Dahlias love sun, water and fertilizer. At the same time, the soil must be well drained and you should avoid planting too close to shrubs or under trees. Most dahlias will thrive even in partial shade. Since plants may vary in height from dwarf bedding types to 7 foot giants, set out the tubers accordingly, the taller varieties to the rear of the garden and the smaller varieties as border decorations.
A basic soil conditioner that will improve any garden is an application of peat moss and manure. If your soil tends to be heavy, add sand or commercial soil conditioner. About two weeks before planting your tubers an application of low nitrogen fertilizer such as 4-10-10 should be broadcast over the planting area and worked into the soil.
Set out tubers after the last frost. In most Northwest gardens this will be between April 15 and June1. Dig 5-inch deep holes, about a foot in diameter and place a support stake in the center of each. The growing point (or “eye”) should be close to the stake. Then cover the tuber with soil. When the plant is about two feet tall, tie the plant to the support stake.
As the growing tips emerge, they become a favorite food of slugs. Place slug bait around the base of the plant on a regular basis. Once mature, dahlia plants are harmed primarily by aphids, spider mites, and earwigs. A spray program may be implemented. With a pesticide. Check local regulations. Most gardens, with just a few dahlia plants, will not require spraying.
Water every two weeks – more frequently if it is very hot and when blooms begin to form. A thorough, deep watering is better than frequent light sprinkling.
PINCHING AND DISBUDDING:
To create a compact, bushy plant with more flowers, pinch out the central growing tip when the dahlia plant is about a foot high. For bigger blooms, better stem and quality form, pinch off the two side buds at the end of each growing branch. Continue this disbudding throughout the growing season. As is true for many other flowering plants, the removal of spent blooms encourages greater flower production and prolongs the blooming season.
Around November 1, cut dahlia plants to within 6 inches of ground level. Carefully dig a circle about two feet in diameter around the plant and gently lift the root ball. Tubers should be washed to remove attached soil. Roots may be soaked in a fungicide such as Captan. The root clumps may be labeled and stored as a clump or divided into individual tubers. Make sure that a piece of the main stem with an ‘eye’ is attached to each tuber. Dahlias will keep best if stored in a cool area not subject to freezing temperatures. Most growers store their tubers in a medium such as slightly damp vermiculite or sand in order to prevent shriveling.