Dahlia (Dahlia) Single, double, and semidouble, 2-inch to 3-inch flowers on plants 12 to 20 inches tall. A few tall varieties, 3 to 5 feet in height with 4- to 5-inch flowers, but most taller varieties are grown as perennials. Soil and Light Full sun. Rich, well-drained soil kept moist. Planting Start seeds 6 to 8 weeks early indoors, and set outside after frost danger. Comments Use dwarf varieties as bedding plants. Taller varieties often need to be staked. Cut flowers of all varieties for fresh arrangements.