I hibernate MY eastern garter snake from Quebec AT 4 celcius ,I think its cold enough for Parietalis ,they tolarate the cold more than eastern.

I just take a look on all my adult eastern in the fridge at exactly 4 celcius ,I have 2 thermometer one with mercury another digital ,just to be sure and all 5 adult are full submerge in water...
I change the water bowl and put cold water from my fridge and when I took them into the new water bowl ,no move at all they are all in deep sleep.I will end the hibernation in february ...

ANd Steve I use aspen wood as beeding buy at Walmart,Its the best stuff Ive ever found for hibernation,I don't have any problem with humidity...and it's very easy to clean,usually I use aspen beeding for my large boa constrictor,but for hibernate my garter its awesome!!!