desert rose (Adenium) no way, plumeria and oleander you're taking a big risk, pachypodium depends on the species and is also risky if you're not careful

all the cactus, jade, dioscorea no problem as long as they are dry

i've never grown ponytail, adenia or fockea

euphorbia is a huge genus, you have to be more specific, my guess is that most should be OK

only water whatever might be actively growing - maybe dioscorea or the jade

The succulents I don't know, except that many succulent Euphorbias are sensitive to the cold.

Ferocactus and other Barrels (Echinocactus? Thelocactus?) should be fine at any temperature above freezing, but I'd be worried about the Myrtillocactus and Pilosocereus. Both are considered sensitive and usually kept well above freezing. They can probably handle brief periods down around freezing if humidity is low and the days are warm, but I'd expect some damage in the conditions you describe. Surface marking at a minimum.

I wouldn't water anything at all when night-time temperatures are expected to be near freezing, which I guess might be for several months continuously.

hello Castorcrap,
I have an unheated GH that went below freezing this last winter(upper/mid 20's?), lost 4-Euphorbias (trigona (10+'! wrapped, not in GH), glochidata, and 2xmilii), others were fine or little damage. My ponytail palm was ok, fockea edulis was fine though the smaller one did receive damage. Pachypodium lamerei came close to loosing but now growing, jade fine (gollum the same).

Perhaps on particularly cold nights you could add slight warmth with Christmas lights or an icechest filled with hot water (lid closed)? This next winter, I'll have some kind of backup plan for those extremely cold nights (for CA).

goodluck,
-Ethan

"I wouldn't water anything at all when night-time temperatures are expected to be near freezing, which I guess might be for several months continuously." Ditto!
I'm going to go one step further...IME, Do Not water pachypodium, ponytail palm and jade after temps have dropped below 40/45. Adenium and plumeria (cousins) *will Not* tolerate temps below 45/50 without damage or death. These two plants would be much better off in your parents' dark, warm home as they go dormant in winter and require little/no light and no water.
If you *need* to place these plants in this gh, I would get clear 7mil plastic and a string of outdoor (large) Christmas lights. Make a tent for your plants and place the plants and light under the tent. You'd be surprised how much heat those dudes put off. Of course if you are not going to be there to attend them, you'll need a timer to turn the lights off and on...or you can leave them running - mine ran for 4-5 weeks last winter. (One string = $2/3 mo) My plants received no water during this period. Ponytail palm looked a bit like a shriveled old potato...but quickly recovered itself when watered again. HTH
Jo

Keep the plants dry, and a fan going 24/7 and you may get away with murder. This is what they do in England, and the back 40 at the Huntington Gardens. The also open the doors on both ends for fresh air circulationo. You also may want to put some insulation on the glass to protect them even more. I use double plastic insulation. But it is costly. Good advise up above, I have used light bulbs as well for heating. 100 Watt
Norma

"The also open the doors on both ends for fresh air circulationo."

Norma - surely they don't open the doors on both ends during a freeze? how would that help?...except for circulation, of course. please advise. TIA - Jo

plants that are "winter-growing" which prefer short days and cool nights in my experience can be watered safely if temps are at or near freezing at night, but it must warm up (to at least 55-60F) and be sunny the next day - the key is to make sure the plant is active so that it uses the water otherwise it may rot.

I have a Dioscorea elephantipes that definitely prefers the colder weather, I water it without restraint in the conditions I described. I used to have a jade plant, I did the same with it, and it flowered in the late fall too.

I should take back what I said about pilocereus and myrtilocactus, I assumed they were regular cereus, they are probably risky at consistently very low temps.

x