Dahlia tubers are enlarged, fleshy stems of the dahlia plant which grow underground and sprout eyes, which may grow into new dahlia plants. Gardeners can grow dahlia tubers by cutting a clump of tubers so that each new tuber contains an eye, which will grow into a sprout. Dahlia tubers can be planted in spring after the last frost by digging a hole 6 inches (15 cm) wide and 6 inches deep. Plants should be no less than 18 to 24 inches (30 to 45 cm) apart. Dahlia tubers must be planted on their side with the eye facing up and should not be watered after planting until after the new plant is about 12 inches (30 cm) above the ground.
Gardeners should plant dahlia tubers in well-drained, moist, aerated soil that has been enriched with humus and a balanced fertilizer about one week before planting. While dahlia tubers need to be watered regularly, soil that is kept too moist can cause disease and rot. Soil should be slightly acidic, with a pH level of around 6.5 to 7. Herbicides should never be used on soil where dahlia plants are growing because they will kill the plant.
Dahlia tubers grow best in part shade to full sun, preferably in an area that receives sun in the morning and shade in the afternoon. Like many other plants, strong sunlight will help the dahlia plant yield the most blooms. Shadier locations yield dahlia plants that are taller, leggier, and produce fewer flowers.
Pests can be a problem for dahlia plants, especially aphids, spider mites and thrips. Snails and slugs should be baited two weeks after planting, since they can begin attacking dahlia tubers soon after they sprout. Since dahlia plants may grow quite tall, they should be staked or supported with plant cages so they stand straight.
Dahlias are native to Mexico so they are not hardy at freezing temperatures. Tubers should be dug up and stored indoors for winter after the first frost but before the first hard freeze. Dahlia leaves and stems should be cut back and the tuber left in the ground for another week or two. The dahlia tuber should be dug up, left to dry for about a day, and stored indoors in a refrigerator until spring. When the temperature warms, dahlia crowns can be divided into tubers that will develop into new plants, provided each dahlia tuber has an eye.