How to Grow Petunias that Will Bloom All Summer

Purple Petunias
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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 now find petunias in just about every color but true blue and with growing habits that mound in borders or trail down containers.

Petunias are annual flowering plants, native to Argentina. They 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.

The 2 oldest types of petunias are grandifloras and multifloras. Both have a somewhat mounding growth habit. Grandiflora has larger flowers, but Multiflora holds up better in the rain. If you grew petunias a few decades ago, you will remember how the flowers turned to mush, when they got wet.

Spreading type petunias, which include The "Wave", "Supertunia", "Cascadia", and "Surfinia" series are some of the most popular petunias because most don’t need any deadheading and they can be used as garden plants, ground covers, or trailing in containers.

"Calibrachoa" or "Million Bells" look like tiny petunias, but they are actually an entirely different plant species.

Trying to categorize petunias by looking at them is difficult, but it’s hard to go wrong with any of the varieties currently being sold. Hopefully, your petunias will be labeled. Look for the traits you most value, things like flower size, flower abundance, or no deadheading required. You should be able to find a suitable petunia in whatever color you choose. Here are examples of each of the types listed above.

Botanical Name

Petunia X hybrida

Common Name

Petunia

Hardiness Zone

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.

Sun Exposure

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

Bloom Period

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.

Tips for Growing Petunias

Petunias do best in full sun but can handle partial shade, especially in hotter areas. They are very slow to grow from seed. If starting from seed, begin at least 10 to 12 weeks before your outside planting date.

Petunia seeds need light to germinate, so don’t cover the seed with soil. Sprinkle it on top of the soil and pat lightly, for good contact. They 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.

Although petunias like cool weather, they are not frost-tolerant. Wait until all danger of frost is past before planting your petunias outdoors.

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.

Petunias will tolerate a range of soil pH. They don’t like to be dry for long periods, but they also don’t like wet feet.

Caring for Your Petunia Plants

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.

Problems To Watch for with Petunias

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

  • Gray Mold and Soft Rot - Usually occurs in rainy climates. Choose weather-resistant varieties.
  • Aphids - Hose off with a strong blast of water.
  • Budworm caterpillar - Small green caterpillars that 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.

Using Petunias in Your Garden Design

Because of their profuse blooms, petunias are excellent in hanging baskets, either alone or as a trailing plant in a mixed planting. They are low growing and need to be planted in large groupings, to make a splash in the garden. But containers of petunias can be placed in strategic areas of the garden, to add color where needed.

The Best Varieties of Petunias 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.