To get rebloom from a daylily one needs to supply that daylily with growing conditions that support strong growth. I have found that I can have a diploid daylily growing in average garden conditions and it never reblooms (not once in 10 years). And then I start to give it fertilizer (high nitrogen number) in liquid form (so it is also watered), keep it weed-free and once it has put on a substantial amount of good growth (about a year's worth) it will rebloom. I am in zone 4. If I stop giving it good growing conditions rebloom will dwindle and then disappear. Every plant I have tried this with (diploid & tetraploid) has rebloomed. I expect that there will be some that do not but information from both California and Florida even from the 1940s indicates that probably all daylilies can rebloom given the appropriate growing conditions and a long growing season is not by any means always needed but good growing conditions that foster strong growth (that do not limit the plant's ability to grow) are needed usually.
