Native to Mexico and Guatemala. Grown in the Zones 1–24, but must be lifted and stored in cold-winter climates (see below).
Through centuries of hybridizing and selection, dahlias have become tremendously diversified, available in numerous flower types and flower sizes (from 2–12 in. across) and all colors but true blue.
Bush and bedding dahlias range from 1 ft. to over 7 ft. tall. The tall bush forms are useful as summer hedges, screens, and fillers among shrubs; lower kinds give mass color in borders and containers. Modern dahlias, with their strong stems, long-lasting blooms that face outward or upward, and substantial, attractive foliage, make striking cut flowers. Leaves are generally divided into many large, deep green leaflets.
Planting. Most dahlias are started from tuberous roots planted in spring after frost is past and soil is warm. Several weeks before planting, dig the soil in the planting area to 1 ft. deep and work in plenty of organic matter, such as garden compost or ground bark.
At planting time, dig a foot-deep hole for each root; make holes 1 1/2 ft. wide for dahlias that grow over 4 ft. tall, and 9 1/2 in. wide for smaller types. Space holes for larger varieties 4–5 ft. apart, smaller ones 1–2 ft. apart.
Incorporate about 1/4 cup of granular low-nitrogen fertilizer into the soil at the bottom of each planting hole, then add 4 in. of soil before placing the root in the center of the hole, with its growth bud pointing up.
Before planting a tall variety, drive a 5–6-ft. stake into the hole just off center, then place the root horizontally in the bottom of the hole, 2 in. from the stake and with the growth bud pointing toward it.
Cover roots with 3 in. of soil, and water thoroughly. As shoots grow, gradually fill the hole with soil.
Dahlias can be started from seed. For tall types, start seeds early indoors; transplant seedlings into garden beds after frost danger is past. For dwarf dahlias, sow seed in place after soil is warm, or buy and plant started seedlings from the nursery. Dwarf dahlias are usually replaced each year, though they can be lifted and stored.
Thinning, pinching. On tall-growing dahlias, thin out shoots when they’re about 6 in. high, leaving only the strongest one or two. When the remaining shoots have three sets of leaves, pinch off tops just above the upper set; dahlias with smaller flowers, such as pompons, singles, and dwarfs, need only this first pinching. For the best show of larger-flowered types, pinch again by removing all but the terminal flower buds on side shoots.
Plant care. After shoots are aboveground, start watering regularly to a foot deep; continue throughout active growth. Dahlias planted in enriched soil shouldn’t need additional food, but if your soil is light or if roots stayed in the ground the previous year, apply a granular low-nitrogen fertilizer when the first flower buds show. Mulch to discourage weeds and to eliminate cultivating, which may injure feeder roots.
Cutting flowers. Pick nearly mature flowers in the early morning or evening. Immediately place cut stems in 2–3 in. of hot water; let stand in gradually cooling water for several hours or overnight.
Lifting, storing. In climates where the ground freezes in winter, dig and store tuberous roots in fall. In other regions, roots may remain in place as long as drainage is excellent and winter temperatures remain above 20°F/–7°C. In borderline climates, mulch with a 4-in. layer of straw or similar material. Gardeners in most areas, however, prefer to dig the roots annually.
To lift the roots, cut stalks to 4 in. above-ground after the tops turn yellow or are frosted. Dig a 2-ft.-wide circle around each plant; carefully pry up the clump with a spading fork, shake off loose soil, and let the clump dry in the sun for several hours. From that point, follow either of the following methods.
Method 1: Divide the clumps immediately. Freshly dug roots are easy to cut, and eyes (growth buds) are easy to recognize at this time. To divide, cut the stalks with a sharp knife, leaving 1 in. of stalk attached to each section; make sure each division has an eye, so it will produce a new plant. Dust cut surfaces with sulfur to prevent rot; bury in dry sand, sawdust, peat moss, or perlite and store over winter in a cool (40° to 45°F/–4° to 7°C), dark, dry place.
Method 2: Leave clumps intact. Cover them with dry sand, sawdust, peat moss, or perlite and store in a cool, dark, dry place as directed in the method above. With Method 2, roots are less likely to shrivel. About 2 to 4 weeks before planting in spring, separate the intact clumps, cutting them apart as described under Method 1. Then place all roots whether fall- or spring-divided in moist sand to plump them up and encourage sprouting.