Dahlias are spectacularly showy flowers with intricate blooms up to 10 inches in diameter. They require a great deal of maintenance and care. Except for small bedding annual dahlias, the plants are grown from large, fleshy tubers. Dahlias are tender plants, and in U.S. Department of Agriculture zones colder than 8, the tubers must be dug up in fall and replanted in spring.
Plant dahlias in a location with full sun and a rich, well-drained soil. Plants that get less than eight hours of sunlight a day develop a leggy growth habit and are susceptible to powdery mildew. The plants need good air circulation, but they also need protection from strong winds that can cause serious damage. Dahlias are thirsty plants that need plenty of water, so plant them in a location with easy access to a water hose and use mulch to prevent moisture evaporation. They are sensitive to herbicides, so don’t plant them in an area chemically treated for weeds.
Dahlias need a rich soil with plenty of organic matter. Work a thick layer of compost into the soil. Add peat moss if necessary, to help loosen heavy soils. Avoid fertilizers high in nitrogen because they weaken the stems.
Dahlias have shallow roots that are easily damaged during cultivation. Plant dahlias in a weed-free bed, and apply a thick layer of mulch around the plants immediately after planting to help keep weeds to a minimum.
Plant dahlias in holes 6 inches deep. Lay the tuber in the hole horizontally with the eye, or growth bud, facing up. Space the tubers 2 to 3 feet apart. From the time you plant the tubers it takes 8 to 12 weeks for plants to flower.
Varieties that grow more than 3 feet tall need stakes. Drive the stake into the ground about 2 inches from the eye before you bury the tuber to prevent injury to the tuber. Continuously tie the stem to the stake as the plant grows. Tomato cages are a good alternative to stakes for large varieties; the foliage will hide the cage.
Most plants need water right after they are planted, but early watering causes dahlia tubers to rot. Unless the soil is extremely dry, wait until the shoots are a few inches tall to begin watering.
Dahlias may become tall and leggy if seedlings aren’t pinched. To prevent legginess, pinch out the tops of large flowering varieties when they have two to four pairs of leaves. Pinch dahlias with flowers that are 6 inches in diameter or smaller when they have four or five pairs of leaves. Pom pom and miniatures are an exception; don’t pinch these varieties.