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Extension > Garden > Yard and Garden Flowers > Growing petunias

Growing petunias

By Deborah Brown

It's no wonder petunias continue to rank among the most popular flowering annuals. They're bright and lively, bloom from spring until frost, and scent the air with lovely fragrance. Best of all, they're amazingly easy to grow, both in the garden and in containers.

Milliflora Petunia 'Fantasy Pink Monr' produces one and a half in flowers in great abundance.

Choosing petunias

There are literally hundreds of named petunia varieties. They fall into four distinct categories, based on flower size and growth habit. Some are more suited to container growing, while others are better for massing in the garden.

Grandiflora petunias, the most popular type, produce large flowers that are three to four or more inches across. They may be singles or ruffly doubles. Some have a pendulous, cascading habit that makes them more suitable for hanging baskets and window boxes, but most are upright plants that develop over the summer into large mounds of flowers which are twelve to fifteen inches tall.

Multiflora petunias are often more compact. Their flowers are smaller than the grandifloras, but they make up for it with the sheer number of blossoms that are open at any given time. Like grandiflora varieties, they're available in single or double forms, though the vast majority are singles. Typically, they're massed together to create big splashes of color in the garden.

Milliflora petunias are compact, miniature plants that produce abundant quantities of perfect, small flowers only an inch to an inch and a half in diameter. They make good edging plants and are also attractive when mixed with other flowering annuals in containers, where they may be viewed close-up.

Groundcover or "spreading" petunias are only about six inches tall, but spread so rapidly that they cover a huge area over one growing season, provided they're watered and fertilized frequently. This makes them ideal for scrambling down a hillside garden, or planting atop a retaining wall. They're also amazing in hanging baskets and window boxes, where they trail two to three feet or more over the summer. When grown in full sunlight, they are so covered by flowers that you hardly see any foliage.

Site and soil

The most important requirement for growing petunias successfully is a location with plenty of light. Petunias need at least five or six hours of good sunlight; they'll perform even better when located in full sun all day. The more shade they receive, the fewer flowers they'll produce. Impatiens are a better choice for blooming in shady places.

While soil needn't be terribly rich to grow good petunias, it must drain decently. It's always useful to improve garden soil by conditioning it with organic matter, such as baled peat moss, well-rotted leaf compost or manure.

Spread the organic matter two to three inches thick. Then incorporate it into the soil to a depth of eight to ten inches, using a rototiller or garden fork. This helps open up heavy clay soil, which improves drainage, but can also increase the ability of light, sandy soil to hold moisture and nutrients.

Fertilizer and water

Work a balanced fertilizer such as 8-8-8, 10-10-10, or 12-12-12 into garden soil at a rate of two pounds per hundred square feet. Later--early to mid-July--begin to use liquid fertilizer every three weeks (weekly for "spreading" petunias). Once the plants have begun to spread, it will be much easier to water this solution into the soil than to side dress with dry fertilizer.

Incorporate timed-release fertilizer into the soil when planting window boxes or other containers. Otherwise fertilize regularly every two weeks with a liquid fertilizer meant specially for flowering plants. ("Spreading" types require weekly fertilizing.)

For a fertilizer recommendation tailor made to your specific garden conditions, have an accurate soil test run. Contact your local County Extension office or the Soil Testing Lab at the University of Minnesota for information.

Petunias tolerate lots of heat, and are relatively undemanding when it comes to water. Except for "spreading" types, which require frequent watering, thorough watering once a week should be sufficient in all but the worst weather. Leave sprinklers on long enough to soak the soil to a depth of six to eight inches every time you water. Hanging baskets and other containers also need more frequent watering, perhaps as often as daily, depending on their size and the volume of soil they contain.

Starting seeds indoors

'Purple Wave' cascades over the sides of large containers.

Although petunias are easy to grow outdoors from transplants, they may prove difficult for beginning gardeners to start indoors. The advantages of starting petunias indoors are that you have a wider choice of varieties from which to choose, and you can raise large quantities of plants for less money. But it takes ten to twelve weeks before petunias are big enough to plant out, so they need to be started early (about March first in northern climates). This means there's ample opportunity for problems to develop between seeding and the final product!

Because of their size, petunia seeds present a challenge, even to experienced gardeners. Not only are they very tiny and fine, but they also need light in order to germinate. Pelleted seeds are easier to handle, but not always available.

Spread seeds sparingly on top of a container of clean, damp potting soil or milled sphagnum moss. Water with a fine mist to wash them into the potting material or press them in gently with your fingers before watering. Then cover the container with clear plastic and store it in a bright, warm (70 to 85 degrees F) place--out of direct sunlight--until seeds begin to sprout. This usually takes seven to ten days after planting. Remove the plastic film once seedlings emerge. Then relocate the container to a bright, but cooler place; 65 degree days, with night temperatures anywhere from 55 to 65 degrees.

For best results, place the petunias four to six inches below a fluorescent light fixture until they're ready to plant outdoors. You needn't invest in expensive lights made specially for growing plants. Ordinary fluorescent tubes usually do just fine. Put the lights on a timer to keep them lit sixteen to eighteen hours daily. Raise the lights as seedlings grow, always maintaining that four to six inch space between plants and lights.

When seedlings have three true leaves, it's time to transplant them into individual peat pots or packs that hold several plants each. Feed them every two weeks (weekly for the "spreaders") with diluted liquid fertilizer. Harden off young plants by putting them outside on sunny, warm days. Then bring them back in at night for several days before planting them outdoors permanently.

Planting tips

Wait until soil warms to about 60 degrees and frost danger has passed before transplanting petunias into the garden. Space grandifloras and multifloras about twelve inches apart in full sunlight, or several inches closer together when planted in a shadier location. Milliflora petunias can be spaced as close as four to six inches, but the spreading ground-cover types of petunias should be planted at least one and a half feet apart. Petunias must be planted much more closely together in containers in order to look attractively full right from the start.

Plan to provide some protection from midday sun for the first few days, if weather is hot or windy with few clouds at transplanting time. When grandifloras or multifloras grow about six inches tall, pinch them back to encourage rapid formation of flowering side shoots. Do not pinch millifloras or "spreading" petunias.

Whenever feasible, it's a good idea to remove faded flowers, including the portion below each flower where seeds will develop. This practice, called "deadheading," encourages blooming by preventing seed maturation. Although it may not be practical to deadhead masses of petunias in the garden, it's a must for flowering annuals in containers. Deadheading not only helps prolong blooming, it also keeps plants looking fresh, healthy and well-groomed.

A final tip: If you have an abundance of petunias blooming in the garden, it won't hurt to cut a length of stem here and there to take indoors for use in bouquets and floral arrangements. Just be sure to remove any leaves that will be submerged in the vase, where they would deteriorate rapidly.

Deborah Brown,
Extension Horticulturist
Department of Horticultural Science

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