For the past 2 winters I'd tried to root some rhodies. First in my house in a thick walled poly storage box with a mini-humidifier powered by USB. (readily found on ebay for only $10-$15 at the time) None of them rooted, and after a while I realized my approach was wrong and I needed bottom heat. I read somewhere that the tops need to be cool and the roots warm. So this winter I moved that box out to my never-below-freezing garage and set a heat mat at 68F. It was near a window but I covered it with sheeting to make sure it never got direct sun. I also switched to professional Dip n Grow from what was probably a lesser rooting product. This time I got a lot of hollies to root, but the couple of rhododendrons struck in December still wouldn't root. I tossed them in frustration in early March when no roots had formed and the edges of the beveled cut on the stem were starting to rot.
However in mid March I noticed the bud swelling on my Nancy Evans on Charles Loomis graft, and decided to cut the competing Charles Loomis branchlet away. I figured, "why waste this" and struck it. Around the same week I accidently snapped a branch off another hyperythrum hybrid , walking past it, and again, figured, why let that go to waste.

(to visualize my garage in winter: picture a 2 car garage about 1/3rd filled with various junk, and 2/3rd so dense with potted plants you can hardly move around LOL)
Well...guess what? The 'Charles Loomis' definitely rooted in 2 months. I can tug it and feel that there are roots. It's sprouting too, as you can see.

The 'Opal Thornton' also seems to be budding up, but I'm afraid to tug on that one yet.

It looks to me like either 1) there was some hormonal signalling at work that meant rooting was more likely later in winter
or 2) the days being longer helped
or 3) the garage being slightly warmer helped. *however*, I had a themometer in the poly storage bin at all times and it was always, IIRC, around the low 60s air temp in there, and obviously 68F soil temp. (I resorted to using an oil/electric radiator to keep the garage above 32F at all times during the deep freezes, so it's logical the box with the 40W heating pad was kept well above that) I don't think it was much warmer in March, generally, because as the garage warmed up the bottom heat mat tended to turn itself off. Last week I completely turned the heat map off, of course, because it was not having to run at all with how hot it was getting. I plan to move everything out to my growing-on protection frame (posted elsewhere on GW) soon after this cool spell.

The only other thing I did differently this time was to hold the cuttings in the rooting solution - recommended dilution which I think was 10:1, for about 3 seconds, instead of a quick instant dip.

Thoughts from anyone else who's gone through the process of trying to master rooting? In any case, I will be rooting many 'Charles Loomis' as it is very heat resistant and apparently accepted a graft of 'Nancy Evans'.