September marks the transition for Southern California gardeners from summer’s extreme heat to the prime fall and winter growing season. Although autumn begins on the calendar, summer temperatures often persist, requiring strategic planting decisions. This month focuses on establishing plants that tolerate the lingering warmth now but will thrive once the weather moderates. Success involves timing, selecting correct varieties, and mitigating heat stress on new seedlings and transplants.
Understanding Southern California’s September Climate
Southern California experiences a unique “false fall,” where high daytime temperatures and soil warmth often extend into October. This pattern distinguishes the region’s gardening rhythm. While warm soil aids root establishment, it poses a risk of heat-induced bolting for many cool-season crops.
Microclimates significantly determine precise planting timelines. Coastal areas benefit from the marine layer, which offers slightly cooler temperatures and humidity, allowing for earlier planting. Inland valleys and areas prone to Santa Ana winds must delay planting or provide extra shade against intense heat. Gardeners must focus on plants with high heat tolerance during their initial growth stages.
Cool-Weather Vegetables to Start Now
September is the time to start cool-weather vegetables. Root vegetables and leafy greens are best direct-seeded this month. Sow carrots, radishes, and beets directly, keeping the soil consistently moist to encourage germination and prevent tough roots. Leafy greens like spinach, Swiss chard, and leaf lettuce can also be seeded now, but require shade protection from the midday sun to prevent premature bolting.
Brassicas, including broccoli, cabbage, cauliflower, and kale, are typically better started from transplants in September. Starting with transplants bypasses the vulnerable seedling stage in the heat and allows these long-growing plants to develop strong root systems. Direct-seed peas and fava beans now, as they prefer warm soil for germination before cooler weather slows their growth.
Herbs and Ornamental Flowers
September allows for the introduction of cool-season herbs and flowers, setting the stage for winter color and flavor. Focus on culinary herbs that prefer cooler conditions, such as parsley, cilantro, dill, and chives. Cilantro and parsley should be succession-sown throughout the month to ensure a continuous harvest before their growth slows in winter. Replace heat-loving summer herbs like basil with these cold-tolerant varieties.
Ornamental planting should focus on annuals that provide color during the winter and early spring. Plant seeds for flowers like sweet peas, which need the warm soil now for strong root development. Transplants of the following can be set out to establish themselves:
- Snapdragons
- Calendula
- Pansies
- Stock
Choosing cool-season annuals now ensures a vibrant display that will tolerate the mild winter temperatures.
Essential Planting Tips for Warm Starts
Successful planting requires specialized techniques to mitigate the effects of lingering summer warmth. Deep and consistent watering is paramount to combat high evaporation rates and heat stress. Water deeply enough to penetrate the warm topsoil and reach the deeper, cooler root zone, avoiding frequent shallow sprinkles. Seedbeds, especially for small seeds, must be kept uniformly moist until germination.
Soil and Moisture Management
Soil preparation is crucial for moisture retention. Incorporate generous amounts of organic compost into planting beds to enhance the soil’s capacity to hold water. Applying a thick layer of organic mulch around new transplants helps keep the root zone cool and reduces water evaporation. Vulnerable seedlings, especially leafy greens, require temporary afternoon shade to prevent scorching and bolting.
Pest and Wind Protection
Vigilance against common late-summer pests is necessary, as many thrive in persistent warmth. Pests like spider mites and whiteflies multiply rapidly in hot, dry conditions and damage new growth. Regularly inspect leaves and use a strong jet of water to dislodge insects, managing early infestations effectively. Protecting plants from hot, dry Santa Ana winds with windbreaks or sheltered locations prevents rapid moisture loss and stress.