Petunias have always offered nonstop summer bloom, but now you're more likely to find a color, size, and growth habit to suit your garden needs. New, vigorous trailing types and miniature-flowered types have rekindled gardeners' interest.
Petunias are low-growing, bushy to spreading tender perennials usually grown as annuals. Their soft, thick leaves are slightly sticky to the touch. Trumpet-shaped flowers have a single set of plain-edged petals, or can be doubled, ruffled, or fringed and come in colors from light pink through dark red, and pale blue through deep purple. You'll also find white, cream, and even a new bright yellow. Many flowers have veins of accenting colors, or alternating stripes.
Plants grow in all climates, preferring the warmth of summer; most die at or just below freezing. Gardeners in mild-winter climates know petunias as winter annuals or even short-lived perennials.
Most modern petunias descend from two wild species from Argentina, a large white one (P. axillaris) and a violet-flowered one (P. integrifolia, formerly P. violacea). The initial hybrids of these two were produced in Germany and England in the early 1800s. Over time, they proved so stable--not to mention popular--that botanists now refer to them simply as P. hybrida.
This nearly 200 years of gene mixing combined with petunias' great popularity (and the modern habit of giving plants a trademarked name) has led to some confusion among types.
When planning a petunia color scheme, keep in mind that purple-flowered varieties tend to be the most vigorous, while yellow ones--except for 'Prism Sunshine'--are the least. Red-flowered varieties of all types also tend to be weak. The breeding required to produce yellow and red varieties results in genetically weaker plants than the naturally hardy purple ones.
Following are descriptions of the basic petunia hybrids, with listings of strains that I've found to be superior.
Grandiflora. Of all petunias, these produce the fewest but largest flowers, 3 to 5 inches across. Since their introduction in the 1930s, they've been the most popular type of petunia.
Plants grow 1 to 2 feet high and 2 to 3 feet wide. The flowers can be single, ruffled, or fringed. Colors include blue, pink, red, rose, salmon, scarlet, white, and pale yellow, plus striped forms of these colors. Double flowers are also available.
Grandifloras are damaged by rain and strong winds. Also, the plants' thick, heavy stems have a tendency to fall over, resulting in unsightly holes in the planting. It may be difficult to find a place in the garden protected enough to show off grandifloras' full splendor all season. I like to grow them in a cutting bed and let the flowers take a starring role in arrangements.
Among the grandifloras, the big news is 'Prism Sunshine'. This 1998 All-America winner is notable for both its bright yellow color and its vigorous growth. (Earlier yellow petunias produced weak plants of modest performance.)
Multiflora. Sometimes called floribundas, multiflora plants are similar in size to grandifloras, but their flowers are only about 2 inches in diameter. Their chief virtue is more abundant flowers that have greater resistance to disease and bad weather. Because their branching is denser, they are less likely than grandifloras to flop over. They are also available in more colors than any other kind of petunia. Overall, they make neat, compact plants that cover themselves in bloom.
Despite their many virtues, multifloras are less vigorous than other petunias and not really suited to center stage in the garden.
Milliflora. These dwarf petunias grow to about two-thirds the size of multifloras. Small, delicate, 1- to 1-1/2-inch blooms cover the compact plants, which are ideal for containers, window boxes, and hanging baskets. I plant millifloras as a border, spacing them about 8 inches apart. Plants form mounds 6 to 8 inches across. Don't expect them to fill into a solid mass like other petunias. To show off the compact, dense mounds, plant them in clusters of three, five, or seven, depending on space. Space plants about 9 inches apart.
In the southern states (USDA Hardiness Zones 7 and warmer), millifloras are hailed as a long-lasting new bedding plant that won't flop over. In zones 5 and cooler, some critics have noted a relative lack of vigor. My experience is different. I have enjoyed them for the last three years in Vineland, Ontario (zone 5), and find that when planted early and given good care, they are very welcome additions to the garden.
Unlike the multitude of series and varieties common to other petunias, only one type of milliflora is available: the Fantasy series, offered in nine colors.
Doubles. Grandifloras, multifloras, and some trailing types are available with double flowers. All of these novelties, but especially the trailers, are striking in mass plantings. My preference is to place them in beds or planters where you can see them at close range and appreciate the blooms' complexity. While most double-flowered petunias are available as seed for home gardeners, seeds of doubles have less vigor so take longer to germinate and grow.
The Doubloon strain includes a solid pink as well as a veined pink, blue, and lilac. The more upright-growing Marco Polo, bred in Australia, comes in four clear colors: blue, rose, white, and a very showy pink. Both Doubloon and Marco Polo petunias are marketed under the Flower Fields brand.
Surfinia and other new trailing types were introduced in 1989, but only now are they becoming a market presence. They represent the first really new type of petunia in a generation.
