Growing Petunias
Petunias are easy to grow, bloom reliably all summer, and are available in a wide range of colors, flower forms, and growth habits. Types of petunias Petunia varieties (cultivars) can be divided into four main groups or categories based on flower and growth habits: grandiflora, multiflora, milliflora, and spreading or groundcover. Grandifloras have large single or double flowers. Single grandifloras produce large blooms up to 5 inches across. Some single varieties have ruffled or fringed petals. Others possess a trailing habit that 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 may become unkempt and straggly by late summer. Popular grandiflora petunias include cultivars in the Supercascade, Dreams, Ultra, EZ Rider, and Storm Series. Multiflora petunias produce smaller flowers than the grandifloras, but in greater quantity. Both single and double cultivars are available. 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 cultivars in the Celebrity, Hurrah, and Carpet Series. Milliflora petunias are compact, miniature plants that produce large numbers of 1- to 1½- inch-diameter flowers. Their compact size makes them an excellent choice for containers and edging beds and borders. Cultivars in the Picobella and Fantasy Series are widely grown milliflora petunias.
Spreading or groundcover petunias are vigorous, lowgrowing plants that spread like groundcovers. By the end of summer, some cultivars may cover an area 3 to 4 feet in diameter. Spreading petunias possess excellent heat and drought tolerance and require little maintenance. They are excellent choices for hanging baskets, containers, and as an annual groundcover. Spreading petunias include those in the Wave, Easy Wave, Tidal Wave, Avalanche, Ramblin’, and Triology Series. Series refers to a group of closely related cultivars with uniform characteristics, such as height, spread, and flowering habit. Generally, the only variable within a series is flower color. Starting seeds indoors Although most gardeners buy transplants, petunias can be started indoors 10 to 12 weeks before the average last frost date. Late February/early March is an appropriate sowing date in Iowa. An excellent medium for starting seeds indoors is a commercially prepared soilless product, such as Jiffy Mix. 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 with the germination medium to within 1 inch of the top and press lightly to firm. (If the germination medium is dry, moisten it before filling the container.) Petunia seeds are very small. There are approximately 250,000 to 300,000 seeds per ounce. To make them larger and easier to sow, petunia seeds are often coated with an inert material. (Seeds that have been coated with an inert material are referred to as pelleted seeds.) Carefully sow the petunia seeds on the surface of the germination medium, then gently press the seeds into the medium.
Petunia seeds require light for germination. They should not be covered with additional material. After the seeds have been sown, moisten the medium by placing the container in a pan of warm water or by applying a fine spray to the surface of the medium. Uniform medium temperatures and moisture are required for optimum seed germination. Place the container in a warm (75° to 80°F), brightly lit location. To maintain uniform moisture levels, place a piece of clear plastic food wrap over the container. Clear plastic domes also are available. Do not set the covered container in direct sunlight. The high temperatures that 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 seedlings emerge. Place the container 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 14 hours. Temperatures should be 65° to 70°F. When seedlings have 3 true leaves, transplant into plastic cell packs, peat pots, or other containers. To produce stocky plants, keep the seedlings under fluorescent lights or in a sunny window, allow the potting soil to dry between waterings, and fertilize every 2 weeks with a dilute fertilizer solution. Harden the seedlings outdoors a few days before planting into the garden. Selecting transplants When buying petunias for spring planting, select compact, stocky plants. Tall, spindly plants take considerably longer to recover from transplanting. Harden plants outdoors for a few days before planting in the garden. Initially place plants in a shady, protected location; gradually expose them to longer periods of direct sun. Bring plants indoors if freezing temperatures are predicted. Planting Petunias can be transplanted into the garden after the danger of frost has passed. It’s usually safe to begin planting petunias in late April in southern Iowa and in mid-May in northern parts of the state. Petunias perform best in sunny locations. They also require a moderately fertile, well-drained soil. Poorly drained soils often can 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 2½ feet apart. Pinch back grandifloras and multifloras to encourage branching. Pinching is especially helpful for tall, leggy plants. Millifloras and spreading petunias usually don’t require pinching. Maintenance During dry weather, a deep watering once every 7 to 10 days should be sufficient for petunias in beds and borders. Plants in containers will need to be watered more frequently. Check containers frequently and water when the soil surface becomes dry. Petunias growing in containers should be fertilized every 2 weeks with a dilute fertilizer solution. To encourage additional blooms and improve plant appearance, remove the spent flowers on grandiflora and double petunias. Milliflora and spreading petunias are selfcleaning and don’t require deadheading. Leggy petunias can be rejuvenated by cutting them back to about half their length in mid-summer. After pruning, fertilize and water the plants to promote new growth.
