Don't let the showy appearance fool you - dahlias are easy to grow. Prepare the soil by digging to a depth of about a foot and amending with compost if necessary before planting. Set the tubers 3 to 4 inches deep about 10 days before the last expected frost. The distance between plants depends on the variety you are planting. A spacing of about 2 feet will work for most varieties.
Tall varieties and those with huge flowers will require staking. For easy support, plant dahlias around a trellis or fence. When planting in a mixed border, it's tempting to plant them in the back because of their height, but make sure they are accessible because in zones 7 and colder they will have to be dug up after the first frost.
Dahlias need full sun and appreciate steady moisture and regular feedings until the flowers start to appear. At this time, taller varieties may need to be staked. For a bushier plant, pinch out the growing shoot after plants have produced 4 to 6 pairs of good, strong leaves. A number of buds will appear at the top of the stems. To get maximum sized flowers, some of these buds should be pinched out. If only one bud is allowed to develop, the largest possible flowers will be produced. It takes about 6 weeks from the time buds are removed until another series of buds is formed and about three weeks longer for the buds to open.
Dahlias As Cut Flowers
Dahlias are beautiful and long lasting cut flowers, and cutting encourages to plant to produce more blooms. Cut fully open flowers early in the morning. Use a sharp knife to avoid crushing the stems, and then dip the stems in hot water to seal and remove foliage from the lower part of the stems below the water level before placing them in the vase. Leaves will yellow if they don't have enough light, so set your arrangement in a sunny window. A solution of 1/4 teaspoon of bleach plus 2 teaspoons of sugar to 1/2 gallon of water will prolong the vase life. Cutting the bottom 2 inches from the stems every other day will help to keep the flowers looking fresh.
Insects and Diseases
Indications: Aphids will be visible on the foliage. Also look for curled leaves and a wet, sticky film on foliage.
Treatment: Soap-Shield, yellow sticky traps and ladybugs.
Indications: Spider mites are tiny and difficult to see. Look for spotted leaves, bronzing leaves, fine webbing on foliage and general loss of vigor.
Thrips: Indications: Thrips are very tiny, but sometimes you can see them on the blossoms. They fly and run rapidly, and are almost impossible to catch.
Treatment: Soap-Shield and yellow sticky traps.
Dahlias are susceptible to fungal and viral diseases and bacterial wilts. Light yellow vein banding, wavy yellow lines and rings or spots of yellow on the leaves are indications of viral diseases. Other indicators include stunted growth or abnormally shaped leaves. Once a plant is infected with these diseases there is nothing you can do but remove it before the disease spreads and destroy the plant. Don't compost diseased plants!
If you live in an area where the ground freezes in winter, you will have to lift your dahlias and store them in a cool but frost-free area over the winter. This usually includes zones 7 and colder.
When bud production slows and flowers are poorly colored it's time to cut the plants back. If you grow several types of dahlias, place a tag around the base of each plant for identification before cutting. It's best to cut the stems through a node where it will be solid. If the weather is dry, let the tubers stay in the ground for a week to ten days, but lift the tubers as soon as possible if you suspect rain or a hard freeze.
To lift the tubers, dig into the soil about 8 to 10 inches away from the stems. Lift gently to pry the roots away from the soil. When all roots have been loosened, dig under the tuber and firmly remove it with the ball of soil. Remove all loose soil, but leave moist soil that clings to the roots and tubers. Brush off the remaining soil as it dries out.
Once clean, store the clumps of tubers in a cool area with good ventilation. The tubers can be placed in dry sand with the stems exposed. Check them monthly to make sure they aren't shriveling, which means they are too warm or too dry. Soft spots should be removed with a sharp knife. To prevent rot, dust with an organic fungicide.
Start seeds indoors in early spring. The seeds will germinate a little faster if you use a heating mat, or place them on top of a refrigerator for heat. Begin to harden off when night temperatures stay above 50 degrees. Seedlings will not tolerate frost, so plant them out after your last expected frost date. Many dahlia seeds available for purchase are grown as annuals because the tubers produced by these plants won't result in flowers similar to the parent plant.
Large tubers can be divided. A tuber won't grow unless it has an 'eye' or a little sprout growing from it, so make sure that all your divisions contain at least one eye.
Cuttings taken when the shoots are 3 to 4 inches long are easily rooted in sandy soil. Use a rooting hormone for best results.