zones 9b to 10b, where elevations remain below 3,500 feet. September is the beginning of the planting window, though residual summer heat often delays the start until mid-month or when daytime highs consistently fall below 110°F. The goal in this zone is to utilize the relatively frost-free mild winter as the main growing season, extending from fall through late spring.
Conversely, the High Desert and Mountain Zones, including areas such as Flagstaff and Prescott, are characterized by higher elevations and colder winter temperatures, typically falling into USDA Zones 5b to 7a. For these gardeners, September is a race against time, as the first frost often arrives in late September or early October. Planting in these cooler regions focuses on short-season crops or establishing hardy perennials before the ground freezes. This window for new plantings is much narrower and more urgent than in the Low Desert.
Key Cool-Season Vegetables for Fall Harvests
September planting in the Low Desert is dominated by cool-season crops that thrive in mild winter temperatures. These vegetables are generally started directly from seed or planted as transplants to bypass the lingering heat. Root crops and most leafy greens are successful when sown directly into the garden bed, allowing them to establish a strong taproot without the stress of transplant shock.
Leafy greens, such as Swiss chard, kale, spinach, and various types of lettuce, can be sown from seed throughout the month. Later plantings provide a continuous harvest into the winter. These crops require minimal soil depth but benefit from consistent moisture during germination to prevent the seeds from drying out in the remaining warmth. Root vegetables like carrots, radishes, beets, and turnips should also be direct-seeded in fine, well-tilled soil for uniform root development. Thinning these seedlings is important to ensure the remaining plants have adequate space to form full, harvestable roots.
For vegetables requiring a longer time to mature, specifically the Brassica family, starting with transplants is a more reliable approach in the Low Desert. Broccoli, cauliflower, and Brussels sprouts have extended maturation times, making them vulnerable to residual heat if started too early from seed. Planting sturdy transplants of these crops in mid-to-late September ensures they are established and growing vigorously as temperatures drop. Hardy vegetables like garlic can also be planted from individual cloves starting in mid-September. Garlic requires full sun and well-draining soil rich in phosphorus for proper bulb development over the winter.
Hardy Herbs and Ornamental Planting
September is an opportune time to establish herbs and prepare for winter ornamental color. Annual herbs that quickly bolt in summer heat will thrive when planted now, benefiting from decreasing temperatures. Cilantro, dill, and parsley are best planted from seed at this time to ensure a fresh supply throughout the cooler months before they flower as the days lengthen in spring.
Perennial herbs like oregano, mint, and thyme are best introduced as small transplants in September, giving their root systems time to expand before winter. These hardy Mediterranean herbs are well-suited to the Low Desert’s climate, preferring well-drained soil and tolerating intense sun once established. Sonoran Desert native plants are excellent choices for adding visual appeal, as they are adapted to thrive with minimal intervention.
Annual flowers that provide color through the fall and winter should be planted as seedlings toward the end of September, particularly after daily high temperatures drop below 100°F. Pansies and petunias are popular choices for cool-season color. They should be watered frequently after planting until their root systems are fully established. Planting native wildflower mixes, such as those including desert marigold, is also recommended in September to encourage spring blooms.
Soil Preparation and Early Watering Strategies
Successful September planting relies heavily on mitigating lingering heat and replenishing the soil after the stress of summer. The intense summer sun depletes organic matter and compacts the soil, making it necessary to amend the planting area before introducing new crops. Garden beds should be tilled to a depth of 8 to 12 inches to break up compaction and allow for better root penetration and aeration.
Incorporating organic matter, such as high-quality compost and worm castings, is necessary to restore soil structure and nutrient content. A balanced organic fertilizer, particularly one with a good ratio of nitrogen and phosphorus, can be worked into the top layer of soil to provide a gentle and steady nutrient supply for young plants. Phosphorus is beneficial for encouraging robust root development in new seedlings and transplants.
Watering protocols in September must balance the need to cool the soil with the plant’s requirement for moisture. While temperatures are still high, deep watering is preferred over frequent, shallow applications. This encourages roots to grow downward and helps cool the lower soil profile. Watering deeply, to a depth of approximately 12 inches, every two to five days is a general guideline for new garden beds. Frequency should decrease as the month progresses and temperatures decline. Applying a thick layer of organic mulch (three to four inches deep) helps insulate the soil from the sun’s heat, reducing water evaporation and protecting the roots of new seedlings.