Dahlias should not be planted until all danger of frost has passed, and the soil temperature reaches 58-60 degrees F.
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
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
Dahlias, are a popular addition to the landscape because they have a wide height range (1 to 6 ft) and a variety of flower shapes and sizes (2 to 12 inches). Color range includes orange, pink, purple, red, scarlet, yellow, and white. Some flowers are striped or tipped with a different color. Dahlias begin blooming in early summer and continue to frost. Flower production may slow with high summer temperatures and moisture stress.
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.
Dahlias are hardy to USDA Zone 8 where they can be cut back and left in the ground to overwinter. In Zone 7b, dahlias sometimes overwinter in the ground depending on soil drainage and severity of the winter. It is recommended the tuberous roots be lifted and stored during the winter. After a killing frost, cut the foliage back 2 to 4 inches above the ground. Lift each clump separately with a spading fork. To avoid spearing tuberous roots, begin digging a foot away from the center of the plant. Gently brush soil off the clump of tuberous roots. Spread the clumps in a well-ventilated, shaded area to cure for a few days. Do not leave the tubers in full sun. Dust injured portions with sulfur to inhibit diseases. Pack each clump upside down in a ventilated box or basket using slightly damp vermiculite, perlite, or peat moss to cover the clumps. During the winter, the tuberous roots should be stored between 35 and 50 F and should not be exposed to hot or cold drafts or damp or very dry conditions. If the tuberous roots begin to shrivel during the winter, sprinkle lightly with water.