Native to Mexico, the dahlia is a perennial flower that is extremely frost-tender but grows well throughout the warmer spring and summer months. Thousands of dahlia varieties exist, with some that grow up to 8 feet tall and others growing to only 12 inches. Dahlias also come in many different bloom colors and shapes. You should plant your dahlia tubers after all danger of frost has passed, or you can start them indoors about two months before the last expected frost.
Plant your dahlias in full sunlight and in sandy, well-draining soil with an optimal pH of 6.0 to 6.5. Plant them in the spring, when soil temperatures are at least 60 degrees F, spacing them about 1½ to 2 feet apart.
Water your dahlias deeply with a garden hose or watering can, soaking the soil thoroughly once or twice each week during the summer. Begin watering only after the dahlias begin to sprout above the soil surface.
Feed your dahlias with a slow-release, low-nitrogen fertilizer once or twice each year, at planting time in the spring and again when the dahlias begin to bloom. A fertilizer with a 5-10-10 NPK (nitrogen-phosphorous-potassium) ratio is best for dahlias.
Cut back the dahlias when they reach about 1 to 1½ feet tall with scissors or pruning shears. Pinch the main stem back to just above the third leaf set to encourage the dahlias to grow stronger and laterally, instead of growing tall and top-heavy.
Remove all spent flowers in the mornings or evenings to encourage reblooming into autumn.
Get rid of slugs on and around your dahlias by setting out slug baits. Control earwigs on your dahlias by spraying the plants with a mixture of 2 tbsp. of liquid dish soap and 1 pt. of water from a spray bottle.