August in Arizona presents a unique challenge to gardeners, where extreme heat and the arrival of the monsoon season demand careful planning. The state is either enduring the peak of summer or facing a quickly closing window before the first fall frost. Successful planting relies on acknowledging regional climate differences and strategically using the remaining warmth to establish fall and winter harvests. This transitional month is the time to start seeds indoors for the cool season or plant a final round of heat-loving crops.
Low Desert Planting (Phoenix and Tucson)
The low desert (below 3,500 feet) uses August as a bridge month to launch the highly productive fall and winter growing season. Gardeners must utilize the intense heat to germinate seeds for crops that will mature when temperatures cool. The first half of the month is the last opportunity for a final planting of warm-season “monsoon crops.”
Last-chance plantings include heat-loving varieties like black-eyed peas, yard-long beans, and summer squash. These thrive in the warm, humid conditions provided by the monsoon weather. Direct-sowing these seeds into well-watered soil is necessary to take advantage of high soil temperatures for fast germination and a final harvest before daylight hours shorten significantly.
The latter half of August marks the shift to preparing for the cool-season garden, which begins indoors. Seeds for slow-maturing brassicas should be started inside under controlled conditions. This timing allows the seedlings to develop a robust root system over six to eight weeks before being transplanted in late September or early October.
Quick-maturing cool-season greens, including Swiss chard, kale, and collard greens, can also be started indoors now. These greens are resilient and will produce leaves throughout the mild desert winter. Basil can still be planted outside from seed or transplant, while perennial herbs like rosemary and thyme should be added to the landscape to establish roots before the cooler months.
High Elevation Planting (Flagstaff and Prescott)
In Arizona’s high-elevation areas, such as Flagstaff and Prescott, the planting strategy in August focuses on speed due to the rapid approach of the first frost, which can occur as early as mid-September. Gardeners must prioritize ultra-quick-maturing crops to ensure a harvest before temperatures drop consistently below freezing, maximizing the use of remaining warm soil.
Direct sowing of fast-maturing root vegetables is a primary August task, with radishes and beets being prime candidates. Selecting varieties with a short “days to maturity,” ideally under 50 days, is essential to beat the frost deadline. Leafy greens like spinach, leaf lettuce, and kale can also be sown directly into the garden.
August is also the time to perform succession planting for crops intended for a late-fall harvest. Beyond vegetables, this month is suitable for planting woody landscape trees and shrubs, such as aspens or pines. This allows for root establishment before the winter dormancy period.
Heat-Tolerant Flowers and Ornamentals
August is an excellent time to introduce new ornamental plants, especially those known for their heat tolerance and low water requirements. Planting them now allows their root systems to establish with the benefit of warm soil.
Lantana is a common choice, offering vibrant color and exceptional drought resilience. The desert marigold and various perennial salvias thrive in the intense sun and heat, requiring minimal supplemental irrigation once established. Many of these plants are ideal for Xeriscape landscaping.
When planting these ornamentals, ensure they are mulched immediately with a three to four-inch layer of organic material. This thick layer helps insulate the root zone, keeping the soil temperature cooler. Immediate mulching and consistent initial watering will reduce transplant shock.
Essential August Gardening Techniques
The intense August heat requires specific techniques to ensure plant survival. The most important consideration is the strategic application of water, following the principle of watering deeply but infrequently for established plants. Deep watering encourages roots to grow downward, where the soil is cooler and moisture is retained longer.
For established trees and shrubs, this means soaking the soil to a depth of two to three feet every five to seven days. Newly planted seeds and transplants require frequent, lighter watering to keep the shallow top layer of soil consistently moist. Checking soil moisture with a probe before watering prevents root rot, which can occur from overwatering during the monsoon season.
Shade cloth is a tool for protecting vegetables and new plantings from the brutal mid-day sun. A 50% to 60% shade cloth is the optimal choice for most fruiting vegetables and leafy greens, blocking enough solar radiation to prevent leaf scorch. Erecting the cloth to block the harsh western sun (10 AM to 4 PM) significantly reduces plant stress.
Monsoon activity, characterized by high winds and torrential downpours, necessitates specific preparations for drainage and stability. Gardeners should ensure their beds and containers have excellent drainage to prevent waterlogging, which suffocates roots and encourages fungal diseases. Trimming mature trees to thin the canopy allows wind to pass through, reducing the risk of uprooting or breakage.