Seed-propagated trailers. The Wave series is another star of the new trailing types. The incredibly vigorous 'Purple Wave' won an All-America Selections award in 1995. 'Purple Wave' was the first, and it continues to set the standard of performance, not only when compared with other trailing petunias, but also compared with sister colors in the Wave strain. Three new Wave colors were introduced in 1997: pink, lilac, and rose.
Wave flowers are multiflora-sized, 2-1/2 to 3-inches across, slightly larger than other trailing types. Although the flowers appear otherwise similar to Surfinia, and both types are hardy to about 28oF, their growth habits and leaf shapes are different. Wave petunias spread very rapidly, forming a low, solid mass studded with flowers. Plants remain about 6 inches tall and spread more than 5 feet across, making them a good choice for hanging baskets or patio pots.
Cutting-propagated trailers. Surfinias result from a cross of P. hybrida with a wild South American trailing type, P. pendula. They are remarkable for their long, trailing, dark green foliage that offsets flowers in five colors: blue-purple, pink, purple, violet, and white. Flowers are about 1-1/2 inches in diameter. Plant them in baskets, tubs, beds, or window boxes. In baskets, they produce trails of flowers. Their vigor can be a problem; it's not easy to find a sufficiently large hanging basket and to keep it watered on hot days. A single plant can cover a 4-foot-diameter space, or trail that far out of a basket within four to five weeks!
Supertunias are modern hybrids of P. hybrida with another wild petunia, P. axillaris. These fast-growing, long-lived, profuse bloomers are ideal for hanging baskets, planting beds, and containers. Eight colors are available: burgundy, fuchsia, bright pink, pastel pink, dark purple, two types of violet-magenta, and white. Plants are slightly hardier--to 27° F--than Surfinias, so might be more likely to survive mild winters and serve as short-lived perennials. Their flowers are about 2-1/2 inches in diameter; plants will spread to 3 feet in six weeks. Both Surfinias and Supertunias are marketed under the Proven Winners brand.
Cascadias are Surfinia look-alikes. They share a similar hybrid history and are sold under the Flower Fields brand. Cascadias are available in 15 colors including several shades of purple, pink, and blue. For instance, uniquely colored 'Charlie' is blue with a soft yellow center. Cascadias also include a double-flowered strain, Doubloon.
Petitunia were introduced in 1998. Also marketed under the Flower Fields brand, it has the same vigorous and wide-spreading habit as the Cascadias, but its flowers are a diminutive 3/4 inch in diameter. The tiny flowers bloom in such profusion that the overall color impact is nearly equal that of its larger-flowered cousins. Colors include pink, purple, and white with darker veins. All four of these similarly vigorous petunias--Surfinia, Supertunia, Cascadias, and Petitunia--are propagated by cuttings only, not seed. Cutting-grown petunias are available at garden centers. As bedding plants, plant one per square yard. They'll form a very dense carpet, with new shoots continuously producing blooms. Additionally, there's no need to pinch off spent flowers, as old flowers dry up and fall off.
Throughout most of the country, petunias are warm-season flowers that can produce a carpet of color from spring until frost. In intermediate- and low-desert areas such as Tucson and Phoenix, and in eastern zone 10, petunias fail in humidity or during extended periods of high heat (temperatures in the 90s), or both. In those regions, grow them as winter annuals, planting in fall and enjoying them through late spring.
Plant in full sun in rich, well-drained soil. If your soil is either very sandy or clayey, amend it by incorporating about 6 cubic feet of composted organic matter per 100 square feet.
Plant grandiflora and multiflora types about 10 inches apart, and Wave and Surfinia types 2-1/2 to 3 feet apart. After plants are established, pinch back halfway for compact growth. For optimum growth, feed monthly with complete liquid fertilizer. If you live in long-season zones 7 or 8, cut back plants halfway in August to force new growth.
Trailing petunias also need sun and well-drained soil, but they need more nitrogen fertilizer than bedding petunias because of their more rapid growth. An iron chelate spray or a nitrogen fertilizer with extra iron will promote attractive dark green leaves.
Pests. In humid weather, gray mold (botrytis) damages flowers and leaves of most types. There's no practical remedy if you live in a wet-summer climate beyond planting resistant types, such as multifloras. The ozone in smog causes silvery spotting on leaves (petunias are so sensitive that they are good indicators of invisible ozone pollution). Tobacco budworm is a problem in some areas. Snails and slugs dine on petunias as well as other plants, and likewise, aphids may gather on succulent new stems.
Peter Kopcinski is a seedsman living in Marlton, New Jersey.
Photos by Suzanne DeJohn/National Gardening Association.
Article published on June 23, 2008.