Originally native from Europe to China, Korea, and Japan, their large showy flowers have made them popular worldwide. There are over 60,000 registered cultivars. Only a few cultivars are scented. Some cultivars rebloom later in the season, particularly if their developing seedpods are removed.
Daylilies occur as a clump including leaves, the crown, and the roots. The long, often linear lanceolate leaves are grouped into opposite flat fans with leaves arching out to both sides. The crown of a daylily is the small white portion between the leaves and the roots, an essential part of the fan. Along the flower stem or scape, small leafy "proliferations" may form at nodes or in bracts. These proliferations form roots when planted and are the exact clones of the parent plant. Some daylilies show elongated widenings along the roots, made by the plant mostly for water storage and an indication of good health.
The flower consists of three petals and three sepals, collectively called tepals, each with a midrib in the same or in a contrasting color. The centermost section of the flower, called the throat, has usually a different and contrasting color. There are six stamens, each with a two-lobed anther. After pollination, the flower forms a pod.
Daylilies can be grown in USDA plant hardiness zones 1 through 11, making them some of the most adaptable landscape plants. Most of the cultivars have been developed within the last 100 years. The large-flowered clear yellow 'Hyperion', introduced in the 1920s, heralded a return to gardens of the once-dismissed daylily, and is still widely available. Daylily breeding has been a specialty in the United States, where their heat- and drought-resistance made them garden standbys during the later 20th century. New cultivars have sold for thousands of dollars, but sturdy and prolific introductions soon reach reasonable prices.
Tawny Daylily Hemerocallis fulva, and sweet-scented H. lilioasphodelus (H. flava is an illegitimate name), colloquially called Lemon Lily, were early imports from England to 17th century American gardens and soon established themselves. Tawny Daylily is so widely growing wild that it is often considered a native wildflower. It is called Roadside or Railroad Daylily, and gained the nickname Wash-house or Outhouse Lily because it was frequently planted at such buildings.
Hemerocallis is one of the most hybridized of all garden plants, with registrations of new hybrids being made in the thousands each year in the search for new traits. Hybridizers have extended the plant's color range from the yellow, orange, and pale pink of the species, to vibrant reds, purples, lavenders, greenish tones, near-black, near-white, and more. However, a blue daylily is a milestone yet to be reached.
Other flower traits that hybridizers develop include height, scent, ruffled edges, contrasting "eyes" in the center of the bloom, and an illusion of glitter or "diamond dust." Sought-after improvements in foliage include color, variegation, disease resistance, the ability to form large, neat clumps and being evergreen or semi-evergreen instead of herbaceous (also known as "dormant" — the foliage dies back during the winter.)
A recent trend in hybridizing is to focus on tetraploid plants, with thicker petal substance and sturdier stems. Until this trend took root, nearly all daylilies were diploid. "Tets," as they are called by aficionados, have double the number of chromosomes as a diploid plant. Only one cultivar is known to be triploid, the brilliant orange 'Kwanzo' or 'Kwanso,' which cannot set seed and is reproduced solely by underground runners (stolons) and division. Usually referred to as a "double," meaning producing flowers with double the usual number of petals (e.g., daylily 'Double Grapette'), 'Kwanzo' actually produces triple the usual number of petals.