Petunia in Zone 7B — Desert Southwest

Petunias are a classic for a reason, and they absolutely earn their place in our Desert Southwest gardens. Their vibrant colors add cheer to containers and beds, plus they bloom almost non-stop if you keep them happy. Who doesn't love that kind of return on investment?

Sure, our extreme heat and dry air can be tough on flowering plants, but with a little planning, you can enjoy petunias throughout our long, 245-day growing season. By starting smart and paying attention to watering, you'll have blooms from spring until the first frost.

Starting petunias from seed indoors is definitely an option, but honestly, buying transplants from a local nursery is often the easiest route, especially if you're new to desert gardening. Petunias are notoriously slow to get going from seed. But if you're up for the challenge, go for it!

If you want to start indoors, sow seeds mid January through early February, about 10 weeks before you plan to transplant. Use seed trays with a good seed-starting mix, provide warmth (a heat mat helps), and make sure they get plenty of light – grow lights are almost essential. Bottom watering is a good way to keep the soil moist without disturbing the tiny seeds. Our very-early spring means starting early is key.

Transplant your petunias outdoors late March through late April, after the danger of frost has passed. Before planting them in the ground or in containers, be sure to harden them off. This means gradually exposing them to outdoor conditions for about a week, increasing the amount of time they spend outside each day.

Space your petunias 10-12 inches apart to allow for good air circulation. Buying starts is the easiest way to get petunias in your garden. Keep an eye on the weather forecast – those late-season temperature swings can still happen, so be ready to cover your plants if needed.

Watering is crucial for petunias in our arid climate. While they need moderate watering, that can be a tricky balance in the Desert Southwest. Container petunias will need daily watering during the hottest parts of the summer, while those in the ground are less demanding.

During the spring and fall, water deeply about 2-3 times per week, providing about 1 inch of water each time. In the extreme heat of summer, increase watering to almost daily for containers and every other day for in-ground plants. Use the "finger test" – stick your finger about 2 inches deep into the soil. If it feels dry, it's time to water. Water at the base of the plants to avoid wetting the foliage, especially as the monsoon season approaches.

Signs of underwatering include wilting and dry, crispy leaves. Overwatering can lead to yellowing leaves and root rot. A layer of mulch around the base of the plants will help retain moisture in our dry air and keep the soil cooler.

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

If your petunias start to look leggy (long, bare stems with few flowers) in midsummer, don't be afraid to cut them back by about half. This will encourage new growth and a bushier shape. As the first frost approaches around mid- November, you can let the plants fade naturally.

Petunias are grown for their beautiful blooms, so there's no real "harvest" in the traditional sense. Instead, your harvest is the enjoyment of their continuous color from late May through mid July. Deadheading keeps them blooming all season.

Keep deadheading spent flowers to encourage more blooms. As the weather cools down in the fall, you might see another flush of blooms. As the first frost approaches in mid- November, you can enjoy the last of the flowers before the plants are done for the season.

Here are three common problems you might encounter with petunias in the Desert Southwest:

Tobacco Budworm

• What it looks like: Small, green caterpillars that bore into the flower buds and eat the petals. You'll see holes in the flowers and sometimes find the caterpillars inside.
• What causes it: Moths lay their eggs on the plants, and the caterpillars hatch and start feeding.
• How to fix/prevent it: Handpick the caterpillars off the plants. You can also use Bacillus thuringiensis (Bt), a biological insecticide, to control them.

Botrytis (Gray Mold)

• What it looks like: A gray, fuzzy mold that appears on the flowers and leaves, especially during periods of wet weather.
• What causes it: A fungal disease that thrives in humid conditions.
• How to fix/prevent it: Improve air circulation around the plants by spacing them properly. Avoid overhead watering, especially during the monsoon season. Remove any infected plant parts immediately.

Leggy Stems

• What it looks like: Long, bare stems with few leaves and flowers.
• What causes it: Insufficient sunlight, excessive heat, or lack of pruning.
• How to fix/prevent it: Plant petunias in a location that gets at least 6-8 hours of sunlight per day. Prune the plants regularly to encourage bushier growth.

Desert Southwest Specific Challenges: Our extreme heat can stress petunias, making them more susceptible to pests and diseases. The arid humidity means that even a little monsoon moisture can quickly lead to fungal problems. Proper watering and air circulation are key to keeping your petunias healthy in our challenging climate.

Petunias make great companions for several plants in your Desert Southwest garden. They can help deter pests and attract pollinators, plus they make everything look prettier!

Good companions include tomatoes, peppers, green beans, and squash. Petunias deter tomato hornworms and other pests that can damage tomatoes and peppers. They also attract pollinators, which are essential for squash and green bean production.

I don't recommend planting petunias near cabbage or other brassicas, as they attract cabbage moths. Avoid planting them near root crops like carrots or radishes, as the petunias' spreading growth habit can shade them out.