Zinnia in Zone 6A — Mountain West

Zinnias are a fantastic choice for Mountain West gardens. Their vibrant colors add a splash of joy to our landscapes, and they make excellent cut flowers for bringing that beauty indoors. The cool nights even in summer here seem to intensify their colors, making them even more stunning.

Our short growing season and alkaline soil can be challenges, but Zinnias are relatively quick to mature, fitting nicely into our 153-day window. By starting at the right time, you can enjoy blooms from mid-summer until the first frost.

Starting Zinnias indoors can give you a head start, especially if you're aiming for earlier blooms. Begin your indoor sowing from early April through early May, about four weeks before you plan to transplant them outside. Use seed trays with a good seed-starting mix.

Keep the trays in a warm spot and provide plenty of light, either with a grow light or in a sunny window. Bottom watering is a great way to keep the soil moist without wetting the leaves, which can help prevent disease. Remember, our spring weather can be unpredictable, so starting indoors offers a safe haven from late frosts.

Transplant your Zinnia seedlings outdoors from mid May through mid June, once the danger of frost has passed. Before transplanting, harden them off by gradually exposing them to outdoor conditions for about a week. This helps them adjust to the intense sun and cooler nights we experience at high altitude.

Space the plants 6-12 inches apart to allow for good air circulation. Be mindful of potential late-season hailstorms, which can damage young plants; consider using row covers or other protective measures if necessary.

Direct sowing Zinnias is a perfectly viable option in our region, and saves the work of starting seeds indoors. Sow seeds directly into the garden from early May through mid July. Make sure the soil has warmed up a bit, as Zinnias prefer warmer soil temperatures for germination.

Prepare the soil by loosening it and amending with compost. Plant seeds 1/4 inch deep and space them 6-12 inches apart. Keep the soil consistently moist until the seedlings emerge. Direct sowing is a good choice if you have alkaline soil as it avoids transplant shock.

Zinnias need moderate watering, but it's crucial to get it right in our dry Mountain climate. Water deeply but infrequently, allowing the soil to dry out slightly between waterings. A good rule of thumb is to stick your finger about two inches into the soil; if it feels dry, it's time to water.

During the hottest part of the summer, they might need about 1 inch of water per week, especially if rainfall is scarce. Always water at the base of the plant to avoid wetting the leaves, which can lead to powdery mildew in our cool nights. Signs of underwatering include wilting leaves and dry, cracked soil. Overwatering can lead to yellowing leaves and root rot.

Mulching around your Zinnias with compost or shredded bark can help retain moisture and keep the soil cooler.

Pruning Zinnias is simple and rewarding. The key is to deadhead spent flowers regularly to encourage more blooms. Simply cut off the faded flower heads just below the flower, at the next set of leaves.

Don't be afraid to cut long stems for bouquets – Zinnias thrive on cutting! This encourages the plant to produce even more flowers. As the first frost approaches in early October, you can cut all remaining blooms to enjoy indoors, as they won't survive a freeze.

You can expect your first Zinnia blooms starting in early July and continuing through early October. Harvest flowers when they are fully open and the petals are firm. The more you cut, the more the plant will bloom, so don't be shy!

To harvest, cut the stem at an angle just above a leaf node. Remove any leaves that will be below the waterline in your vase. Zinnias are excellent cut flowers and have a long vase life.

As the first frost approaches, cut all remaining blooms to bring indoors. They won't continue to ripen off the vine, so enjoy them while they are fresh.

Here are some common problems you might encounter with Zinnias in our region:

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. Surprisingly, dry leaf surfaces actually favor it, which is common with our low humidity.
• How to fix/prevent it: Improve air circulation by spacing plants adequately and pruning. Remove affected leaves promptly. Neem oil or potassium bicarbonate sprays can help. A milk spray (1 part milk to 9 parts water) is also surprisingly effective. Plant resistant varieties if possible.

Bacterial Leaf Spot

• What it looks like: Small, water-soaked spots on the leaves that turn brown or black. Spots may be surrounded by a yellow halo. In severe cases, the spots can merge, causing large sections of the leaf to die.
• What causes it: Bacteria that thrive in warm, humid conditions. Splashing water from rain or irrigation can spread the bacteria.
• How to fix/prevent it: Avoid overhead watering to reduce leaf wetness. Improve air circulation. Remove and destroy infected leaves. Copper-based fungicides can help prevent the spread, but are most effective when applied early.

Alternaria Blight

• What it looks like: Dark brown or black spots on leaves, often starting on the lower leaves. Spots may have concentric rings, resembling a target. Leaves may turn yellow and drop off.
• What causes it: A fungus 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 copper can help prevent the spread.

Mountain West Specific Challenges: Our intense sun can sometimes stress plants, making them more susceptible to disease. While our low humidity generally helps prevent fungal diseases, the cool nights can still create favorable conditions for powdery mildew. Also, alkaline soil can affect nutrient availability, so amend with compost regularly.

Zinnias make great companions for several garden vegetables. Planting them near tomatoes and peppers can attract pollinators, boosting fruit production. The bright colors of Zinnias also attract beneficial insects that prey on common garden pests that bother these crops.

Squash and cucumbers also benefit from having Zinnias nearby for the same reason. Avoid planting Zinnias near plants that are highly susceptible to powdery mildew, as this can increase the risk of spreading the disease.