September 2009 - Petunias

The petunia has been, without a doubt, one of the most popular annual flowers ever to grace our gardens, porches and patios. Whether edging a flower bed, covering a bare area like a ground cover, spilling out of a container or trailing from a hanging basket--petunias help keep the gardening season at its most colorful from late spring to fall.

A Bit of History

The petunias we know today, however, are a far cry from those that first appeared in 19th century gardens, although they're all based mainly on two species that were discovered in South America in the mid-1700's and early 1800's: White-flowered Petunia axillaris and purple-flowered Petunia violacea. Introduced into Europe in the early 1800's, these species weren't spectacular garden flowers--they were lanky and rather small-flowered--but breeders even then, especially in Germany and England, began crossing them in search of larger flowers and more colors. The result was the garden petunia--a group of plants in exciting colors, some with large, sometimes double flowers, others with fringed single flowers. Referred to as Petunia x hybrida, the plants weren't hybrids as we know the term; they were chance crossings of species. Burpee's 1888 catalog listed a 'Black-throated Superbissima', which had deeply veined, dark crimson-purple petals and a black throat.

Double flowers, as Vaughan's Seed Catalog of 1900 noted, occurred in only 20 to 30 percent of the plants grown from seed; the rest would be large singles. It took until the 20th century for hybridizers to bring the genetics of Mendel to bear on petunia plant breeding.

At the beginning of this century, breeders in Japan began researching petunias, and in 1934, Sakata Seed Corporation bred the first consistently fully-double petunias. 'All Double Victorious' mix, considered a breeding breakthrough, was an All-America Selections Winner that year. Flowers were as large flowered as double petunias of today. It was a grandiflora type with fringed petals. Sakata Seed Corporation managed to interpret and apply Mendel's law of gene dominance in the search for a fully-double petunia, one that would come true to type from seed.

During the 1930's, German seed companies bred grandiflora petunias--all open-pollinated varieties--and greatly expanded the diversity of the plants, especially in the area of colors. The 1939 Benary Seed Growers catalog offered an open-pollinated, dark purple, white-edged petunia--only recently, in the '90's, has this unusual bicolor combination been introduced as a hybrid. The history of the hybrid petunia involved an exchange of information--individuals and companies learning from, and building on, what others had done.

In the late '30's, Charles Weddle, of W. Atlee Burpee & Company, applied the same law in his search for the fully double petunia. When he discovered the key--that the gene for doubleness was a dominant gene and crossing a true breeding double-flowered petunia with a compatible petunia would yield seeds that produced all double-flowered offspring--the production of modern-day petunias was on the way.

After the interruption of World War II, work began again in earnest. Doubleness wasn't the only characteristic breeders were looking for. They also wanted larger flowers, and more of them for a longer time, more compact plants with better branching habits, better disease- and weather-resistance--many petunias, even today, look bedraggled after a rainstorm. Fred Statt, of Harris Seeds, worked on disease-resistance, while still coming up with petunias that looked beautiful. In the early '50s, Weddle and Claude Hope, founders of Pan American Seed Company, went on to hybridize double and single grandifloras and multifloras--these really revolutionized the bedding plant industry.

In 1949, Weddle won an AAS award for the first F1 single-flowered multiflora, 'Silver Medal'. Crossing a grandiflora with a multiflora produced a hybrid plant that was vigorous and desirable, and in 1952, PanAmerican's 'Ballerina', the first F1 grandiflora, won an AAS award.

The breeding work involved with F1 hybrids and the seed production made the seed very expensive, but it also allowed hybridizers to greatly improve the plants over their species or open-pollinated relatives. The cost and size of the seed increased the chances for failure on the part of the home gardener and made the purchase of bedding plants more desirable. Coincidence or not, garden centers began to spring up around the country in the 50's, and petunias quickly became gardeners number one choice for annual color.

Changes and improvements have continued for decades. The first truly red petunia, a multiflora called 'Comanche'--bred by PanAmerican Seed--was brought out in 1953. The first yellow petunia, called 'Summer Sun', was bred by Claude Hope and introduced in 1977 by a relatively new company, Goldsmith Seeds. A new class of petunias, called floribunda, was created by Ball Seed Company in 1983, and introduced the 'Madness' series. In 1995 'Purple Wave' was introduced as an AAS Winner and began a new class of spreading petunias. The new variety was bred by Kirin Brewery in Japan and introduced by PanAmerican Seed Company. An additional new class, milliflora, was bred by Goldsmith in 1996. These different classes may bring about a revolution in petunias, leading to changes that completely transform the garden petunia we now know.

Petunia Particulars

Petunias are members of the Solanaceae, or nightshade, family--which includes relatives like tomatoes, peppers, Salpiglossis (which the early petunias closely resembled) and Nicotiana. Many of the different classes of petunias can be used interchangeably in the garden, but some are especially suited for containers and ground covers. Many petunias have a light, sweet fragrance, particularly noticeable in blue petunias.

