The daylilies comprise the small genus Hemerocallis of flowering plants in the family Hemerocallidaceae. The name Hemerocallis is based on the Greek words for day and beauty, which reflects the fact that the individual flowers last for only one day. They open at sunrise and wither at sunset, to be replaced by another one (sometimes two or none) on the same stem the next day. Because of this, they do not make good cut flowers for flower arranging.

Originally from Eurasia, native from Europe to China, Korea, and Japan, their large showy flowers have made them popular worldwide, and there are over 60,000 registered named cultivars. Only a few cultivars are scented; some will rebloom later in the season, particularly if their developing seed pods are removed.

The alternating lanceolate, alternating leaves are grouped into fans (a clump also containing the roots and the crown). The crown of a daylily is the small white portion of the stem, between the leaves and the roots. This crown is an essential part of the fan. Along the scape, 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 spindlelike widenings at the roots, used mostly for water storage.

The flower consists of three petals and three sepals, 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 all 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 above all in the United States, where the heat- and drought-resistant qualities of Hemerocallis made them garden standbys during the later 20th century. New cultivar introductions have sold for thousands of dollars, but sturdy and prolific introductions soon reach reasonable prices.