Zinnia in Zone 10B — Desert Southwest

Zinnias are a must-grow for any Desert Southwest gardener wanting a burst of color. Their vibrant blooms are perfect for cut flower arrangements, attracting pollinators to your garden, and adding a cheerful touch to our sometimes harsh landscape. Plus, they thrive in both our spring and fall growing seasons.

While the extreme heat and intense sun of our summers can be a challenge, with careful timing and attention to watering, you can enjoy a long season of zinnia blooms. Our 349-day growing season gives you plenty of opportunity to get them established and blooming before the worst of the summer heat hits.

Starting zinnia seeds indoors is possible, but not always necessary in our climate. Given our very-early spring character and long growing season, direct sowing usually works just fine. However, if you want a head start or live in a microclimate with late frosts, starting indoors can be beneficial.

Sow seeds late December through mid January, about four weeks before you plan to transplant. Use seed trays filled with a good seed-starting mix, and keep them in a warm location with plenty of light – a sunny windowsill or under grow lights works well. Bottom watering is key to prevent damping off.

Transplant your zinnia seedlings outdoors late January through late February. This allows them to get established before the intense summer heat arrives.

Before transplanting, harden off your seedlings by gradually exposing them to outdoor conditions for about a week. Choose a sunny spot with well-draining soil, and space your plants 6-12 inches apart. Keep an eye on the weather – unexpected cold snaps or late frosts can still occur, so be prepared to cover your seedlings if necessary.

Direct sowing zinnias is a simple and effective method in our Desert Southwest climate. You can sow seeds late January through mid October, taking advantage of both our spring and fall growing seasons.

Prepare your soil by loosening it and amending with compost. Zinnia seeds need warm soil to germinate, so wait until the soil temperature is consistently above 60°F. Sow seeds about ¼ inch deep and space them 6-12 inches apart. Keep the soil consistently moist until germination.

Zinnias need moderate watering, but proper watering is crucial for summer survival in our extreme heat. Water deeply but infrequently, allowing the soil to dry out slightly between waterings.

During the cooler spring and fall months, water about once a week, providing about 1 inch of water. In the heat of summer, increase watering to 2-3 times per week, providing up to 2 inches of water each time. Use the "finger test" – stick your finger into the soil about 2 inches deep. If it feels dry, it's time to water.

Always water at the base of the plants, avoiding overhead watering. Our arid climate helps to minimize fungal diseases, but wetting the leaves can still create problems. Signs of underwatering include wilting and yellowing leaves, while overwatering can lead to root rot. A light layer of mulch around the base of the plants can help retain moisture and regulate soil temperature.

Deadheading is essential for keeping your zinnias blooming throughout the season. Simply pinch or cut off spent flowers just below the flower head.

This encourages the plant to produce more blooms instead of putting energy into seed production. Don't be afraid to cut long stems for bouquets – this actually encourages more flowers to grow. As the first frost approaches in late December, you can let the last blooms go to seed if you want to collect seeds for next year.

You can expect your first zinnia harvest late March through late December, about 60 days after planting. Cut the flowers when they are fully open and the petals are firm.

The more you cut, the more they bloom, so don't be shy! Harvest the stems in the morning, and place them in water immediately. Zinnias are excellent cut flowers with a long vase life.

As the first frost approaches in late December, harvest any remaining blooms. You can also allow some flowers to dry on the plant to collect seeds for next year's garden.

Zinnias are generally easy to grow, but they can be susceptible to a few common problems in our Desert Southwest gardens. Powdery mildew, bacterial leaf spot, and alternaria blight are the most common issues.

Powdery Mildew

• What it looks like: White or grayish powdery coating on leaves, stems, and sometimes flower buds. Leaves may curl, yellow, and eventually die.
• What causes it: Various fungi that thrive in warm days, cool nights, and moderate humidity. Ironically, dry leaf surfaces actually favor it (unlike most fungi).
• How to fix/prevent it: Improve air circulation around your plants by spacing them properly and pruning out crowded growth. Remove any affected leaves promptly. Neem oil or potassium bicarbonate sprays can help control the spread. A milk spray (1 part milk to 9 parts water) is surprisingly effective. Plant resistant varieties when possible.

Bacterial Leaf Spot

• What it looks like: Small, water-soaked spots on leaves that turn brown or black. Spots may have a yellow halo. In severe cases, leaves may drop off.
• What causes it: Bacteria that thrive in warm, humid conditions. Splashing water from irrigation or monsoon rains can spread the bacteria.
• How to fix/prevent it: Avoid overhead watering to minimize leaf wetness. Improve air circulation. Remove and destroy infected leaves. Copper-based fungicides can help prevent further spread, but won't cure existing infections.

Alternaria Blight

• What it looks like: Dark brown or black spots on leaves, often starting on the lower leaves. Spots may have concentric rings, giving them a target-like appearance.
• What causes it: A fungal disease that thrives in warm, humid conditions. Spores are spread by wind and splashing water.
• How to fix/prevent it: Remove and destroy infected leaves. Improve air circulation. Avoid overhead watering. Fungicides containing chlorothalonil or mancozeb can help prevent further spread.

Desert Southwest Specific Challenges: Our extreme heat and intense sun can stress zinnias, making them more susceptible to these diseases. While our arid humidity generally helps to minimize fungal problems, the monsoon rainfall can create periods of high humidity that favor disease development. Be vigilant about monitoring your plants and taking preventative measures to keep them healthy.

Zinnias make excellent companions for many vegetables in your Desert Southwest garden.

Good companions include tomatoes, peppers, squash, and cucumbers. Zinnias attract pollinators, which are essential for fruit production in these vegetables. Plus, their bright colors can help to deter some pests.

Avoid planting zinnias near plants that are susceptible to powdery mildew, as they can spread the disease. Also, while not strictly "bad", be mindful of spacing – zinnias need plenty of sun, so don't plant them where they will shade out other sun-loving plants.