Daylilies (Hemerocallis spp.) comprise about 15 species of herbaceous wildflowers, but gardeners over the years have created nearly 50,000 named cultivars. Certain cultivars grow better in very cold winter regions, and some prosper even in the subtropics.
According the American Hemerocallis Society, daylily cultivars exist that bloom anytime from early spring to autumn. For example, extra-early blooming varieties flower in March in the Deep South, while those near the Canadian border bloom in June. Very late-blooming selections flower in August in the South, but in late September in the North.
Most daylilies open for only one day and are classified as either diurnal or nocturnal. Diurnal daylilies open their flowers in early morning and close by dusk. Nocturnal daylily flowers open in late afternoon, remain open overnight and close the next dawn. Extended-type daylilies open their flowers for at least 16 hours at a time.
Many modern cultivars are dubbed re-blooming or remontant, meaning they have a primary flowering period, such as late spring, and then sporadically produce more flowers later in the year, such as in early fall.