Select a sunny (minimum of 6 hours) location protected from strong winds. Dahlias grow best in a deep, fertile, well-drained soil with a pH of 6.0 to 6.5; they are not tolerant of water-logged soils. Dig and prepare a 12 inch diameter by 12 inch deep planting hole. Mix a shovel full of compost, a handful of bone meal, and a little Dolomite lime to the soil which was removed.
Fill the planting hole with the soil mixture until it is about six inches deep. Then place the tuber horizontally in the bottom of the hole with the eye pointing upward. Tall varieties will need staking, so this is a good time to set an appropriate size stake into the ground next to the tuber (near the eye). This will prevent damage which can result if it is added after the tuber has begun to grow. Cover the tuber with about two inches of your soil mixture and water thoroughly. When the sprout begins to emerge from the soil, gradually add more soil mix until the hole is entirely filled. Once the plant attains sufficient height, secure it loosely to the stake. (Add more ties as the stem grows until the plant is supported approximately 24 inches below the eventual top of the plant.
Dahlias are very sensitive to freezing temperatures. Large tuberous roots may be planted about 2 weeks before the last spring frost date. Small tuberous roots and transplants should not be planted until all danger from frost has passed. Spacing between plants depends upon the cultivar and method of growing: rows, beds, or borders. Large-flowering dahlias should be spaced 3 to 4 ft apart; smaller dahlias can be spaced 2 ft apart.
A Dahlia in bloom is a heavy feeder, so you may want to consider using a water soluble “bloom type” fertilizer about a month before the plants begin to bloom.
Dahlia Growing Tips
Apply a 3- to 4-inch layer of mulch using either compost, pine straw, or pine bark. Dahlias are heavy users of water; keep the soil moist but not saturated. Each tuberous root usually produces multiple shoots. While you can leave all the shoots to grow, thinning will produce flowers of higher quality. Leave one to four of the strongest shoots.
When plants are about a foot tall and display 3 to 4 pairs of leaves, pinch out the terminal shoot to encourage branching. One pinching is sufficient for most dahlias; small growing types can be given an additional pinching. Tall-growing dahlias should be staked to support the long stems and large flowers. Begin staking when the plants are about a foot in height. Fertilize monthly with a water soluble fertilizer or apply 2 to 3 lb of 5-10-10 per 100 sf. in July. Excessive nitrogen can result in foliage production at the expense of flower production. A second application may be needed on sandy soil or during rainy seasons.
Flower size can be increased by removing lateral flower buds. When the three buds that form at the end of each branch reach the size of small peas, remove the two side buds. Small-flowering types should not be disbudded. Faded flowers should be removed to encourage continuous blooming.
Check plants on a regular basis for diseases, insects, and other pests. The most likely pests are aphids, spider mites, leafhoppers, thrips, and European corn borer. Disease problems include tuber rots, ring spot, vascular wilts, mosaic virus, powdery mildew, and botrytis blight. Disease problems can be reduced by following recommended cultural practices and by providing good air circulation. Remove or treat affected plants or plant parts when first observed.