Multiflora: This class performs better than most others in adverse weather conditions, especially hot, wet spells, during which they continue to flower freely. The flowers, which may be single or double, are produced in abundance all season. Single multifloras are ideal for mass plantings and border plantings; double multifloras make spectacular container and window box plantings. Flowers range from 1-1/2 to 3 inches (4 to 7.5 cm) in diameter and come in a rainbow of colors, often with contrasting centers or stripes.

Grandiflora: The single-flowered grandiflora has been the most popular type of petunia for years. A sometimes sprawling plant, it is excellent for mass plantings and for containers. The double-flowered grandiflora is the class that helped to bring on "petunia mania." Its 3 to 4 inch (7.5 to 10 cm) blooms look wonderful in porch or window boxes and large tubs. Both single- and double-flowered grandifloras come in numerous color variations, with cultivars that are solid or bicolor, deeply veined, striped or edged in a contrasting shade called picotee types. These petunias do best in cool temperatures; in high heat, the stems tend to stretch. Newer cultivars, though, are more compact and more rain- and disease-tolerant than many of the older ones.

Spreading: Low-growing, spreading plants that reach only 4 to 6 inches (15 cm) in height, spreading 'Wave' petunias can be used as a flowering ground cover, in full sun, as well as trailing in hanging baskets. Flowers form along the entire length of each stem and are produced prolifically all season without the stems being trimmed back. The flowers, 2 to 3 inches (5 to 7.5 cm) in diameter, are available only in a deep magenta purple or pink at this time. Spreading petunias stand up particularly well to heat and humidity. Not all trailing petunias are 'Wave' petunias. Some trailing petunias are propagated from cuttings. 'Wave' petunias are grown from seed.

Floribunda: Available in single- and double-flowered hybrids, the floribunda petunia is basically an improved multiflora: Flowers are somewhat larger than those of a multiflora, but they're produced with the same abundance. The plants flower earlier, like a grandiflora, but are more weather-tolerant--they perk up quickly after a rain shower. Floribundas are excellent for mass plantings in the landscape and for container plantings in pots and hanging baskets.

Milliflora: A new class of petunias named 'Fantasy.' The term was first coined in 1996 to accommodate hybrids that are about two-thirds the size of a normal petunia. The flowers are only 1 to 1-1/2 inches (2.5 to 4 cm) across, but they are produced abundantly so that they can literally cover the plant with color. These petite beauties bloom earlier and do not stretch. Well suited to containers and hanging baskets, millifloras require little maintenance because they don't have to be pruned back in midsummer to continue their flower show.

Growing Petunias

Selecting a Site. Petunias flourish in full sun, but they will grow adequately in part shade. In part shade, the stems will stretch more, and the plants will flower less, but they will still add color to your landscape.

Full sun translates into 6 or more hours of direct sun daily. That may actually be too much for petunias grown in the deep south and southwest, however; in those areas, try to pick a site that has some midday shade to protect the plants from the hottest rays.

If you're planting window boxes, remember that overhanging eaves will produce some shade and will also shelter the plants from rain--beneficial to the blooms, but detrimental to the plants if you forget to water them before they wilt!

Preparing the Soil. Even though petunias are very adaptable and will grow in almost any kind of soil--rocky, sandy or clay--they do best in a light but rich soil that has good drainage. When you have selected a site, amend the soil by digging in compost or peat moss before planting: a 1- to 2-inch (2.5 to 5 cm) layer should do.
Starting from Seed. Petunia seed is very tiny, so starting your plants from seed may seem daunting, but it really isn't that difficult. Plan to sow the seed indoors 8 to 12 weeks before the average last frost date in your area; in frost-free areas, count back from the date when you'll be planting your impatiens and other warm-weather annuals in the garden. Double-flowered petunias may take more time to bloom, so start them at least 12 weeks before planting-out time.

1. Fill a shallow container, or flat, with a commercial seed-starting mix. Moisten the mix and let it drain.

2. Tap the seeds out of the packet gently, trying to get even distribution over the mix; blending the seeds with an equal amount of dry sand may help. Do not cover the seeds: they need light to germinate. Press the seeds down so that they come in contact with the mix, then water the container from the bottom. Label container with variety name.

3. Cover the flat with a sheet of clear plastic wrap, or place it in a plastic bag closed with a twist tie, to keep the mix from drying out while the seeds are germinating.

4. Set the flat in a warm, bright location, or under grow-lights. Keep the growing medium between 70 and 75º F (21 to 24º C)--in a multi-shelf light garden, the shelf above one tier of the lights may provide the necessary warmth. When seedlings emerge in 10 to 14 days, remove the plastic cover. Keep the mix evenly moist--not soggy--by watering the flat from the bottom.

