Siting and Planting
Dahlias prefer full sun in all but the hottest climates and sandy, fertile loam with a pH between 6.5 and 7.0. Where summer temperatures sizzle, give them morning sun and afternoon shade. To lighten the soil and improve fertility, dig 2 inches of composted manure into the dahlia bed's top 1 foot of soil at least a couple of weeks before you plant them. For each tuber, make a hole 1 foot deep. After tossing a handful of bone meal into the hole, cover the meal with 4 inches of soil. Lay a dahlia tuber horizontally on top of that soil with its eye facing upward, and insert a 5- or 6-foot stake into the ground about 2 inches from the eye. Shovel 3 inches of soil over the tuber before making the next hole 3 to 4 feet from the first one.
Watering and Fertilizing
If the soil is already damp, don’t water it until the dahlia shoots begin to emerge. Irrigate dry soil only enough to keep it lightly moist as dahlia tubers rot easily at this stage. After dahlias send up shoots, gradually fill in the remaining soil around those shoots. When the holes are full and the plants are up, make sure they receive at least 1 inch of water per week. Once a week, feed them with 2 tablespoons of an organic liquid tomato fertilizer such as 4-6-2 mixed into 1 gallon of water.
Mulching and Weeding
Don't mulch dahlias until after their holes are full and their shoots have emerged. At that point you can spread 2 inches of wood chips, straw or another organic material around them, keeping it away from their stems. If you can't mulch, it's best to weed dahlias by hand to avoid disturbing the tubers.
Thinning and Supporting
When the shoots have reached 6 inches, thin them to leave only the two most vigorous ones on each plant. When those reach 1 foot, tie them loosely to their stakes with thick twine or other broad, soft ties. After each stalk has three sets of leaves, pinch back its tip to the topmost set of leaves to encourage branching out. As the plants grow, keep an eye on the flower buds, which will appear in clusters of three at the tip of each stem. Once they are the size of peas, remove the two side buds on each stem -- leaving only the bud in the middle -- to promote larger blooms. Continue to add more ties as needed to support the plant and its heavy flowers.