I have a bigger problem now. The two large cuttings I got are unrooted. And it is almost December. I put one into gritty mix and another into pure Pumice, but I think they will not root at this time of the year? I gave them full sun exposure to at least maximize their chances of rooting. But are they going to survive the rain and sub-40F temperatures without roots?

What is my best strategy at this point to get these to survive Winter and start to root in Spring?

I wouldn't have high hopes for large unrooted Euphorbia cuttings in full sun, and exposed to rain and temps lower than 40°,

I agree with Howard. The more warmth you can give them the better, and keep them pretty dry and out of strong sun.

Best advise was given above. I had a 12 inch cutting I purchased on Ebay last year. It took a whole year for it to root. It just started to grow this year before I brought it in for the winter. Some euphorbia takes that long. The bigger the cut the harder it is. So keep warm and indoors for now. Next year maybe you might see some growth. If you water it with out roots, it will rot. So if you have to, I would spray mist from time to time, no watering.

Based on the fact that it might take a year to root, I will certainly get these out of the direct sun when Spring brings more serious heat. Right now peak heat during a sunny day is under 75F and I think there is very little dehydration going on there. It also sounds like gritty mix is definitely a no-no for rooting these plants, given the risk of moisture / rain.

I am very tempted to leave at least one of the cutting in pure pumice in the rain, just as a test for how well this species does in pumice / rain / < 40F zone 9a Winters. Most of my other Euphorbias survive these conditions but only in pure pumice, not in gritty mix.

I would wait until the plant has roots and is well established before testing how it does in rain and under 40°F.

Gritty mix is very close to pumice. I agree that pumice is best to root but only to root and not left to the elements. Got to keep this one sheltered or you will lose it. They are expensive and would be a shame to loose it even if it was a gift.

@Stush2049, in my experience gritty mix is much different than pumice. Gritty mix can leave a lot of water around in the form of adhesion to the smaller Turface particles, plus the bark and Turface are both very water retentive and keep the plant roots pretty moist for weeks at a time in a cool environment with little evaporative pressure. More to the point, I have lost a lot of Euphorbias in the rain in gritty mix. For many species in that genus, gritty mix and rain and cold are just lethal. I have much better luck with pure pumice in cold and rain for Euphorbias.

If I kept it sheltered in gritty mix and did not water, things might be fine. Gritty mix is a good summer soil for Euphorbia. But I am pushing the limits of what the plant can tolerate by leaving it out in the rain and cold in zone 9a. For those harsh conditions, pumice has worked for me.

I understand. In my case, I rooted mine is pure grit (poultry grit). When rooted, just sat around doing nothing until transplanted into some thing more better. Then it took off. Grew almost 2 inches in about 3 months. This after sitting for over a year.