Dahlias offer flamboyant flowers on lush plants from summer through fall, right up to the first frost. Fully hardy to zone 8, these hybrids of species native to Mexico and Colombia may overwinter in Zone 7 with a thick blanket of mulch. Gardeners in colder zones can get a head start by planting tubers in pots 4 to 6 weeks before the last spring frost date. Plant tubers several inches deep in a light, soilless potting mix and water sparingly until new growth appears, then more freely. Place pots in a sunny window or under grow lights, and then plant outside after danger of frost has passed. Taller varieties will need staking, with stakes placed carefully so as not to injure the tuber or roots. If planting, either indoors or in the ground, is delayed, store the tubers in their bags in a cool, dry, dark location.
Light/Watering: Dahlias are at their best when grown in full sun in the North, afternoon shade in the South. Do not water until growth appears above the ground; once plants are established, a deep watering twice a week will get them through summer dry periods.
Fertilizer/Soil and pH: Soil temperature at planting should be 60°F. Dahlias prefer an open, well-drained soil with a pH of 6.5 to 7.0. A fertilizer low in nitrogen is best (5-10-10, for example), first applied 30 days after planting and then side-dress monthly. Do not overfeed, and avoid high-nitrogen, water-soluble products.
Pests/Diseases: Aphids can be killed by spraying with insecticidal soap or Neem. If Dahlia stems show breakage and wilting, borers may be present; to deter, keep weeds away from the planting, and cut off and destroy any larvae-infested stems. If leafhoppers are a problem, spray plants with a mix of one tablespoon isopropyl alcohol to one pint of insecticidal soap, repeating 3 times at 3-to-5-day intervals. Watch for spider mites during hot, dry weather; spray leaves with Neem or a forceful jet of cold water, particularly on the undersides. If powdery mildew appears as a whitish coating on the leaves, spray with wettable sulfur or other appropriate fungicide. Next year, give plants more space for better air circulation. If stems rot at the soil line or plants suddenly wilt and die, a bacterial or fungal agent may be present. Remove and destroy any affected plant parts; avoid this problem by planting in well-drained, light soil and do not overwater. Keep mulch several inches away from the plant stems.
Companions: Dahlias can hold their own among Roses and Oriental Lilies, are lovely with many annuals, and pair beautifully with ornamental Goldenrod (Solidago) and fall-blooming Asters.
Pruning: Dahlias make excellent cut flowers; to achieve nice stems for cutting and bushier, compact plants, pinch out the center shoot just above the third set of leaves. These shoots can be rooted, if desired. To get the most out of your cut flowers, place them in very hot water (160°F) until it cools.
Reflowering: If dead flowers are diligently removed, flowering will continue until the first frost. To develop large, exhibition-size blooms, remove side buds and allow only one bud per stem to develop.
Dividing/Transplanting: Dahlia tubers may be divided in spring or fall. Use a sharp knife to cut the tubers apart—only those tubers with an eye (a dormant bud) will grow stems. Not all tubers will have eyes, so cutting a clump into halves or quarters is safer than cutting off individual tubers. Allow roots to dry for a day before storing or planting. If you have more than one variety, label each tuber.
End-of-Season Care: Wait a few days after the foliage is blackened by frost before digging the tubers to store for the winter. If plants are in a frost-free area, dig by mid-November. Cut the stalk to 4–6″ tall, rinse off the soil, and allow the clump to air dry under cover for 24 hours. Line cardboard boxes or terra-cotta pots with newspaper and layer tubers with barely moist sawdust, sand, or peat. Do not store in plastic. Keep boxes cool (40–50°F) and dry for the winter in a dark spot and check for rot or shriveling on a monthly basis; if shriveling occurs, mist the packing material lightly with water. Remove all old foliage from the garden area.
Calendar of Care
Early Spring: Dahlia tubers may be divided now. Plant tubers 6″ deep when soil has warmed after frost or start early indoors in pots. Water tubers sparingly once after planting and then do not water until new growth appears. Fertilize with 5-10-10 when growth reaches 2″ tall.
Mid-Spring: Continue feeding every month, side-dressing with 5-10-10. Watch for aphids and other insect pests and treat accordingly. Keep weeds away from plantings.
Late Spring: Mulch if desired but keep material away from crowns and stems. Water thoroughly if season is dry. Tie larger varieties to stakes as they grow. Pinch out center shoots just above the third set of leaves, or leave shoots and remove side buds for extra-large flowers.
Summer: Watch for signs of fungal wilt and remove and destroy affected plant parts if it occurs, then sterilize pruners with bleach solution. Monitor plants for aphids and other insect pests and treat accordingly. Continue to water if conditions indicate, and deadhead diligently for continuous bloom.
Fall: Mulch plants heavily if overwintered in the ground in Zones 7 and above. Further North, wait until a few days after frost has killed the foliage, then dig tubers, air dry under cover and store cool, dry and dark in newspaper-lined cardboard boxes or clay pots. Cover tubers with lightly moistened sand or peat. Tubers may be divided before storing if desired. If you have more than one variety, label each tuber.