I live in Manitoba, Canada. I have a wild radix I almost ran over back in mid November last year, temp was like 4C and it was sunbathing on the road. It was just a baby, so I kept it, the next day it dipped so cold and a layer of snow was on the ground. This year I was finding a bunch of snakes out with the temperatures 6C, I also notice that if the temp is too hot, you won't see many snakes out, I typically find lots when the temp is like 18 to 22.

Garters are very adaptable... kinda why I say 40F is t-shirt weather lol