Unfortunately, there does not appear to be any fixed definition for a "cool morning" or for determining CMOs. (Yes, I know, it's frustrating...
)
FWIW, I had maiden bloom early this season on a seedling out of 'Ballerina on Ice' (as pollen parent), and I noted that it did not open properly after a 49 F night, but did after a 55 F night. (I have other notes from perusing the web to the effect that 'Ballerina on Ice' is reputedly a CMO, which is why I used it in some crosses. Whether "Ballerina..." is indeed a CMO, I invite others to comment on. My accompanying note on the seedling was that after that 55 F night, it opened better than 'Ballerina...' did.)
So one might hazard a guess that there is a hard limit somewhere in/below the low 50s, Fahrenheit, where even the staunchest CMOs will be unable to perform. Then somewhere around 54-55 F, the CMOs can open (they may or may not open all the way reflexed (if that is their normal habit), but they can at least open flat), whereas other daylilies may be sulking and struggling.
However, having watched a few plants early this season, I am coming to the conclusion that it is perhaps not so much about *only* "cool mornings" or "cool nights", but daytime temperatures in the preceding day(s). And the more cool days (in a row) there are, the harder it is for even the supposed/suspected CMOs to open fully. I am not yet fully convinced, but I am leaning in that "cool preceding day(s)" direction.
Perhaps Maurice or someone more knowledgeable can chime in on that.
That said...
'One Fine Day' (which blooms early in the season) has been a good CMO here. Nighttime temperatures can still be cool when
'Osterized' begins bloom, and it has been a reliable opener for me. (I suspect that a lot of UFs and spiders would be good CMOs.) Iirc, Richard Norris also had a CMO (it might have been 'Super Seventy Three').
'Pennysylvania Bucktails' is reputedly a CMO and it did open well here early in the season, but the colors were washed out/not fully formed, and the scapes were weak.
In that vein, it is not just opening that is affected by cool/cold weather, but also pigment development - at least such has been my experience. That being the case, you may want to look for yellow or near-white CMOs for your colder months; the color will be paler and less saturated than it should be, but it will still be in the correct color range even when the weather is cool (as opposed to a "pink" daylily which looks like flesh or straw or pale yellow).
