Flowering season for daylilies will always depend not only on where the daylily was hybridized but also on the weather each year in the location where it is grown, as well as the actual growing conditions provided by the gardener.
Daylilies do not use any specific conditions (usually related to the weather) to start the development of their scape and flowers. When they are a certain size they will start to produce a scape and produce a new growing point. When the new growing point is large enough the next scape will be started. Since temperature very strongly affects how fast plants grow and develop it will strongly affect how quickly a daylily produces its scapes and therefore when it flowers.
Temperature will affect the flowering of daylilies in much the same way that it affects the amount of pigment in flowers. There will be an optimum temperature for the development of the scape.
Arisumi, a researcher, grew the cultivar 'Purity' at 55F, 65F, 75F, 85F and 95F. He brought groups of plants inside to grow at the specific temperatures on different dates. Those brought inside in November and December are described below.
The average time to first flower depended on the temperature. The plants grown at 55F (growth was too slow) and 95F (growth was too poor) did not flower during his observations.
At 65F one set of plants (brought inside in November) flowered but after the end of the observations and when the temperatures were no longer controlled and were above 65F. Arisumi estimated that they would have started flowering after 170 days (November). At 75F the plants started flowering about 135 days (November). At 85F the plants (November) started flowering about 109 days.
For plants brought inside in December, those grown at 65F were estimated to start flowering in about 140 days. Those grown at 75F started flowering about 107 days and the one that flowered at 85F started after 109 days.
Temperatures of 85F and 95F were detrimental to the growth of the daylilies, specifically the cultivar 'Purity'. Presumably, that may have been because the temperatures were constant for both day and night. Arisumi wrote "At 85° and 95° the plants grew rapidly during the first 3 to 4 weeks and then became progressively chlorotic and the older leaves dried prematurely"
