Daylilies as a group are hardy from U.S. Department of Agriculture planting zones 2 to 10, which include most of North America. Not all varieties of daylilies, however, are hardy in every zone. Several factors determine how specific plants react to temperature.
Siloam hybrids were originated by an Alabama breeder for Southern summers.
Hybridizers select plants that survive in hot or cold climates. To get the proper daylily for your garden, buy from daylily growers in areas with climate conditions similar to yours.
Deep colors may fade in the heat.
Plants for hot climates must have "sun-fast" flowers–meaning their color does not change if they receive too much sun. Dark colors, such as red or purple, may need more shade in hotter zones to remain consistent, according to the Bay Area Daylily Buds website, while most pastel daylilies can take the sun. Long growing seasons encourage continued blooming or re-blooming when spent flowers are deadheaded promptly.
Daylilies can be fully evergreen only in the South.
Fern-leaf and evergreen daylily plants are less cold-tolerant than other varieties. Evergreen plants may be semi-evergreen or deciduous in growing zones where winter temperatures fall below freezing.
The fungus Puccinia hemerocallidis, one of the daylily’s few pests, does not over-winter in zones 6 and colder. The University of Georgia found that germination of the virus declined sharply beginning at 86 degrees F, suggesting that the fungus is most dangerous between 72 and 86 degrees.
Pollen is most viable when temperatures are moderate in early summer.
Daylily seed fertilization depends on pollen viability. A 2007 American Hemerocallis Society paper, "Pollen (Dead or Alive?)," suggests that pollen begins to decrease in viability from 85 to 95 degrees.