Through centuries of hybridizing and selection, dahlias, which originated from Mexico and Guatemala, have become tremen- dously diversified, available in numerous flower types and flower sizes (from 2 to 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 llers 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, are striking cut flowers. Leaves are generally divided into many large, deep green leaflets. Some forms have burgundy leaves; a well- known example is 'Bishop of Llandaff'. Not browsed by deer.
Dahlia flower forms. Dahlia flowers are composite (daisylike) blooms containing many individual flowers called florets. One type of dahlia flower is composed of ray florets (which look like petals) surrounding a central cluster of petal-less disk florets. A second type has ray florets only. The American Dahlia Society has classified dahlias according to the flower forms described below. Blooms range in size from giant (over 10 in.), large (810 in.), and medium (68 in.) to small (46 in.), miniature (24 in.), and mignon (under 2 in.).
Anemone form. Single or multiple layers of rays surround tubular disk florets that form a pincushion center.
Collarette. One layer of long rays and a second, inner layer of shorter ones that form a collar around the center of the flower.
Orchid form. A single layer of rays with inrolled margins for two-thirds or more of their length, giving the flower a pinwheel appearance.
Peony form. Central disk florets are surrounded by two or more rows of rays; innermost rays may be curled or twisted.
Single. One layer of rays arranged in a plane around a central cluster of disk florets.
The following flower forms are composed of ray florets only.
Ball. Flower looks spherical, though it's flattish in profile. Rays have inrolled margins for at least half their length.
Cactus form. Ray margins roll downward; tips are pointed. Straight cactus rays radiate in all directions from center; may be straight or curved downward, margins rolled for over half their length. Incurved cactus rays are similar, but they curve upward. Semicactus flowers have broad-based rays with margins rolled along outer half (portion farthest from center); the rays may be straight or curve upward or downward.
Decorative. Full flowers of two types. Formal decorative has many overlapping layers of symmetrically arranged, fairly flat rays that tend to curve downward. Informal decorative is just as full, but rays are curved, curled, or twisted and are often arranged in a more irregular pattern.
Fimbriated. All rays are split at tips with split portions twisted, giving the flower a fringed look.
Pompon. Similar to ball form, but rays are inrolled along their entire length, giving them a tubular appearance.
Waterlily form. Broad rays curve slightly upward; flower profile is flat to saucer shaped, resembling a waterlily bloom.
Novelty. Any flower form not covered in previous categories.
D. imperialis. TREE DAHLIA. Zones LS, CS, TS; USDA 8-11; hardy in MS, but frost there usually kills flower buds before they open. Multistemmed tree grows each year from permanent roots to a possible 1020 ft. tall, 46 ft. wide. Daisylike, 4- to 8-in.-wide lavender flowers with yellow centers bloom at branch ends in late fall. Leaves divided into many leaflets. Frost kills tops completely; cut back to ground afterward. Annual dieback relegates tree dahlia to tall novelty class; available from specialists. Grow from cuttings taken near stem tops (or from side shoots) in fall; root in containers of moist sand kept in a protected place over winter. Or dig root clump and divide in fall. 'California Angel' has double white blooms.