Dahlias are tuber-grown flowers that are prized for their large, colorful, showy blooms. Dahlias come in a vast array of flower shapes, colors and sizes, some dahlias growing much taller than others. There are hundreds of cultivated varieties and hybrids of dahlias, many of which are grown especially for use as cut flowers. Although dahlias are sometimes grown as nursery transplants, they are most commonly grown from tubers.
Select a planting location that receives full to partial sunlight and has well-draining soil. If you're growing tall dahlia types, choose a spot that has some protection from high winds.
Plant dahlia tubers in the spring after all chance of frost has passed and the soil temperature has warmed up to at least 60 degrees F. Remove all grasses, weeds and debris from the planting site.
Loosen the soil to a depth of about 12 inches using a pitchfork or rototiller. Spread a 2- to 4-inch-thick layer of organic compost on the planting bed and mix it into the soil to a depth of at least 6 inches.
Dig planting holes for the dahlia tubers that are about the same width as and three times as deep as the diameter of the tubers. Space the planting holes up to 3 feet apart or according to the recommended spacing for the dahlia variety you're planting.
Place the dahlia tubers into the planting holes so that they're resting horizontally with the buds, or "eyes," facing upward. Fill in the holes with the displaced soil and firm down the soil with your hands.
Insert wooden stakes beside the planted dahlia tubers; you'll use the stakes later to support the dahlias when they begin to grow. Don't water the dahlia tubers until they sprout above ground.
Water the sprouted dahlias deeply and thoroughly once each week during summer when rainfall is less than 1 inch. Spread a 2- to 4-inch-thick layer of organic mulch around the dahlias to keep weeds at bay and preserve soil moisture.
Tie the dahlia's stems to the wooden stakes with soft string or twine after they begin to grow tall. Pick off the flowers after they finish blooming and begin to fade.