Sunflower in Zone 6B — Mountain West

Sunflowers bring a burst of sunshine to our Mountain West gardens, offering not just beauty but also delicious seeds and cut flowers. Their towering height and bright faces thrive in our intense sunlight, and even with our cool nights, they reward us with a cheerful display.

Our short growing season and occasional hail can be challenging, but with careful timing, you can absolutely grow gorgeous sunflowers here in Zone 6B. With our roughly 163-day growing season, we can easily fit in sunflowers with a little planning.

Starting sunflower seeds indoors gives you a head start, but it's generally not necessary unless you're aiming for an extra-early bloom or dealing with heavy bird pressure. If you choose to start indoors, begin in early to late April, about three weeks before you plan to transplant.

Use seed trays with a good seed-starting mix, and keep them warm and well-lit, either with a grow light or in a sunny south-facing window. Bottom watering helps prevent damping-off; just set the tray in a shallow dish of water and let the soil absorb it from the bottom. Remember, our springs here in the mountains can be slow to warm up.

Transplant your sunflower seedlings outdoors in early to late May, after the danger of hard frost has passed. Before transplanting, harden them off by gradually exposing them to outdoor conditions for about a week.

Choose a sunny spot and space the seedlings 18-24 inches apart. Keep an eye on the weather forecast; a late spring snow or hailstorm can still happen, so be prepared to cover your young plants if needed.

Direct sowing is the easiest and most reliable way to grow sunflowers in our climate. Sow seeds directly into the garden from late April through early July, ensuring the soil temperature is at least 60°F.

Prepare the soil by loosening it and adding compost to improve drainage and fertility. Plant seeds about 1 inch deep and space them 18-24 inches apart. Sunflowers are easy to grow and germinate quickly, so you'll see sprouts in no time!

Sunflowers are surprisingly drought-tolerant once established, thanks to their deep taproots, but they do need consistent watering, especially when young. During our dry Mountain West summers, a good rule of thumb is to water deeply but infrequently.

Check the soil moisture about two inches down – if it's dry to the touch, it's time to water. Aim for about 1-2 inches of water per week, either from rainfall or irrigation. Water at the base of the plant to avoid wetting the foliage, which helps prevent fungal diseases in our low-humidity environment.

Signs of underwatering include wilting leaves, while overwatering can lead to yellowing leaves and root rot. Mulching around the base of the plants helps retain moisture and suppress weeds, which is especially helpful in our dry climate.

You can expect your first sunflower harvest from mid-July through early October, depending on when you planted. For seeds, wait until the back of the flower head turns brown and the seeds are plump and starting to loosen.

To harvest seeds, cut the entire head off the stalk and hang it upside down in a dry, well-ventilated place to dry completely. For cut flowers, cut the stems when the petals first open. As our first frost approaches in early October, harvest any remaining heads, even if they aren't fully mature. You can still dry the seeds indoors.

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

Birds Eating Seeds

• What it looks like: Sunflower heads stripped bare of seeds, often with birds actively feeding on them.
• What causes it: Birds love sunflower seeds, and they're abundant in our region.
• How to fix/prevent it: Cover the heads with netting or burlap bags once the seeds start to mature. You can also try planting extra sunflowers to share with the birds.

Sunflower Moth

• What it looks like: Webbing and frass (insect droppings) on the flower heads, with small caterpillars feeding on the seeds.
• What causes it: Sunflower moths lay their eggs on the developing flower heads.
• How to fix/prevent it: Inspect your plants regularly and remove any caterpillars you find. You can also use insecticidal soap or Bacillus thuringiensis (Bt) to control infestations.

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.

Mountain West Specific Challenges: Our moderate-at-altitude heat, low humidity, and dry rainfall generally reduce disease pressure on sunflowers. However, intense UV radiation can sometimes stress plants. Providing adequate water and partial afternoon shade during the hottest part of the day can help mitigate this.

Good companions for sunflowers include cucumbers, squash, green beans, corn, and lettuce. Cucumbers and squash provide ground cover, helping to retain moisture and suppress weeds. Green beans fix nitrogen in the soil, benefiting the sunflowers. Corn provides a natural trellis for climbing sunflowers. Lettuce benefits from the partial shade cast by sunflowers.

Avoid planting potatoes near sunflowers, as they compete for nutrients in the soil.