Where Dahlias Grow
Dahlias are hardy in U.S. Department of Agriculture plant hardiness zones 7 to 10, but grow best in certain planting zones within these USDA zones. They thrive in full sun in cool, moist areas. Dahlias will grow in hot summer climates, but they should be planted where they get direct sun in the morning with shade in the afternoon. An area with protection from strong winds is best, but do not plant them beneath trees or large shrubs. The soil must have plenty of organic matter, drain quickly and have a pH of 6 to 7. Begin preparing the planting site in the fall for spring dahlia planting.
Soil Drainage Test
Dig a hole that measures 1 foot wide and 1 foot deep to see how fast the soil drains. Fill it with water; wait for the water to soak into the soil, and then fill it again. Soil that absorbs the water within two hours the second time it's filled drains fast enough. When there's still water in the hole after three hours, you can sufficiently improve the soil's drainage with the addition of organic matter. If the water has not drained completely within four hours, it drains too slowly. Choose another area, or build a 1- to 2-foot high raised bed for the dahlias.
Soil pH Test
Test the soil pH with a soil test kit or digital meter, both commonly sold at garden centers. Use lime to raise the pH or sulfur to lower it if the soil test results indicate either is necessary. Test the soil in the fall prior to planting dahlias. It can take time to change soil pH, especially if it has to be raised. Lime applied in the fall will have all winter to be absorbed into the soil. The amount of sulfur or lime needed depends on whether the soil is sandy, loam or clay. Adjusting the pH up from 5.5 to 6 will require .625 pounds of lime per 25 square feet of sandy soil or 1.25 pounds per 25 square feet of loamy soil. To bring the pH down from 7.5 to 7, use .125 pounds of aluminum sulfate per 25 square feet of sandy soil or .25 pounds for loamy soil. Till the soil to a depth of 1 foot with a shovel or rototiller; sprinkle the aluminum sulfate or lime over the soil, and till it in.
Work a 4-inch depth of organic matter into the top foot of soil. Mix it in thoroughly with a rototiller to avoid pockets of soil and organic matter, which will interfere with water movement. You can do this in the fall, or in the spring as soon as the soil can be worked. Well-aged cow manure, leaf mold, composted pine bark mulch and sphagnum peat moss all work well. Sphagnum peat moss will lower the soil pH.
Plant the dahlia tubers in the spring when the soil temperature rises above 55 degrees Fahrenheit. Plant tall dahlia tubers 6 to 7 inches deep and shorter dahlia tubers 2 to 3 inches deep. Dig the hole, and lay the tuber in the hole with the eye facing up. Then fill the hole with soil. The eye is a small, raised bump where the stems grow from the tuber. Plant 1-foot-wide cultivars 2 feet apart, 2-foot-wide cultivars 3 feet apart and 3-foot-wide cultivars 4 feet apart.
Dahlia Care After Planting
Do not water dahlia tubers right after planting. They won't be able to use the moisture and will rot. Water them when the new stems emerge. Spread a 2-inch depth of mulch over the soil around the dahlias after the stems are a few inches high. Water the dahlias once or twice each week, as necessary, to keep the soil uniformly moist. They usually need a minimum of 1 to 1 ½ inches of water each week but may need more as temperatures rise during the summer.