garters are very cold tolerant snakes....45 won't kill him. He could have been injured by the planter. he could have been injured without showing outward signs of injury, could have been internal. get him back in his cage and keep him warm. garter snakes won't die in an overnight temp of 45...I brumate mine in the low 50's, but used to do it in my garage where it stayed in the mid 40's. didn't bother them. I quit doing in when we had a cold spell and it was getting down in the low 30's.
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Don
www.donsgartersnakes.net