How to Grow and Care for Petunia Plants

Purple Petunias (Solanaceae Petunioideae)

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Petunias are one of the most popular garden flowers for both borders and containers. They are prolific bloomers, although some forms require deadheading to keep them going. However, most petunia varieties will bloom throughout the summer, except in extreme heat. You can find petunias in just about every color but true blue and with growing habits that mound in borders or trail down containers.

Petunias have wide trumpet-shaped flowers and branching foliage that is hairy and somewhat sticky. Within the petunia family, there is great variety: single and double blooms, ruffled or smooth petals, striped, veined or solid colors, mounding and cascading habits and even some with fragrance. Most of the petunias sold today are hybrids, developed for specific design purposes. They grow easily when you transplant them to the garden, and this should be done in the spring when the threat of frost has passed.

  • Botanical name: Petunia x hybrida
  • Common name: Petunia
  • Plant type: Annual flower
  • Mature size: Six to 24 inches tall and spreads of up to three feet
  • Sun exposure: Full sun
  • Soil type: Well-draining, clay or sandy
  • Soil pH: 6.0-7.5
  • Bloom time: Spring and summer
  • Flower color: Pink, purple, yellow, various
  • Hardiness zones: n/a
  • Native area: Argentina
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How to Grow Petunias

Although some species of petunias are tropical perennials, today’s hybrids are usually grown as annuals, so hardiness zones are not a factor in growing them. Although petunias like cool weather, they are not frost-tolerant. Wait until all danger of frost is past before planting your petunias outdoors.

Petunias repeat bloom throughout the summer. Some varieties will require frequent deadheading and some stem pruning to continue setting flower buds. Extreme heat can cause petunia plants to stop setting flowers until the temperature drops.

Older varieties of petunias require diligent deadheading or they will stop blooming. This is not always a pleasant task since the foliage is sticky and blossoms that have been rained on will turn into slimy mush.

Even the newer varieties that say they don’t require deadheading will benefit from a pinching or shearing mid-season. When the branches start to get long and you can see where all the previous flowers were along the stem, it’s time to cut them back and refresh the plant.

Monthly feeding or foliage feeding will give your petunias the energy they need to stay in bloom. But be judicious with water and make sure the soil is well drained. Too much water will cause the plants to become "leggy", with lots of stem and few flowers.

Light

Most petunia varieties prefer full sun, but in the heat of summer, partial shade will keep them refreshed and blooming better.

Soil

Petunias require a light, fertile soil that provides good drainage. They like a slightly acidic soil pH.

Water

Like many flowering annuals, petunias don’t like to be dry for long periods. But they also don’t like wet feet. Weekly, deep watering is sufficient—except for spreading and container-grown petunias, which may require daily watering.

Temperature and Humidity

Petunias also prefer warmer temperatures for germination. Start the seeds on heating pads or on top of your refrigerator. Once the seed has germinated, move them from the warm area and let them grow on in cooler temperatures.

Fertilizer

Garden petunias like a balanced fertilizer such as 8-8-8, 10-10-10, or 12-12-12. In early to mid-July, start using a liquid fertilizer every two to three weeks. Spreading petunias may need weekly fertilization, while container-grown plants will respond well to a time-release fertilizer.

Best Varieties to Grow

New petunia varieties come out every year, making older varieties obsolete, but here are some particular favorites.

  • "Blue Spark" cascadia: Trailing violet flowers with a sweet scent.
  • "Supertunia silver": White with lavender throat and veins. Good weather tolerance and very floriferous.
  • "Prism sunshine": AAS winning hybrid with buttery yellow grandiflora sized flowers with multiflora weather tolerance. Can be grown from seed.

Pruning

When planting, pinch the seedling back to encourage more branching and a fuller plant. How far back to pinch depends on the plant. If it is a short, stocky seeding, just pinch an inch or less. If the seedling has gotten gangly, you can pinch back by half.

Container Growing

Because of their profuse blooms, petunias are excellent in hanging baskets, either alone or as a trailing plant in a mixed planting. Containers of petunias can be placed in strategic areas of the garden, to add color where needed. When potting, use a free-draining soilless potting mix fortified with a slow-release fertilizer.

Growing From Seeds

It might be worth the challenge to grow petunias from seed, especially if you're trying for a particular variety. Just remember that starting from seed takes longer so begin at least ten to 12 weeks before your outside planting date. Petunia seeds are tiny and fine but they need light to germinate; don’t cover the seed with soil but rather sprinkle the seeds on top of the soil and pat lightly, for good contact.

Common Diseases and Pests

Petunias are usually carefree growers although they can get pummeled by rain.

  • Gray mold and soft rot: This usually occurs in rainy climates. Choose weather-resistant varieties.
  • Aphids: Hose off with a strong blast of water.
  • Budworm caterpillar: Small green caterpillars attack late June and July and feed on the flower buds. Often you won’t see the actual caterpillar, but you may notice small black dropping and small holes in the leaves and buds. They’ll disappear in July, but you could use Bt on them if it’s a real problem.