When I started this, we had had below freezing weather for too many days in a row. I lost count, but suffice it say I have worn my long underwear under my jeans to stay warm in and out of the house. I probably don’t need them in the house, but it saves having to dress and undress when going outside to check the greenhouse, walk the dogs, whatever. My little greenhouse has insulation, but when it drops into the low teens, like it will tonight and has done for several nights running, I turn on two heaters full blast to keep it warm.
Now those of you living in Wyoming and Minnesota and such aren’t phased by a puny 8 or 18 degrees; I understand that. I can hear you snorting and rolling your eyes already. But it is all relative. And still darn cold. And still an issue. So cactus lovers must do what’s necessary to protect their collections. One medium-sized space heater has worked in the past, but when we have several days when it doesn’t warm up even during the day above freezing, it’s time to drag out the second heater.
Rest assured I do know that I break every safety rule recommended for space heaters. I use one of those industrial-size orange extension cords to get power to the second one. They both run all night and part of the day unattended. I do check on things during the day and at least once before bedtime; so far, so good. I think the plan is to install a gas heater next year. I’ll let you know how that works.

I have been told by the experts that simply keeping the temperature above 32 degrees and keeping the plants dry will work just fine. I’m not comfortable with that; anything in the 30s just seems too cold. I am much happier with the warmth between 40 and 50 degrees, and the plants seem happier.
It must be working; this senecio cephalophorus is blooming, several mammillarias are ringed with pink flowers, euphoribia milii is blooming, and the kalanchoes are getting ready. You heat your greenhouse to a temp that makes you happy. But do give them some heat, or you will be replacing plants in the spring.