Propagate by division or separation - Divide and plant overwintered tuberous roots in spring. Make sure that each cut piece has an "eye" from which growth can occur.

For larger plants and earlier blooms, plant the tuberous roots indoors 6 weeks before last spring frost. Give the plants adequate light and well-drained soil, and transfer them to the garden when frost danger has passed.

For direct outdoor planting, plant the roots in a 6- to 8-inch deep hole, and refill the hole half way. Add additional soil as plant grows, until the hole is filled in level with the ground.

Keep well-watered but not soggy. Avoid wetting leaves as this can promote foliage diseases. Even during wet seasons, supplemental watering may be needed to produce maximum growth and bloom.

Plants usually require staking. Install stake at planting to avoid damaging roots later.

Dahlias are heavy feeders and perform best in rich, well-drained soil. They usually benefit from high nitrogen fertilizer early in the season until flower buds form, and high potassium fertilizer from midsummer to early fall. Over-fertilization with nitrogen can produce lush foliage but poor blooms, and may encourage the tuberous roots to rot in storage overwinter.

When the plants have have 4 leaves, pinch off the top of the plant to encourage bushiness. Pinch back excess buds for larger flowers. Deadhead faded flowers.

After the foliage dies back following the first fall frost, cut the plants back to 6 inches, gently dig the tuberous roots and bring them indoors. Allow the soil to dry out, then remove the excess soil and carefully inspect the roots and discard any which look unhealthy. Store in a cool (not freezing) well-ventilated location for the winter. Inspect the roots regularly for disease during overwintering.

More growing information: How to Grow Annuals

More growing information: How to Grow Bulbs

Aphids
Stem borers
Spider mites
Caterpillars
Earwigs
Cucumber beetles
Capsids
Flower thrips
Planthoppers
Slugs and snails

Powdery mildew
Impatiens necrotic spot
Smut
Fungal leaf spot
Dahlia mosaic viruses
Soft rot
Crown gall
Blossom blights
Gray mold
Tomato spotted wilt virus

There are hundreds of varieties of Dahlia to choose from. Below are some of the more common classes of Dahlia, based on the size and form of their flowerheads.

For more variety information: American Dahlia Society

Ball-flowered cultivars have rounded, dense, 2- to 8-inch-wide blooms composed of wide, slightly curled petals. The flowers sometimes have flattened tops.   Plants grow 3 to 4.5 feet tall.

Pompom-flowered cultivars have rounded, 2- to 4-inch-wide, globular blooms composed of wide, rounded petals. The outer petals are wholly curled, the inner petals are often gently curled. Plants grow 2.5 to 4 feet tall.

Collarette cultivars have unusual 2- to 6-inch-wide blooms that consist of an outer ring of nearly flat, large petals, and an inner ring of short, small petals that surround the central disk as a "collar." Plants grow 2.5 to 4 feet tall.

Fimbriated cultivars have large, 6- to 8-inch-wide blooms with split petals that give the flowers a fringed look. Plants grow 4 to 5.5 feet tall.

Formal decorative cultivars have large, 2- to 10-inch-wide flattened blooms composed of wide, flat petals. Plants grow 4 to 5.5 feet tall.

Informal decorative cultivars have 2- to 10-inch-wide (or larger) blooms comprised of wide petals that may be curled or twisted to give the overall bloom a casual, full look. Plants grow 4 to 5.5 feet tall.

Some groups have flowers that resemble other species:

Anemone-flowered cultivars have blooms that consist of a ring of flat outer petals and an inner ring of tubular inner petals. The blooms resemble anemone blooms, hence the common name. Plants grow 2 to 4 feet tall.

Orchid-flowered cultivars have exotic and unusual, 2- to 4-inch-wide, star-like blooms composed of a single row of tightly curled petals. Plants grow 3.5 to 4.5 feet tall.

Peony-flowered cultivars have 2- to 6-inch-wide blooms composed of two to many rows of wide, rounded petals that often gently curve inward. Plants grow 3 to 4 feet tall.

Single-flowered cultivars bear daisy-like, 1- to 6-inch-wide blooms. The flowers consist of a single row of petals around a central disk. Plants grow 1.5 to 2 feet tall.

Waterlily cultivars bear 4- to 8-inch-wide blooms that consist of flat, cupped, wide petals that resemble water lilies. Plants grow 3 to 4.5 feet tall.

Some groups have flowers that resemble cactus flowers:

Straight cactus-flowered cultivars have 2- to 8-inch wide blooms composed of downward curled petals that are arranged in either a flat or recurved manner. Plants grow 4 to 5.5 feet tall.

Semi-cactus-flowered cultivars have 2- to 10-inch-wide (or larger) blooms composed of petals that are curled for up to one half their total length. The petals may be arranged in a flat, incurved, or recurved manner. Plants grow 4 to 5.5 feet tall.

Incurved cactus-flowered cultivars have 2- to 8-inch-wide blooms composed of curled, nearly tubular petals that curve inward. The outer petals are only gently incurved, and the inner petals are increasingly incurved as they near the center. Plants grow 4 to 5.5 feet tall.