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Horticulture and Home Pest News
Horticulture & Home Pest News is filled with articles on current horticulture, plant care, pest management, and common household pests written by Iowa State University Extension specialists in the Departments of Entomology, Horticulture and Plant Pathology.

Selecting and Planting Petunias

This article was published originally on 2/19/1999

Selecting and Planting Petunias

Petunias have been one of the most popular flowering annuals for years. Their popularity can be attributed to several desirable traits. Petunias are easy to grow, bloom reliably all summer, and are available in a wide range of colors, flower forms, and growth habits.

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Petunia

Petunia varieties can be divided into four main groups or categories based on flower and growth habits. The four categories are grandiflora, multiflora, milliflora, and the spreading or "wave" petunia.

Grandifloras are large-flowered petunias consisting of both single and double flowering forms. Single grandifloras produce large blooms up to 5 inches across. Some single varieties have ruffled or fringed petals. Others possess a trailing habit which make them ideal for window boxes and hanging baskets. Double grandifloras produce double, fringed flowers and are best suited to containers. Generally, grandifloras (single and double) don't flower as heavily as multifloras. Also, the flowers don't hold up as well during rainy weather. Plants often become unkempt and straggly by late summer. Popular grandiflora petunias include varieties in the Supercascade, Supermagic, Ultra, and Falcon Series. (A series is a group of closely related varieties with uniform characteristics, such as height, spread, and flowering habit. Generally, the only characteristic which varies within a series is flower color.)

While the flowers on the multiflora petunias are smaller than the grandifloras, they produce many more flowers. There are both single and double flowered multiflora petunias. Multifloras are generally more compact and resistant to wet weather than the grandifloras. Single multifloras are excellent in mass plantings in flower beds. Double multifloras are most suitable for containers. Excellent multiflora petunias include varieties in the Celebrity, Primetime, and Carpet Series.

Milliflora petunias are compact, miniature plants that produce large numbers of 1- to 1 1/2- inch-diameter flowers. Their compact size makes them an excellent choice for containers and edging beds and borders. Varieties in the Fantasy Series are widely grown milliflora petunias.

The fourth type of petunia is the spreading or "wave" petunia. The first introduction in this category was 'Purple Wave.' A 1995 All-America Selection, 'Purple Wave' grows 4 to 6 inches tall, but can spread 2 to 4 feet. The burgundy-purple flowers measure 2 inches across. Recent additions to this group are 'Pink Wave,' 'Rose Wave,' 'Misty Lilac Wave,' and 'Coral Wave.' The "wave" petunias possess excellent heat and drought tolerance and require little maintenance. They are excellent choices for hanging baskets and as an annual groundcover.

Starting Seed Indoors

Though most gardeners buy transplants, petunias can also be started indoors in late winter. Sow petunia seeds 10 to 12 weeks before the last frost. Early March is an appropriate sowing date in Iowa.

A commercially prepared medium, such as Jiffy Mix or Redi-earth, is a good germination medium. Containers used for starting seeds should be clean and have drainage holes in the bottom. Previously used containers should be washed in soapy water and then disinfected by dipping in a solution containing one part chlorine bleach and nine parts water.

Fill the container to within 1/2 to 1 inch of the top and then firm lightly. Next, moisten the medium by partially submerging the container in water. When the surface becomes wet, remove the container, allow it to drain for 1 or 2 hours, and then sow the seeds. The medium can also be moistened with a rubber bulb sprinkler.

The seeds of petunias are very small. (There are approximately 250,000 to 300,000 seeds per ounce.) For many home gardeners, sowing the fine petunia seeds is difficult. Fortunately, pelleted petunia seeds are often available. Pelleted seeds are seeds coated with a material to make them larger and easier to handle. Carefully sow the seeds on the soil surface, then gently press the seeds into the germination medium with a pencil or a small block of wood. Since petunia seeds require light for germination, don't bury them in the germination medium or cover them with additional material. After sowing, thoroughly moisten the medium with a fine mist from a rubber bulb sprinkler or by partially submerging the container in water.

Uniform medium temperatures and moisture are required for optimum seed germination. Place the container in a warm (75 to 80Å¡F) location in bright light. To maintain uniform moisture levels, place a piece of clear plastic over the container. Do not set the covered container in direct sunlight. The high temperatures which may develop in direct sunlight may inhibit or prevent germination. Petunia seeds should germinate in 10 to 12 days.

Remove the plastic covering as soon as the germination occurs. Then place the seedlings under fluorescent lights or in a sunny window. Fluorescent lights should be no more than 4 to 6 inches above the growing plants and should be left on for 12 to 16 hours. Temperatures should be 55 to 65Å¡F. Transplant the seedlings into plastic cell packs, peat pots, or other containers when the seedlings have 3 true leaves. To produce stocky plants, continue to grow the plants in a cool location under fluorescent lights or in a sunny window, allow the potting soil to dry between waterings, and fertilize every two weeks with a dilute fertilizer solution. Harden the seedlings outdoors a few days before planting into the garden.

Selecting Transplants

When buying petunias in the spring, select compact, stocky plants. Tall, spindly plants take considerably longer to recover from transplanting. Once purchased, harden the plants outdoors for a few days. Initially place plants in a shady, protected area and then gradually expose them to direct sun. Bring them indoors at night if freezing temperature are predicted.

Planting

Petunias can be transplanted into the garden after the danger of frost has passed. In central Iowa, the last frost usually occurs around May 5-10.

Petunias are easy to grow if a few requirements are met. Petunias perform best in sunny locations. They also require a moderately fertile, well-drained soil. Poorly drained soils can often be improved by incorporating organic matter, such as compost, peat, or well-rotted manure.

Plant petunias about 12 inches apart. The spreading types should be planted 2 to 2 1/2 feet apart. Pinch back grandifloras and multifloras to encourage branching. Pinching is especially helpful for tall, leggy plants. Millifloras and spreading petunias don't require pinching.



This article originally appeared in the February 19, 1999 issue, pp. 14-15.

Year of Publication: 
1999
Issue: 
IC-481(3) -- February 19, 1999