Petunia in Zone 4B — Mountain West

Petunias are worth growing here in the Mountain West for their vibrant color and long bloom season. Their cheerful flowers add pops of color to containers and garden beds, and they thrive in our intense sunlight. Plus, those cool nights we get even in summer seem to intensify their fragrance.

Our short growing season and occasional summer hail can be a challenge, but with a little planning, you can enjoy petunias from late summer until frost. With our roughly 128-day season, starting seeds indoors or buying starts gives you a jumpstart.

Starting petunias from seed indoors is possible, but honestly, buying starts from a local nursery is often the easiest route in our short season. If you want to try it, start your seeds indoors mid-March through early April, about 10 weeks before you plan to transplant outdoors.

Use seed trays with a good seed-starting mix and provide warmth (around 70-75°F) and plenty of light. Bottom watering helps prevent damping-off, which can be a problem with tiny seedlings. Remember our spring weather can be unpredictable, so be prepared to keep seedlings indoors longer if needed.

Transplant your petunia seedlings outdoors late May through late June, once the danger of frost has passed. Harden them off for about a week before transplanting by gradually exposing them to outdoor conditions. This helps them adjust to our intense sun and cooler nights.

Space them about 10-12 inches apart to allow for good air circulation. Buying starts is a very viable option, as petunias can be slow to get going from seed. Protect young transplants from unexpected late-season hail with row covers if necessary.

Petunias need moderate watering, but it's easy to over or underwater them in our climate. Container-grown petunias will need daily watering, especially during the hottest part of the summer. Garden-planted petunias are less demanding, but still need consistent moisture.

Check the soil moisture by sticking your finger about 2 inches deep. If it feels dry, it's time to water. Aim for about 1 inch of water per week, either from rainfall or irrigation. Water at the base of the plants to avoid wetting the foliage, which can lead to fungal problems in wetter summers.

Signs of underwatering include wilting leaves and dry soil. Overwatering can cause yellowing leaves and root rot. A layer of mulch around your petunias will help retain moisture and keep the soil cool.

Deadheading is key to keeping your petunias blooming all season long. Pinch off spent blooms regularly to encourage new growth.

In midsummer, if your petunias start to look leggy, pinch or cut back the stems by about half. This will encourage them to bush out and produce more flowers. As the first frost approaches in mid- September, stop deadheading to allow the plants to set seed (though they likely won't have time to mature).

Petunias are grown for their beautiful flowers, so there's no real "harvest" in the traditional sense. Expect your petunias to start blooming late July through mid September, about 60 days after transplanting.

Simply enjoy their continuous color all season long! Deadheading spent blooms encourages even more flowers. As the first frost approaches in mid- September, let your petunias go to seed, or simply pull them out and compost them.

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 buds and flowers, causing damage and distorted blooms.
• What causes it: Moths lay eggs on petunias, and the larvae hatch into budworms. They are more prevalent during warm, dry periods.
• How to fix/prevent it: Handpick the caterpillars off the plants. Bacillus thuringiensis (Bt) is an effective organic control.

Botrytis in Wet Weather

• What it looks like: Gray mold on flowers and foliage, especially during periods of high humidity or rain.
• What causes it: A fungal disease that thrives in cool, moist conditions. While our humidity is generally low, periods of wet weather can create favorable conditions.
• How to fix/prevent it: Remove infected plant parts promptly. Improve air circulation by spacing plants properly. Avoid overhead watering.

Leggy Stems

• What it looks like: Long, bare stems with few leaves and flowers.
• What causes it: Insufficient sunlight, overcrowding, or lack of pruning.
• How to fix/prevent it: Ensure your petunias are planted in a location with full sun (6-8 hours per day). Pinch back stems regularly to encourage bushier growth.

Mountain West Specific Challenges: Our intense sun can sometimes scorch petunia flowers, especially if they are not properly hardened off. The alkaline soil can also affect nutrient uptake, so consider amending your soil with compost or using a fertilizer formulated for acid-loving plants.

Good companions for petunias include:

• Tomatoes: Petunias can help repel some pests that bother tomatoes.
• Peppers: Similar to tomatoes, petunias can offer some pest protection.
• Green Beans: Petunias can attract beneficial insects that prey on bean pests.
• Squash: The sprawling habit of squash can provide some shade for the base of petunias, helping to retain moisture in our dry climate.

Avoid planting petunias near plants that are susceptible to the same pests or diseases, such as other members of the nightshade family (potatoes, eggplants).