5. When seedlings are large enough to handle (they should have at least two sets of true leaves), usually around 3 to 5 weeks from sowing, transplant them into individual 2-1/4-inch pots. Plants may have stretched under low light conditions. If they reach 6 inches in height, pinch back the growing tips to encourage branching and compact growth. Petunias can be grown cool, between 55-60º F to encourage compact, branching plants.

Plant the petunias outdoors when the weather and soil have warmed up, about the time you plant impatiens or peppers.

Planting Petunias

If you buy plants at the nursery or garden center, they will undoubtedly be in bloom, so you can see the colors. In addition to flower color, look for bedding plants with clean, green foliage--no dried out or spotted leaves, no powdery mold evident. The soil shouldn't be water-logged. The plant should have buds as well as flowers. For planting in window boxes and containers, fill with a commercial soilless mix because it's lighter in weight.

The best time to plant petunias is on a cloudy, breezeless day. Set them in the ground or in a container at the same level they were growing in the nursery pot. Space plants in the ground as follows: Multifloras, grandifloras and floribundas, 10 inches (25.5 cm) apart; spreading petunias, 15 to 20 inches (38 to 51 cm) apart, because they can spread as much as 3 to 4 feet (.9 to 1.2 m); and millifloras, 6 to 10 inches (15 to 25.5 cm) apart. Plants in containers are usually spaced closer together, so that the pots overflow with color.

Consider the heights of the plants when you're deciding where to put them. Most petunias grow 12 to 14 inches (30.5 to 35.5 cm) high, but floribundas may reach 16 inches (41 cm), millifloras are more diminutive at 10 to 12 inches (25.5 to 30.5 cm) and spreading petunias are the true low-growers, reaching only 6 inches (15 cm).
You may want to spread a layer of mulch around the plants, especially the double-flowered ones, to prevent mud from spattering up on the blooms. The mulch helps retain soil moisture and discourage weeds.

Petunia Maintenance

Petunias don't require a lot of care, but they do benefit from some attention.

Fertilize the plants monthly with a balanced fertilizer; double-flowered cultivars appreciate a bit more, perhaps once every two to three weeks.
Because they're quite drought-tolerant, petunias seldom need daily watering other than what they receive with rain; in prolonged periods of drought, however, watch that the soil doesn't get too dry. And, if you're growing the plants in window boxes or other containers--where soil can dry out quickly--check the soil daily in very hot weather and water as needed.

The stems of most petunias have a tendency to stretch out by midsummer and bear fewer flowers, since blooms are formed at the ends of the stems. Prune them back quite severely so they will produce new shoots and more flowers. The exceptions are milliflora and trailing petunias--they don't need to be pruned at all, which makes them really easy-care.


Calibrachoa Millon-Bells	Purple Wave	Surfinia Sky Blue

Gardening With Petunias

Petunias have many uses in the landscape. Edge a perennial border with the more compact multifloras or floribundas. Plant them on the ends of vegetable beds, especially those that contain trellises of beans or cucumbers--they'll fill the ground-level space with color that looks particularly attractive against the vegetables' green foliage. Put petunias in mixed plantings in window boxes or in containers attached to the deck railing: the cascading or spreading types combine well with salvias and geraniums; grandifloras mix well with sweet alyssum, ivy-leaved geraniums and portulaca.

Petunias make excellent cut flowers; like pansies and zinnias, the more you cut, the more the plants seem to produce. Because their stems are somewhat lax and their leaves are sticky, the flowers are best cut with short stems. Place them in small vases or flower rings, where they'll last for four to six days. As a cut flower, they may add fragrance to a room. The flowers can be pressed, either whole or with petals taken apart and pressed individually.

Don't use the blooms as an edible decoration, because the plants are toxic, as many members of the Solanaceae family are.

Problems?

Petunia cultivars nowadays are pretty disease-resistant, but they can have a few problems you'll want to deal with--or take precautions to avoid.

Newly germinated seedlings can fall prey to damping off, a fungus that attacks at the soil level and is irreversible. The seedlings will wilt and die almost overnight. Avoid damping off by using a sterile commercial seed-starting mix. Use only clean, sterilized containers for starting seeds.

Seedlings are susceptible to Botrytis, a fungus that is also soil-borne and spreads quickly from infected plants to healthy ones. It thrives in cool, moist conditions and forms a powdery mold on stems, leaves and flowers. Avoid it by watering the soil from the bottom, not the plants from above. Don't crowd the plants together; make sure they have good air circulation around them.

Plants in the garden can be bothered by white fly and flea beetles; the latter may eat holes in the leaves, the former is more of a nuisance than a dire threat. Avoid them, to some extent, by not growing petunias near other members of the Solanaceae family, especially tomatoes and potatoes.

Plants may look "down in the dumps" after a hard rain. Newer cultivars in all classes, but especially the floribundas, spreading and multifloras, perk up within hours. Note: Petunias have naturally sticky leaves and stems, so don't panic and think the condition is disease- or pest-related.