Maurice - So cold won't damage a plant as much as heat in many cases? I live in a hot zone and have no experience with freezing and snowy conditions. I assume the soil will protect the plants in cold temps, but can not protect them in high temps?
Each plant species is different. A tropical plant might be damaged by 40F but a daylily would not be. It depends on the type of plant.
If you take a daylily that is happily growing at 75F and put it into 25F it will suffer. If you slowly lower the temperature day by day to 25F the daylily will stop growing and usually not suffer. If you took that same daylily growing at 75F and put it into 100F it will probably suffer. Again, if you raised the temperature day by day the daylily will probably acclimate and might not suffer.
We do not have substantial climatological information about how and where the original daylily species grew. Some of them grew at high elevations. Some of them grew at northern latitudes. Only a few of them grew at more southern latitudes. I expect that in general daylilies do better in what we might consider to be moderate temperatures rather than hot temperatures.
More modern cultivars, particularly those bred in Florida and southern areas might do better in warmer temperatures than older cultivars but that depends on just how many hybridizers in Florida (and other warmer locations) use shade cloth and how many do not use daylilies that do not do well at high temperatures in their hybridizing.
