Dahlias (Dahlia spp.) are frost-sensitive bulbs that bloom with showy flowers in shades of white, yellow, orange, red, pink, lavender and purple. Sunny spots and organically rich, moist, freely draining soil provide the best results with most dahlia varieties. Grow 1 to 6 feet tall and 1 to 3 feet wide, depending on the variety, dahlias are hardy in U.S. Department of Agriculture plant hardiness zones 7 through 10. In colder zones, dahlia bulbs are often lifted and stored in frost-free areas during the winter.
Selecting the Right Spots
The best growing spots for dahlias are sheltered from strong winds, and receive at least six hours of direct sunlight every day. Dahlias will grow in partially shaded sites, but flowering is reduced. In hot climates, these plants benefit from filtered shade, such as from a tree, during the hottest part of the day. Dahlias grow in most freely draining soil types, but the plants grow best in slightly acidic, sandy loam.
Providing of plenty of growing room for dahlias gives them the airy conditions that encourage healthy growth. Space dahlias that grow 1 foot tall and wide 1/2 to 1 foot apart, and space plants that grow 1 to 3 feet tall and 1 to 2 feet wide about 2 feet apart. Large dahlias, which grow up to 6 feet tall and 3 feet wide, should be spaced 3 feet apart.
Plenty of water encourages dahlias to grow quickly and healthily, but don't overwater these plants. Water dahlias when the soil is dry to a depth of 1 to 2 inches, and apply enough water to moisten the soil to the depth of the bulb roots, but don't make the soil soggy. Water around the plant base and avoid wetting the foliage, which encourages mildew. To help conserve soil moisture and control weeds, spread a layer of straw 3 to 4 inches deep around dahlia plants, but not touching their stems. In late summer through fall, a layer of black polyethylene helps warm the soil.
Low-nitrogen fertilizers promote good growth without reducing flowering in dahlias. Apply 1/3 to 1 cup of 5-10-10 or 2-12-12 fertilizer to each plant after the sprouts appear. Apply 1/3 cup of fertilizer to small to medium dahlia plants and 1 cup to large dahlias, and use 2-12-12 fertilizer in rich, fertile soil. Evenly sprinkle the fertilizer around the plants, but do not sprinkle it on the plants. Water the fertilizer into the soil.
Staking for Support
Tall dahlias must be staked or they flop and their stems break, which damages the blooms. Push bamboo canes or other plant stakes into the soil behind the plants after planting. When the plants are 1 foot tall, loosely tie the main stems to the stakes with soft twine. Tie the main stems to stakes every 1 foot as the plants grow taller.