The American Dahlia Society recognizes 16 types of dahlias (Dahlia spp.) in 15 different colors. Dahlias are flowering perennials that grow in U.S. Department of Agriculture plant hardiness zones 8 through 10 -- but if you live in zone 8, you should cover them with a 4- to 6-inch layer of organic mulch to protect them from the cold.
Dahlias grow less than 1 foot to over 6 feet tall. Varieties with two to four stalks grow large blooms. Bushy forms bear clusters of miniature to medium-sized flowers.
Location, Soil and Spacing
Plant dahlias in a spot that gets at least six hours of sun each day and is protected from strong winds. The soil should have a pH of roughly 6.5.
Spade 1 to 1 1/2 pounds of 5-10-5 water-soluble, granular fertilizer into the top 6 inches of 50 square feet of soil two to three weeks before planting dahlias. Water it in well.
When planting, place the dahlia tuber with its eye up to 4 inches below the soil in warm climates and 2 inches deep in cool climates. Space small varieties about 2 feet apart. Allow 3 to 4 feet between large cultivars, with 3 to 4 feet of space between rows.
Put a 1- to 2-inch layer of grass clippings, pine needles, pine bark or compost around the base of dahlias. This mulch will help retain soil moisture and discourage weeds.
You’ll need to stake dahlias varieties more than 3 feet tall, especially if you live in a hot climate. Drive a 5- to 7-foot-tall stake 1 foot deep.
Fertilizer and Water
After the dahlias sprout, rake 1/2 to 1 cup of 5-10-10 or 1-12-12 water-soluble, granular fertilizer around each plant, being careful not let it touch the stems. Water well. After that, fertilize at the same rate monthly until mid-August, and then stop.
Water dahlias enough so that the soil does not dry out. Soak the soil around the bottom of the plant instead of using a sprinkler.
When a plant reaches about 12 inches high and has three or four pairs of leaves, prune the tip above the top two leaves to encourage the growth of strong side branches.
Sterilize the cutting blades of your pruning shears by soaking them for five minutes in a solution of 1 part household bleach to 3 parts of water. Let them air dry.
If you prune the lateral buds from varieties that grow large flowers, you can make the flowers even larger. You’ll see three buds the size of small peas on the end of each branch. Remove the two side buds to allow the center bud to yield a larger flower. Do not do this with varieties that yield small flowers, but remove all flowers as they die.
If your area gets hard frosts, dig the tubers up after the first frost in fall and store them in a cool, dry place for the winter. Cut them back, leaving a 3- to 4-inch stem on the tubers, and then dig them up and hang them upside down to dry. Knock off the soil, and then put them in a box with a few inches of vermiculite, peat moss or sawdust at the bottom. Store the tubers in a dry location at 40 to 55 degrees Fahrenheit.