Petunia in Zone 5A — Mountain West

Petunias are worth growing here in the Mountain West for their vibrant color and long bloom season. They thrive in our intense sunlight and provide cheerful pops of color in containers, hanging baskets, or garden beds. Plus, those cool nights we have even in summer seem to intensify their fragrance.

High altitude gardening presents unique challenges, like our short 138-day growing season and intense UV. But with proper timing and a little attention, you can absolutely enjoy beautiful petunias all summer long.

Starting petunias from seed indoors is doable, but be aware they're slow growers. If you're keen to try, begin in early to late March, about 10 weeks before you plan to transplant. Use seed trays with a good seed-starting mix, and provide warmth (a heat mat helps) and plenty of light.

Since our spring weather is often unpredictable, starting early indoors gives you a head start. Bottom watering helps prevent damping-off, a common issue with seedlings. Just set the tray in a shallow dish of water and let the soil absorb it from the bottom.

Transplant your petunias outdoors in late May through late June, after the danger of frost has passed. Before transplanting, harden them off by gradually exposing them to outdoor conditions for about a week. Start with an hour or two of shade, increasing the time and sunlight each day.

Space them 10-12 inches apart in a sunny location. Keep an eye on the weather forecast – even in late May, we can get unexpected cold snaps or even hail. Honestly, for the easiest results, many of us in the Mountain West just buy petunia starts from a local nursery.

Petunias need moderate watering, but it's crucial to get it right in our dry Mountain climate. Container petunias are particularly thirsty and will likely need daily watering, especially during the hottest part of the summer. Garden-planted petunias are less demanding, but still need regular attention.

A good rule of thumb is to check the soil about two inches deep – if it's dry, it's time to water. Aim for about an inch of water per week, either from rainfall or irrigation. Water at the base of the plants to avoid wetting the foliage, which can encourage disease in any humidity we might get.

Signs of underwatering include wilting leaves and dry soil. Overwatering can lead to yellowing leaves and root rot. A layer of mulch around the base of the plants will help retain moisture and keep the soil cool, which is especially beneficial in our intense sun.

Deadheading is key to keeping your petunias blooming all season long. Simply pinch or cut off the spent blooms just below the flower head. This encourages the plant to produce more flowers instead of putting energy into seed production.

In midsummer, if your petunias start to look leggy – long stems with few flowers – you can pinch or cut back the stems by about half. This will encourage bushier growth and more blooms. As the first frost approaches in late September, you can let the plants fade naturally or take cuttings to overwinter indoors.

Petunias are grown for their beautiful blooms, so there's no "harvest" in the traditional sense. The goal is continuous color throughout the growing season. With regular deadheading, your petunias should provide a vibrant display from late July through early September.

As the first frost approaches in late September, you can either let the plants go or bring them indoors if you want to try overwintering them. However, most gardeners in our region simply enjoy them for the season and then replace them with cool-season flowers.

Here are some common problems you might encounter with petunias in the Mountain West:

Tobacco Budworm

• What it looks like: Small, green caterpillars that bore into the flower buds, causing them to be damaged or fail to open. You might also see dark droppings on the foliage.
• What causes it: Moths lay their eggs on the petunia plants, and the larvae hatch into budworms. They thrive in warm weather.
• How to fix/prevent it: Inspect your plants regularly for signs of budworms. Handpick them off if you find them. You can also use Bacillus thuringiensis (Bt), a natural insecticide, to control them.

Botrytis in Wet Weather

• What it looks like: Gray mold on the flowers and foliage, especially during periods of wet weather.
• What causes it: A fungal disease that thrives in humid conditions. While our humidity is typically low, we can get periods of rain that create a favorable environment for botrytis.
• How to fix/prevent it: Remove any infected flowers or foliage immediately. Improve air circulation around the plants by spacing them properly. Avoid overhead watering.

Leggy Stems

• What it looks like: Long, bare stems with few leaves and flowers.
• What causes it: Insufficient sunlight or excessive heat. While petunias love sun, our intense UV can sometimes stress them.
• How to fix/prevent it: Prune back the leggy stems by about half to encourage bushier growth. Make sure your petunias are getting at least 6 hours of sunlight per day, but consider some afternoon shade during the hottest part of the summer.

Mountain West Specific Challenges: Our moderate-at-altitude heat, low humidity, and dry rainfall generally benefit petunias, reducing disease pressure. However, the intense sun can sometimes be too much, so consider providing some afternoon shade. Also, be prepared for occasional hail, which can damage the flowers and foliage.

Good companions for petunias in the Mountain West include tomatoes, peppers, green beans, and squash. Tomatoes and peppers benefit from the petunias' ability to attract beneficial insects that prey on pests. Green beans and squash help to suppress weeds and keep the soil cool, which is especially helpful in our intense sun.

Avoid planting petunias near plants that are susceptible to the same diseases, such as certain types of squash that are prone to powdery mildew. This can increase the risk of disease spreading to your petunias.