Sunflower in Zone 8B — Desert Southwest

Sunflowers are a fantastic addition to any Desert Southwest garden. Their towering height and bright blooms bring cheer, and you can harvest the seeds for snacking or roasting. Plus, they attract pollinators, which is always a win for your other plants.

Sure, our extreme heat and alkaline soil present challenges, but with careful timing, sunflowers thrive here. Our long growing season – nearly 280 days! – gives you two opportunities to plant these beauties.

Starting sunflower seeds indoors is an option, especially if you want a head start. Sow seeds indoors in early to late February, about three weeks before our transplant window opens. Use seed trays with a good seed-starting mix, and keep them warm and well-lit – a sunny windowsill or grow lights work well.

Remember to water from the bottom to prevent damping off, and keep the soil consistently moist. Since our spring warmth arrives early, indoor starting isn’t essential, but it can give you larger plants sooner.

Transplant your sunflower seedlings outdoors in early March through early April, once the danger of frost has passed. Before transplanting, harden them off by gradually exposing them to outdoor conditions for about a week. This prevents transplant shock.

Space them 18-24 inches apart to give them room to grow. Keep an eye on the weather; even in early March, we can still get unexpected temperature swings, so protect your seedlings if needed.

Direct sowing is the easiest and most common method for growing sunflowers in the Desert Southwest. Sow seeds directly into the ground from late February through late August. Sunflowers are very easy to direct sow.

Prepare the soil by loosening it and adding compost to improve drainage and fertility. Plant seeds 1-2 inches deep and space them 18-24 inches apart. Keep the soil consistently moist until the seedlings emerge. Soil temps need to be at least 60°F for good germination.

Sunflowers are moderately drought-tolerant once established, thanks to their deep taproots, but they still need consistent watering, especially in our arid climate. During the spring and fall growing seasons, water deeply about once a week, providing about 1-2 inches of water per week. In the extreme heat of summer, increase watering to 2-3 times per week.

Use the "finger test" to check soil moisture: stick your finger about 2 inches into the soil. If it feels dry, it's time to water. Water at the base of the plant to avoid wetting the foliage, which can encourage fungal diseases in our monsoon season.

Signs of underwatering include wilting leaves and stunted growth. Overwatering can lead to root rot, indicated by yellowing leaves and a mushy stem base. A good layer of mulch around the base of the plants will help retain moisture and regulate soil temperature.

Your first sunflower harvest should arrive from mid-May through early December, depending on when you planted. For seeds, wait until the back of the flower head turns brown and the seeds are plump and slightly loose.

To harvest seeds, cut the entire flower head and hang it upside down in a dry, well-ventilated place until the seeds are completely dry. For cut flowers, harvest when the petals first open. Cut the stem at an angle and place the flowers in water immediately.

As our first frost approaches in early December, harvest any remaining flower heads with mature seeds. You can also leave some flower heads standing to provide food for birds during the winter months.

Here are three common problems you might encounter growing sunflowers in the Desert Southwest:

Birds Eating Seeds

• What it looks like: Missing seeds from the flower head, often with signs of pecking or damage.
• What causes it: Birds are attracted to the ripe sunflower seeds.
• How to fix/prevent it: Cover the flower heads with netting or burlap bags once the seeds start to mature. You can also try using bird feeders away from your sunflowers to distract them.

Sunflower Moth

• What it looks like: Webbing on the flower head, seed damage, and small caterpillars inside the flower.
• What causes it: Sunflower moths lay their eggs on the flower heads, and the larvae feed on the developing seeds.
• How to fix/prevent it: Inspect flower heads regularly for signs of infestation. Remove and destroy any infested flower heads. Bacillus thuringiensis (Bt) can be used as a preventative measure.

Downy Mildew

• What it looks like: Yellow patches on leaf tops with fuzzy gray-purple growth on undersides. Leaves brown and die from the bottom up.
• What causes it: Oomycete pathogen that spreads in cool, humid conditions via wind-carried spores.
• How to fix/prevent it: Remove affected leaves immediately. Improve air circulation. Water at the base, never overhead. Copper-based fungicides for prevention. Plant resistant varieties where available.

Desert Southwest Specific Challenges: Our extreme heat can cause sunscald on the flower heads, so provide afternoon shade during the hottest months. Arid conditions can stress plants, making them more susceptible to pests and diseases, so ensure adequate watering and soil drainage. Our monsoon rainfall can create humid conditions that favor fungal diseases like downy mildew, so monitor plants closely during this time.

Sunflowers make great companions for several plants in the Desert Southwest garden. Cucumbers and squash benefit from the sunflower's shade, protecting them from our intense sun. Green beans thrive when climbing up the sunflower stalks. Corn benefits from the sunflowers attracting pollinators. Lettuce appreciates the partial shade provided by sunflowers, which prevents bolting in warmer weather.

Avoid planting potatoes near sunflowers, as they can compete for nutrients in our already nutrient-poor soil.