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need highlander temperature advice

I recently bought a Craigslist mini-fridge for putting young highlanders in overnight until they get more mature. I figured mini-fridges are usually pretty crappy and would barely cool at all on the lowest setting. It turns out this one is very good at its job. I've been moving a thermometer around inside it to get a feel for the zones. The back and the floor get very cool, and it's about 42F there. The warmest area is inside the door which is about 48F. At higher settings I have no doubt it would become a freezer. I had intended not to go below 50F for this project, so I'm wondering how safe this will be for the various species:

- glabrata
- talangensis
- hamata
- inermis
- flava
- dubia
- jacquelinae
- mikei

They're all small, but if I can't use the full space of the fridge I won't be able to fit them all in.

Are there any species in this list that can definitely handle 42F nights? Are there any species that should definitely be kept closer to 48F? Are there any species that shouldn't go in the fridge at all, or will barely benefit from it?

Someday I may try installing my own thermometer switch or something. I've been considering ripping its guts out and just using the parts to cool my entire grow tent at night. I paid almost nothing for it, so I don't mind ruining it.

EDIT: I didn't mention my daytime temps. Anywhere from 72F to 80F, I'd say.
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hooke the fridge up to a temp controller with probe that will turn on/shut off the fridge as needed :)
All of those species would do very well at 48 F, but would be fine at 42 F if the days were warm enough (all of my highland plants have experienced temperatures in the 30s on numerous occasions). Still a good idea to keep them away from those areas should any frost start to build up.
I would echo what Myles said and get an accurate probe thermostat to control the mini fridge. They are notoriously inaccurate and the design is such, as you noted, the temperature can vary considerably from one part to another and may very easily freeze your plants. Perhaps you could place a small cp fan inside to circulate the air. That may help even out the temperature differences, as well as keeping the temperature closer to the set point.

edit: Are you using an accurate min/max thermometer to measure the low temperatures?
The thermometer I'm using is just a simple outdoor mercury thermometer. It's hard to say how accurate it is. I'm liking the idea of a temperature controller, though. They're common on eBay and it would offer me another layer of protection. Both the controller and the fridge thermostat would have to fail for my plants to be frozen. Plus the controller has an alarm -- if the fridge malfunctions I won't know until it's too late.
The thermometer is probably pretty accurate but it's difficult to know without a min/max thermometer exactly how cold it got. You could be looking at the thermometer after it's already warmed a few degrees once the unit has shut down. Plus the action of simply opening the door to check it will warm it up pretty quickly. The min/max type records the lowest temperature reached and will hold that reading until it's reset.

A double layer of protection sounds like a good idea. That would be my biggest concern with one of those dorm mini fridges.. freezing the plants accidently.