Petunias (Petunia spp.) are one of the most popular garden flowers for both borders and containers. They are prolific bloomers, and you can find them in just about every color but true blue. Petunias have wide, trumpet-shaped flowers and branching foliage that is hairy and somewhat sticky. Within the petunia genus, there is great variety: single and double blooms; ruffled and smooth petals; striped, veined, and solid colors; mounding and cascading growth habits; and even some with fragrance. Most of the petunias sold today are hybrids, developed for specific design purposes. They grow easily and quickly when you transplant them into the garden, which should be done in the spring once the threat of frost has passed.
|Botanical Name||Petunia spp.|
|Mature Size||6–24 in. tall, up to 36 in. wide|
|Soil Type||Well-drained, moist|
|Bloom Time||Spring, summer, fall|
|Flower Color||Pink, purple, yellow, red, orange, green, white|
|Hardiness Zones||10–11 (USDA)|
|Native Area||South America|
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. Extreme summer heat also can cause a temporary cease in blooming. Older petunia varieties typically need deadheading (removing spent blooms) for them to continue blooming. Many newer varieties don’t require deadheading, though they’ll still benefit from it to maximize their blooms.
Petunias also 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 container plants to a protected area or setting up a temporary cover over garden beds.
Watch Now: How to Care for Planting Petunias in a Hanging Basket
Most petunia varieties prefer full sun, meaning at least six hours of direct sunlight on most days. But in the heat of summer, partial shade (especially from strong afternoon sun) will help to keep them refreshed and blooming better.
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.
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 a lot of stem but few flowers. In general, weekly watering when you don't have rainfall will be sufficient. However, some spreading varieties and plants grown in containers typically need more frequent 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 to 75 degrees Fahrenheit during the day and 55 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.
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 varieties need weekly fertilization, so be sure to check your plant’s individual care instructions.
New petunia varieties come out every year. Here are some favorites of growers:
- 'Blue Spark Cascadia': This variety has trailing violet flowers with a sweet scent.
- 'Supertunia Silver': This plant features white flowers with lavender throats and veins. It's a profuse bloomer with good tolerance for extreme weather.
- 'Prism Sunshine': This variety sports large, buttery yellow flowers, and it also has good weather tolerance.
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.
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 seeds at least 10 to 12 weeks before your area's projected last frost date.
Spread the tiny seeds on top of a moist seed-starting mix. Gently press them down, but do not bury them as they need light to germinate. 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. Once they emerge, remove the plastic. When the seedlings have three true leaves, they can be transplanted into their own pots.
Petunias generally don’t have any serious issues with pests or diseases. They can be damaged by heavy winds and rain, so aim to offer them some protection from extreme weather. They also 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. Plus, some pests that might bother them include aphids, flea beetles, slugs and snails which 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.
Botrytis blight outbreaks expected in greenhouses due to weather. Michigan State University Extension.
Pentunia (Group). Missouri Botanical Garden.