Any summer flower show will feature beautiful dahlias
Dahlias are at their peak in late summer, and there are varieties ideal for every sunny garden situation from containers and the front of borders to specimen plantings and the back of borders.
In addition to full sun, dahlias require well-drained soil. Their dormant tubers can be started indoors a few weeks before planting outside or planted directly in the garden after all danger of frost has passed in the spring. Plant tubers 3 to 5 inches deep.
Young plants can also be purchased at garden centers. These should be planted at the same depth they were growing in their pots.
Space them according to their size. Larger varieties need a lot of space; keep them at least 2 feet apart, 3 feet may be better. Mid-size types should be spaced 18 to 24 inches apart, and small varieties 10 to 12 inches apart.
Feed them with a fertilizer high in nitrogen in spring. In early summer, switch to a fertilizer high in potassium to promote the best flowering. Many taller varieties will require staking to keep them standing tall. Deadhead all types to encourage a nonstop supply of flowers.
Don't be afraid to cut some of their blossoms for bouquets. Plants provide plenty and their flowers are as spectacular indoors as they are in the garden, lasting up to a week in a vase.
The huge flowers of dinner plate varieties are often featured at flower exhibitions, but dahlia flowers come in many sizes, from just 2 inches across to nearly a foot in diameter. Dahlias bloom in just about every color except blue.
Their flowers are as diverse in shape as they are in color. Their form may be single or double, flattened or round, with unusual petals or not. Cultivars are classified by their flower types: peony-flowered, ball, single, double, semidouble, cactus, semi-cactus and decorative. Regardless of their form, blooms are held on strong stems.
Dahlias don't begin flowering until midsummer but continue blooming until a hard frost.
In addition to beautiful blooms, their foliage remains fresh and attractive all season. Most varieties feature dark green leaves, but some boast burgundy foliage.
If plenty of space is available, plant taller, large-flowered dahlia cultivars like Purple Gem, Kelvin Floodlight or Arabian Knight. Purple Gem sports intense, cactus-type flowers, Kelvin Floodlight offers bright yellow dinner plate blooms and Arabian Knight shows off dark burgundy decorative flowers.
Dahlias with medium stature such as Bishop of Llandaff are perfect for the middle of perennial borders. They grow up to 4 feet tall and their scarlet flowers dazzle against bronze-red foliage. Fascination features small, deep-pink blooms on 20-inch plants with bronze foliage.
Figaro Mix is suited for the front of a garden bed or containers. It grows just a foot tall and is covered with double and semidouble flowers in a mix of orange, pink, red and yellow. Sunshine shows off golden yellow, single flowers over dark foliage. Hello Rose is another dwarf with rose-colored, double flowers.
The tender tubers of dahlias cannot survive outside in the garden over winter. Many gardeners treat dahlias as annuals, but they can also be dug and saved indoors. After frost, cut the foliage back to no more than 6 inches. Dig them up, remove as much soil as possible, and let them dry before storing them in vermiculite or shredded newspapers in a cool, dry spot. Some recommend dusting them with fungicide before storage.
• Diana Stoll is a horticulturist, garden writer and speaker. She blogs at gardenwithdiana.com.