Cold tolerance isn't related to the type of daylily. Hardy evergreens and tender dormants exist, but tender evergreens and hardy dormants are more common. Dominant and semi-evergreen daylilies require a cold period. Without this cold period, they will gradually decline, reduce flowering and may die. Breeding efforts have resulted in new daylily cultivars that are tetraploid instead of the standard diploid.
Tetraploid daylilies have twice as many chromosomes as the diploids.
The tetraploid daylilies have larger flowers, more intense but fewer colors, and sturdier scapes that better withstand wind. However, the tetraploid size may be overwhelming in small gardens. Many people still prefer the diploids because of their fine form, grace and diverse colors. Bloom Type Reblooming daylilies, those that flower more than once during the season, are available. Some flower in the spring and then rebloom in the fall.