Sunflower in Zone 5B — Mountain West

Sunflowers bring a unique joy to the Mountain West garden. Their towering height and bright blooms thrive in our intense sun, and the seeds are a treat for both you and the local birds. Plus, those cool nights even in summer help develop a richer flavor in the seeds, making your harvest extra rewarding.

Our short growing season and alkaline soil present challenges, but careful timing makes growing sunflowers very doable. With a growing season around 149 days, starting seeds at the right time is key to enjoying these beauties before that first frost in early October.

While sunflowers do best when direct sown, starting them indoors gives you a little head start, especially if our unpredictable spring weather lingers. If you choose this route, aim to start your seeds indoors from mid-April through early May, about three weeks before you plan to transplant them outside. This gives them a jump, but be sure they don't get too root-bound.

Use seed trays with a good seed-starting mix, and keep them in a warm spot with plenty of light – a south-facing window or under grow lights works well. Remember to water from the bottom to encourage strong root growth and avoid damping off. Keep in mind that with our late springs, direct sowing is usually just as effective and less work.

Transplant your sunflower seedlings outdoors from mid-May through early June, after the risk of frost has passed. A week before transplanting, harden them off by gradually exposing them to outdoor conditions. Start with an hour or two of sun and wind, increasing the time each day.

Plant them 18-24 inches apart to give them plenty of room to grow. Keep an eye on the forecast – a late-season hailstorm can devastate young plants, so be ready to cover them if needed.

Direct sowing is the easiest and most recommended method for growing sunflowers in our Mountain gardens. Wait until early May through late June, when the soil has warmed up a bit – ideally around 60°F. Sunflowers sprout quickly, and you’ll avoid the risk of transplant shock.

Prepare your soil by loosening it and mixing in some compost to improve drainage, especially with our alkaline soil. Plant the seeds about 1 inch deep, spacing them 18-24 inches apart. Water gently but thoroughly after planting.

Sunflowers are surprisingly drought-tolerant once they get established, thanks to their deep taproot. However, consistent watering is crucial, especially when they're young and during flowering. In our dry Mountain climate, regular deep watering encourages strong growth.

Water deeply but infrequently, aiming for about 1-2 inches of water per week. The "finger test" is your friend: stick your finger about 2 inches into the soil near the base of the plant. If it feels dry, it's time to water. Avoid overhead watering, which can encourage fungal diseases in our cool evenings, and focus on watering at the base of the plant.

Yellowing leaves can signal overwatering, while drooping leaves indicate underwatering. A layer of mulch around the base of the plants helps retain moisture and regulate soil temperature, which is especially useful with our intense sun.

You can expect your first sunflower harvest from late July through early October, about 80 days after planting. For seeds, the key indicator is the back of the flower head turning brown and the seeds themselves looking plump and well-formed.

To harvest the seeds, cut the entire flower head off the stalk, leaving about a foot of stem. Hang the head upside down in a dry, well-ventilated place to dry completely. As the first frost approaches in early October, harvest any remaining heads, even if they're not fully dry, and bring them indoors to finish ripening.

Here are three common problems you might encounter with sunflowers in the Mountain West:

Birds Eating Seeds

• What it looks like: Sunflower heads stripped of seeds, often with birds actively feeding on them.
• What causes it: Birds are attracted to the readily available seeds, especially as other food sources dwindle.
• How to fix/prevent it: Cover the flower heads with netting or burlap bags once the seeds start to mature. You can also try planting extra sunflowers specifically for the birds.

Sunflower Moth

• What it looks like: Webbing on the flower head, with small caterpillars feeding on the seeds.
• What causes it: Sunflower moths lay their eggs on the flower heads, and the larvae burrow into the seeds.
• How to fix/prevent it: Inspect your plants regularly for signs of infestation. Remove and destroy any affected 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 can be used for prevention.

Mountain West Specific Challenges: Our moderate summer heat and low humidity generally reduce the risk of fungal diseases compared to other regions, but our intense UV radiation can stress plants, making them more susceptible to pests. Consistent watering and proper soil preparation are key to healthy, resilient sunflowers.

Sunflowers are generally good neighbors in the garden. Cucumber and squash benefit from the partial shade provided by sunflowers, which helps prevent sunscald in our intense Mountain sun. Green beans and corn can use the sunflower stalks as a natural trellis, saving space and improving air circulation. Lettuce enjoys the shade during the hottest part of the day, preventing it from bolting too quickly. Avoid planting potatoes near sunflowers, as they can compete for resources and inhibit each other's growth.