The many types of dahlia like a fertile well-drained soil with a constant supply of moisture. Mulch the bed and avoid giving the plants high nitrogen. On poor sites the plants are leggy. Best growth occurs in full sun but the plants tolerate partial shade. Do not plant Dahlias until the soil has warmed up as the roots rot in cold wet soil. Tall varieties may need a support stake installed at planting time. Removal of side buds gives larger flowers. Remove the old flowers as they fade to promote blooming. Dahlias do not tolerate high summer temperatures so may not be suited for southern most regions.
Planting month for zone 7: May
Planting month for zone 8: May
Planting month for zone 9: Mar; Apr; Oct; Nov
Planting month for zone 10 and 11: Sept; Oct; Nov; Dec
Origin: not native to North America
Uses: container or above-ground planter; cut flowers; border
Shaded area represents potential planting range.
Leaf arrangement: whorled
Leaf type: simple
Leaf margin: entire
Leaf shape: elliptic (oval)
Leaf venation: pinnate
Leaf blade length: 2 to 4 inches
Leaf color: green
Flower color: yellow; orange
Flower characteristic: showy
Light requirement: plant grows in part shade/part sun
Soil tolerances: clay; sand; acidic; loam
Soil salt tolerances: unknown
Plant spacing: 18 to 24 inches
Roots: not applicable
Winter interest: not applicable
Outstanding plant: not particularly outstanding
Invasive potential: not known to be invasive
Pest resistance: long-term health usually not affected by pests
Use and Management
Large varieties are spaced two and one-half feet apart, pompoms two feet apart, and dwarfs 12 to 18 inches apart. Most are planted 6 to 10 inches deep. When shoots are three or four feet tall, thin them to the three most vigorous shoots. Frosted plants are cut down to four to six inches above ground level. Dig the roots and allow them to dry. Cover them with sand or peat and store them at 50-degrees F. The roots may be overwintered outdoors in USDA hardiness zones 9 and 10. In USDA hardiness zones 9b, 10, and 11, Dahlia is generally planted in the fall for winter and spring color.
Dahlias are propagated by division or cuttings. For cuttings, use new shoots and make cuttings three to four inches long. Remove the basal leaves and insert the cuttings in sand, perlite or vermiculite. The root clumps are divided in the spring. Each division needs part of the old stem on it.
Aphids suck plant juices.
European corn borer larvae feed on young leaves and flowers causing them to be distorted and brown. They bore in the stem causing death of plant parts above the bored area.
Stalk borer also bores in the stems.
Leafhoppers cause leaf discoloration along the leaf margins which spreads toward the mid-vein. The affected area is at first yellowish but becomes brown and brittle. If not controlled, the plants are stunted.
Thrips cause the under surfaces of petals to turn white and wither.
Mites cause paling of the leaves.
Pests and Diseases
Crown gall causes large abnormal growths at the base of the infected plant. The plants are stunted and the shoots spindly. Infected plants are destroyed.
Bacterial wilt causes stem drooping and wilting. Infected stems have a wet soft rot near the soil. Destroy infected plants and rotate plantings.
Bacteriosis causes browning and softening of the stem. The pith is moist and blackened with the rot extending into the bark. A foul odor is associated with the disease. The roots may be infected.
Botrytis blight is worse in dull cloudy weather. The buds rot and infected parts may be covered with grayish powdery mold. Remove infected plant parts.
Powdery mildew causes white or grayish mold. The disease is worse late in the season and badly infected leaves are distorted and drop.
Verticillium wilt causes brown or black streaking in the conductive tissue of the stem. Plants wilt and die. Stored roots rot if infected. Destroy infected plants, rotate plantings.
Dahlias growing in heavy wet soil, and infected with stem rot, wilt and die. A white mold encircles the stem at the soil line. Improve soil drainage and do not crowd the plants. Rotate plantings.
Dahlia mosaic virus causes leaf mottling. Pale green bands develop along the midribs and leaves are dwarfed and show general mosaic or yellow spotting. Destroy infected plants and control the aphids that spread the disease.
Source: U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS