Native to Mexico, dahlias grow as a tender annual that reproduce by forming large tubers under the soil. Tubers must be lifted and stored for the winter and replanted in the spring. Dahlia blooms range in size from tiny pom-pom varieties of less than 2 inches in diameter to giant, 12-inch dinner plate varieties. Colors include shades of white, yellow, pink, red and purple with some variegated varieties. Plants typically reach a height of 3 to 5 feet and produce abundant blooms from mid-summer to frost. Plant year-old dahlia tubers in full sun and in amended soil.
Select an area that receives full sun 6 to 8 hours a day. Although dahlias tolerate partial shade, it inhibits blooms.
Test soil in the fall to determine the pH and soil makeup. Home soil tests provide a quick analysis, but a test done through your local cooperative extension is more thorough and provides a summary of your specific soil with specific measures for amending the soil.
Till the soil to a depth of 6 to 10 inches and amend with the appropriate materials outlined in the soil test results, adjusting the pH to a level of 6.5.
Add a 2- to 3-inch layer of well-rotted manure or compost to improve drainage and increase aeration. Organic amendments release nutrient slowly, feeding dahlias as they grow.
Apply 2 pounds of 5-10-10 fertilizer per 100 square feet of soil and work in well with the existing soil.
Plant tubers in spring after the danger of frost has passed in your area, spaced 2 to 3 feet apart. Dig a hole large enough to accommodate the tuber and its emerging roots. Plant tubers in the soil so the eyes (the small indentation where buds form) or sprouts are slightly below the surface of the soil. Cover with soil and firm down with your hands to secure the tuber and remove air pockets. Water thoroughly to moisten the soil to the root level.
Pinch out the center leaves of dahlias once they reach a height of 4 inches to encourage dense, compact growth. This creates a second main stem that produces blooms in mid-summer.
Remove buds to encourage large blooms, if preferred. Each stem produces buds in groups of threes. Removing the side buds and allowing only the center bud to grow and bloom increases bloom size, but lowers the number of overall blooms.
Fertilize once a month with foliar feeder from late spring to August to encourage lush growth and abundant blooms.
Water when soil dries. Dahlias require adequate water and suffer if soil remains dry for extended periods. Watering deeply once a week is preferred to frequent shallow watering.