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.