How to Grow and Care For Petunias

Petunias (Petunia spp.) in flower beds and pots are one of the most popular garden flowers. They are prolific bloomers, and you can find them in just about every color but true blue. They have wide, trumpet-shaped flowers and branching foliage that is hairy and somewhat sticky. Within the petunia genus, there is great variety. Most are sold as hybrids and can have single or double blooms; ruffled or smooth petals; striped, veined, or solid colors; mounding or cascading growth habits; and even fragrance.

Petunias are fast-growing plants that will reach full size by late spring. When to plant petunias depends on your area's projected last spring frost date; the plants must not be exposed to any frost. You can keep petunias blooming all summer by giving them enough sun, water, and fertilizer; deadheading the spent blooms; and pruning back scraggly growth. In cold climates, petunias are annuals and only last one growing season. Within their warmer growing zones, petunias will come back every year but still don't last very long—about three years. However, they will self-seed for continued growth.

Types of Petunias

There are many types of petunias that range in appearance and care needs. Based on their characteristics, the petunia varieties are split into five main groups:

• Grandiflora: These types of petunias have especially large flowers. But they struggle in hot, humid climates.
• Multiflora: These petunia varieties have smaller flowers than the grandiflora types, but they make up for it by producing more flowers. They also have a higher tolerance for wet conditions. 
• Floribunda: These petunias fall somewhere in the middle of grandiflora and multiflora. They produce many moderately sized blooms.
• Milliflora: The flowers on these types of petunias stretch less than 2 inches across. But they are abundant and long-lasting. 
• Trailing/spreading: Trailing petunias have a low-to-the-ground, spreading growth habit. They look great as ground cover or spilling over the edges of containers.

Petunia Care

Petunias like the sun versus shade. However, extreme summer heat with harsh sun can cause a temporary cease in blooming.

The plants will require regular watering and feeding throughout the growing season (spring to fall). And they might appreciate some protection from extreme weather, which can involve moving petunias in pots to a protected area or setting up a temporary cover over petunias in flower beds.

Light

Most petunia varieties like full sun, meaning at least six hours of direct sunlight on most days. But in the heat of summer, partial shade (especially from the strong afternoon sun) will help to keep them refreshed and blooming better.

Planting and Soil

Petunias prefer a light, fertile soil that provides good drainage. They can tolerate a variety of soil types as long as they are well-draining. Plus, they like a slightly acidic soil pH. Plant petunias outside after the threat of frost has passed, as the soil must be warm and workable. Plants should be spaced roughly a foot apart.

Water

Like many flowering annuals, petunias don’t like to be dry for long periods. But they also don’t like to sit in soggy soil, which can rot their roots. Plus, too much water can result in leggy plants with few flowers. In general, it's sufficient to soak beds weekly with 1 to 2 inches of water when you don't have rainfall. However, some spreading types of petunias and petunias in pots typically need more frequent and deep watering. Try not to let the soil dry out more than 2 inches down.

Temperature and Humidity

The ideal temperatures for petunias are roughly 60 degrees Fahrenheit to 75 degrees Fahrenheit during the day and 55 degrees Fahrenheit to 65 degrees Fahrenheit at night. They can tolerate temperatures all the way down to about 40 degrees Fahrenheit, but frost and freezing temperatures will damage and ultimately kill the plants. Low to moderate humidity levels are best for these flowers. 

Fertilizer

Feed petunias at the time of planting with a balanced fertilizer. It’s also helpful to work some compost into the soil. Then, starting in July and continuing until the plants decline in the fall, fertilize every two to three weeks with a liquid fertilizer made for flowering plants. Some of the spreading types of petunias need weekly fertilization, so be sure to check your plant’s individual care instructions. 

Pruning

When planting young petunias, pinch back the stems to encourage more branching and a fuller plant. How far back to pinch depends on the plant. If it is a short, stocky seedling, just pinch an inch or less. But if the seedling is gangly, you can pinch back the stem by half.

Propagating Petunias

Petunias can be propagated via stem cuttings. Gardeners often do this if they want to save a particular variety—especially one that’s hard to find at nurseries—and cultivate it indoors over winter until it can be planted outdoors after frost ceases in the spring. Take the cutting from a healthy plant in the fall prior to any frost. Here’s how:

1. Trim off a healthy portion of stem that’s around 6 inches long. Opt for a green, supple stem over one that’s older and more woody.
2. Remove foliage from the bottom half of the stem. 
3. Dip the cut end in rooting hormone.
4. Plant the stem in a small container filled with moistened soilless potting mix, and place it in bright, indirect light.
5. Keep the growing medium moist, and you should have root growth in a few weeks.

How to Grow Petunias From Seed

It's most common to purchase young petunia plants from a nursery. But it can be worth the challenge to grow petunias from seed, especially if you're trying for a particular variety. Start your petunia seeds at least 10 to 12 weeks before your zone's projected last frost date. Here are the steps for planting petunias from seed:

1. Spread the tiny petunia seeds on top of a moist seed-starting mix. Gently press them down, but do not bury them as they need light to germinate.
2. Then, cover the container with clear plastic, and put it in a warm spot but out of direct sunlight. You should see seedlings within seven to 10 days.
3. Once seedlings emerge, remove the plastic.
4. When the seedlings have three true leaves, they can be transplanted into their own pots until they are ready to be transplanted outdoors.

Potting and Repotting Petunias

Use a quality all-purpose potting mix for petunias. Petunias in pots can be spaced slightly closer than when they're in flower beds for a fuller look. But no more than three plants can go in a container that's 12 inches wide and deep. The container must have ample drainage holes. Any pot material should be fine, but opt for a light color to help prevent the roots from overheating.

It's best to start with a container that can accommodate your petunias' mature size to avoid having to repot. Disturbing the roots during the growing season might hinder blooming. 

Common Pests & Plant Diseases

Some pests that might bother petunia plants include aphids, flea beetles, slugs, and snails that feed on the stems and leaves. Often you can just hose pests off the plants with a strong blast of water. But if the infestation is severe and impeding flowering, you can use an insecticide. 

Petunias can be susceptible to fungal diseases, such as gray mold, especially in rainy climates. Opt for a variety that has a higher tolerance for moisture if you live in wet conditions.

How to Get Petunias to Bloom

The primary blooming season for petunias is in the summer, though they can start in the spring and stretch into fall until the temperature drops and frost arrives.

Older petunia varieties typically need deadheading (removing spent blooms) for them to continue blooming at their best throughout the summer. However, many newer varieties don’t require deadheading, though they’ll still benefit from it to maximize their blooms. If you'd like for your petunia to self-seed, leave the spent blooms on the plant. But if you want all of the plant's energy to go toward optimal flowering for one season—which is typically the case if you're growing it as an annual—deadheading is ideal.

Trimming back a leggy petunia plant in midsummer also can encourage fresh, healthy growth, which in turn can produce more branching and blooms.  

Common Problems With Petunias

Petunias are easygoing plants that bloom often, but they occasionally have issues you likely can keep under control.

Wilted Flowers or Leaves

There are a number of reasons for wilted petunia flowers or leaves, but most of the reasons come down to water: too much or too little. Check the soil. If it's not damp, water your petunias. If it's moist, ease up on your watering routine.

Leggy Stems

Petunias often develop leggy stems, but it's easy enough to remedy: Deadhead flowers regularly, and pinch back the stems. If this doesn't help your petunias fill out, you can prune the stems back to 2 to 3 inches long. As the plant regrows, it should be less leggy.