I'm not sure that there are "dormants" that require winter cold hours. I think some "dormants" probably require summer temperatures that are lower than those in some parts of Florida and other southern regions. I know of no good tests of daylily cold temperature requirements for growth that show that there are any daylily cultivars that require cold.
Hybridizers in locations which do not have as much summer heat as other locations cannot select or hybridize for the ability of daylilies to adapt to such high heat stresses. So "Northern" hybridizers who have or had "dormant" lines (without introducing southern high-summer-heat-bred cultivars into those lines) will have plants that do not grow well in those southern high heat stress locations.
I think northern bred "dormants" will have a tendency to be unable to survive high southern summer heat stress rather than needing cold during winters.
That follows from what Arisumi found when he tested different growing temperatures on daylily growth and flowering. He tested 55°, 65°, 75°, 85°, and 95° F.
He found "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."
For flowering he found, "Except for 4 plants at 85° F, flowering was obtained only at 75°." That is, only four plants out of 24 flowered when grown at 85F; none of 24 plants flowered when grown at 95F; yet 24/24 plants flowered when grown at 75F. Growth is always slower at low temperatures so there was not enough time to check for flowering at 55F and 65F within the time limits of his experiment. However, 8/16 plants that had been grown at 65F had scapes present when the experiment ended.
So it is unlikely that daylilies hybridized in the north (without the input of southern-selected plants) would be good candidates for high heat southern gardens. Typically northern hybridizers would produce more "dormant" than evergreen introductions and so those plants would not grow well in the south but not because they are "dormant" but because they cannot handle the high heat stresses.
I would think that good examples of northern-bred lines that cannot handle high heat stress of southern locations would be introductions hybridized by Burkey, perhaps Hite, etc.
