Dozens of colors and color combinations, plant sizes and growth habits make the daylily popular. Even the most outrageously ruffled double flowers, however, are descended from single flowers. The transition from single- to double-petal daylilies takes generations.
Daylilies have three sepals, the tough bud skins that open to form the basis for the flower. They have three petals that surround six stamens and a pistol in the eye of the bloom. Breeding a double flower requires selecting plants whose stamens have extra tissue, making them "petaloids." Over several growing seasons, the petaloids become large enough to form a second whorl of petals.
Diploids with two sets of 11 chromosomes provided early breeding stock and breed most easily. Some cultivars, including the double-flowered Kwanso, naturally grow double but are, unfortunately, triploids and are incompatible with other plants. Late-20th-century breeders produced large, bright tetraploid blooms, some of which were doubles.
Classic Mendellian genetics dictates that plants with matching recessive traits must be matched for the trait to become dominant and appear in the offspring. The tendency to form petaloids is a recessive trait that must be identified repeatedly to encourage the development of new petaloids.