Petunia

Prepared by Karen Russ, HGIC Horticulture Specialist, and Bob Polomski, Extension Consumer Horticulturist, Clemson University. Revised by Karen Russ. (New 03/99. Revised 09/07. Images added 09/07.)

HGIC 1171

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Petunias are one of the most popular flowering annuals. They have a long flowering period, are easy to grow and are available in many forms and colors.

Tidal Wave Silver petunia growing through silver sage
'Tidal Wave Silver' petunia growing through silver sage.
Karen Russ, ©2007 HGIC, Clemson Extension

Height/Spread

Height can vary from 6 inches to 18 inches. Spread can be from 18 inches to 4 feet. The size of the flowers varies from an inch in diameter from 5 to 6 inches.

Ornamental Features

Petunias can be found in every color of the rainbow in solids, contrasting veins or edges, and star patterns. The flowers may be large or small, ruffled, fringed, or double. They bloom from spring until frost. Many petunias, especially white and lavender cultivars have a very sweet fragrance.

Landscape Use

Petunias are versatile annuals. They can be used for color masses, borders, containers, hanging baskets or as a seasonal groundcover.

Petunias should be planted in full sunlight. They will become spindly and have few flowers if grown in shade.

Petunias grow well in most soils. Best growth occurs in well-drained, light soil of medium fertility. If the soil or the area is poorly drained, you can build raised beds to grow good petunias. They prefer soil with a pH of 6.0 to 7.0.

If you grow petunias from seed, start them eight to 10 weeks before planting in beds. The very fine dust-like seed of petunias require light for germination.

Purchase healthy young petunias that are short and compact. Leggy, thin plants are slow to adjust to outdoor conditions. Young plants not yet in bloom often settle in faster.

Plant petunias as soon as the danger of frost is past in the spring. Plants should be hardened off before planting into exposed beds. Place the plants in a protected place such as a cold frame or sheltered area close to buildings to adjust them to outdoor conditions. Keep them well-watered during this period.

Pinch off the top inch before planting to encourage good branching. For good ground cover, space petunias 12 to 18 inches apart.

Plants need at least 1 to 2 inches of water every seven to 10 days once established. Avoid frequent light watering that encourages shallow rooting. Fertilize petunias monthly with a balanced fertilizer to support their rapid growth and heavy blooming.

If plants become leggy or stop flowering, prune the shoots back to about half their length. You can cut back to within a few inches of the base if needed, but do not remove all their leaves. Fertilize with a liquid fertilizer. Water well to force out new growth and flowers.

Large-flowered and double petunias will need to be deadheaded (removal of old and dying flowers) to improve appearance and bloom production. Many of the smaller-flowered cultivars are self-cleaning.

Petunias often reseed in the garden, but will not return true to type. They usually revert to a mix of small white, lavender and rose flowers.

Problems

Petunias have few serious insect or disease pests. Aphids may become a problem. Slugs feed on petunias and can be controlled by the use of baits. Avoid wetting the foliage and flowers when watering to help prevent disease. Petal blight can be a problem in rainy and very humid weather. Viruses occasionally affect petunias.

Types and Cultivars

The types of petunias best suited to growing in South Carolina are the multifloras, millifloras and spreading petunias.

Sugar Daddy a grandiflora type petunia
'Sugar Daddy', a grandiflora type petunia.
Karen Russ, ©2007 HGIC, Clemson Extension

Grandifloras: Grandiflora petunias generally do not thrive in South Carolina because their large heavy blossoms are prone to damage and rot during hot, humid summers. Grandifloras have large flowers that are often frilled or ruffled along the edges. If you grow most grandiflora petunias, be aware that they will need extra care to look t heir best. The following series are well adapted to growing in the South.

  • 'Storm' series petunias are weather-tolerant and have high disease tolerance, uniformity and 3-to 4-inch blooms.
  • 'Ultra' series petunias are compact and low-spreading with masses of 3-to 4-inch ruffled blooms.
  • 'Daddy' series petunias have large flowers with distinctive dark veining. They are rain tolerant.

Multifloras: Multiflora petunias are durable and prolific. Their flowers are not as large as the grandiflora types, but they are very free-flowering and vigorous. Multifloras produce masses of color through summer until frost. They are resistant to petal blight. There are single-and double-flowered multiflora petunias.

  • 'Carpet' series petunias are available in many colors. They are compact, early blooming with 1½-to 2-inch blooms.
  • 'Primetime' series stay compact and uniform, covered with 2¼-inch flowers.
  • 'Heavenly Lavender' is an early, compact, double, deep lavender blue with 3-inch blooms on 12-to 14-inch plants.

Floribundas: Floribundas are intermediate between the grandiflora and the multiflora groups. They are free-flowering like the multiflora varieties and produce medium-sized blooms.

Madness Red Improved a floribunda type petunia
'Madness Red Improved', a floribunda type petunia.
Karen Russ, ©2007 HGIC, Clemson Extension

  • 'Celebrity' series petunias are compact and rain-tolerant. The flowers reach 2½ to 3 inches across.
  • 'Madness' series petunias have big, 3-inch flowers in many veined and solid colors. They are compact and bloom until frost. They bounce back well after rain.
  • 'Double Madness' petunias are compact and floriferous with big, 3-inch flowers all through the summer. Like their single counterparts, 'Double Madness' petunias bounce back within hours of a rainstorm.

Millifloras: Milliflora petunias are much smaller than any other petunias on the market. The flowers are only 1 to 1½ inches wide, but what they lack in size they more than make up in number and duration. They rarely need to be pruned back in midsummer to rejuvenate.

  • 'Fantasy' forms neat, compact mounds 10 inches high in the garden. They are excellent for containers and baskets.

Fantasy Pink Morn a milliflora type petunia
'Fantasy Pink Morn', a milliflora type petunia.
Karen Russ, ©2007 HGIC, Clemson Extension

Spreading or Trailing Petunias: These are low-growing but spread as much as 3 to 4 feet. They form a beautiful, colorful groundcover because the flowers form along the entire length of each stem. They can be used in window boxes or hanging baskets.

  • 'Purple Wave' was the first cultivar in the class of spreading petunias. It produces large blooms of deep rose-purple. It is tolerant of summer heat, drought and rain damage. 'Purple Wave' remains under 4 inches tall.

Purple Wave Petunia
'Purple Wave' Petunia
Karen Russ, ©2007 HGIC, Clemson Extension

  • 'Wave' series petunias are available in a multitude of colors. Most are not quite as ground-hugging as the original. They are weather-tolerant, disease resistant and heavy-blooming.
  • 'Tidal Wave' series petunias are spreading, reaching 2 to 4 feet in width, but they are much taller than other spreading types, growing between 16 and 22 inches tall. 'Tidal Waves' are sometimes called hedge petunias because of their height and dense growth. They share the other good features of regular 'Wave' series petunias.
  • 'Opera Supreme' series petunias have large flowers and bloom profusely and reliably over a long season.

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