Dahlias come in many sizes ranging from small species that reach less than a foot tall when fully grown to massive varieties that can reach 6 to 8 feet at maturity. These summer blooming bulbs (or tubers) produce flowers in numerous colors to match your landscaping from summer until well into the fall season, and the blossoms range in sizes from 2 inches to more than 12 inches across. Dahlias grow best in USDA cold-hardy zones 7 to 11.
Find a site for your dahlias that provides full sun and well-draining soil. Make plans for planting dahlias after the last chance of frost passes.
Plan enough room for the full size of the dahlia plants. Space the smallest varieties 9 to 12 inches apart; varieties that reach 3 feet tall should be spaced 2 feet apart; and the tallest dahlias require 3 feet of spacing between plants.
Increase the nutrients in the soil by adding compost, peat moss and decomposed manure into the planting areas. Use hand tools or a mechanized tiller to work several inches of organic matter to a depth of at least 12 inches.
Dig holes 4 to 5 inches deep for each dahlia tuber. Place the tubers in the hole with the side that has the most "eyes" facing upward.
Cover the dahlia bulbs with soil and water lightly. Supply an inch of water weekly once dahlias emerge but only if rainfall does not.
Apply a low-nitrogen fertilizer, such as 5-10-10 or 10-20-20, one month after planting dahlia bulbs. Follow the directions on the label and reapply monthly, but not after the middle of August.
Hand pull any weeds emerging around the growing dahlias as needed.