Zinnia in Zone 5A — Mountain West

Zinnias are a fantastic choice for high altitude gardens. Their vibrant colors bring a burst of life to our short growing season, and their ability to attract pollinators is a huge bonus for your other crops. Plus, the cool nights here in the Mountain West seem to intensify their colors, making them even more stunning in bouquets.

We face challenges like intense UV and a short season, but with careful timing, Zinnias thrive. Our 138-day growing season means we need to be strategic about starting and protecting our plants, but the payoff is well worth it.

Starting Zinnias indoors gives you a head start, especially important with our short season. Begin in mid-April through early May, about four weeks before you plan to transplant. Use seed trays with a good seed-starting mix, providing warmth and plenty of light – a grow light is really helpful.

Bottom watering is key to prevent damping off. Place the tray in a shallow dish of water until the soil surface is moist. Given our late springs in the Mountain West, indoor starting provides crucial extra weeks of growth.

Transplant Zinnias outdoors in late May through late June, after the risk of frost has passed. Harden them off for about a week before transplanting. Gradually expose them to outdoor conditions for increasing periods each day.

Space them 6-12 inches apart, giving them room to grow and good air circulation, which helps prevent disease. Be mindful of potential late-season cold snaps or hail, and protect your young plants if necessary.

Direct sowing Zinnias is definitely an option if you prefer. Sow seeds mid-May through mid-July, once the soil has warmed up. Zinnias need a soil temperature of at least 60°F to germinate well.

Prepare the soil by loosening it and mixing in some compost. Sow seeds 1/4 inch deep and keep the soil consistently moist until they germinate. Space them 6-12 inches apart after seedlings emerge. The downside to direct sowing is that you might get a later start and shorter bloom time compared to starting indoors.

Zinnias need moderate watering, especially during our dry Mountain West summers. Established plants are somewhat drought-tolerant, but consistent moisture encourages the best blooms. During the peak of summer, water deeply about once a week, providing around 1 inch of water.

Use the "finger test" – stick your finger about 2 inches into the soil. If it feels dry, it's time to water. Always water at the base of the plants to avoid wetting the foliage. Our low humidity means powdery mildew is less of a problem than in wetter climates, but it's still smart to avoid overhead watering.

Signs of underwatering include wilting leaves and stunted growth. Overwatering can lead to yellowing leaves and root rot. A light layer of mulch, like shredded bark or straw, can help retain moisture in our dry climate and reduce the need for frequent watering.

Pruning Zinnias is simple but makes a big difference. Deadhead spent flowers regularly by cutting the stem just below the flower head. This encourages the plant to produce more blooms throughout the season.

Don't be afraid to cut long stems for bouquets – Zinnias respond well to cutting, and each cut encourages more flower production. As the first frost approaches in late September, you can cut all remaining blooms to enjoy indoors.

You can expect your first Zinnia harvest from mid-July through late September, depending on when you started your seeds. Cut the flowers when they are fully open and the stem is firm. The more you cut, the more they will bloom.

Harvest by cutting the stem at an angle, leaving as much stem as possible on the plant. Zinnias make excellent cut flowers with a long vase life. As the first frost approaches in late September, harvest all remaining blooms to bring the vibrant colors indoors.

Here are a few common problems you might encounter with Zinnias in our Mountain West gardens:

Powdery Mildew *What it looks like:* White or grayish powdery coating on leaves and stems. Leaves might curl, yellow, and die. *What causes it:* Fungi thrive in warm days and cool nights, which we often have, and surprisingly, it's favored by dry leaf surfaces. *How to fix/prevent it:* Improve air circulation by spacing plants properly. Remove affected leaves promptly. Neem oil or potassium bicarbonate sprays can help. Even a milk spray (1 part milk to 9 parts water) works.

Bacterial Leaf Spot *What it looks like:* Small, water-soaked spots on leaves that enlarge and turn brown or black. Spots may have a yellow halo. *What causes it:* Bacteria spread by splashing water, often during overhead watering or heavy rain. Our dry climate usually helps keep this in check, but occasional summer storms can trigger it. *How to fix/prevent it:* Avoid overhead watering. Improve air circulation. Remove and destroy infected leaves. Copper-based fungicides can offer some protection.

Alternaria Blight *What it looks like:* Dark brown or black spots on leaves, often starting on lower leaves. Spots may have concentric rings, giving a target-like appearance. *What causes it:* A fungal disease that thrives in warm, humid conditions. While our humidity is low, extended periods of wet weather can create favorable conditions. *How to fix/prevent it:* Remove and destroy infected leaves. Improve air circulation. Fungicides containing chlorothalonil or mancozeb can be used as a preventative measure.

Mountain West Specific Challenges: Our intense UV light can sometimes stress plants, making them more susceptible to disease. The alkaline soil can also affect nutrient uptake, so amending with compost is always a good idea.

Zinnias make great companions for several vegetables in our Mountain West gardens.

• Tomatoes and Peppers: Zinnias attract pollinators, which are essential for fruit set in tomatoes and peppers.
• Squash and Cucumbers: Again, pollinators are key! Plus, the bright colors of Zinnias can help deter some pests.

I don't recommend planting Zinnias near plants prone to powdery mildew, as it could encourage the spread of the disease.