Hibernation outdoors should ony be done if you live in the turtle's natural range and only after much consideration of the hibernation sites available in your outdoor pen. Not all pens have suitable spots for hibernation. Most people will make a "hibernation pit" for their turtles. You can learn how to make one from my new book, or online-at my website or do a search for hibernation information.
A turtle does not forget how to hibernate, but their immunity to cold might be suppressed if they have lived the cushy life of an indoor turtle. Only very strong, healthy, heavy turtles should be hibernated outdoors. My are very healthy, but I still use a hibernation box kept in an outbuilding because hibernation is no picnic, and I like to keep an eye on my turtles as they hibernate. Some people use dorm fridges for the same reason.
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Tess Cook
Box Turtle Care and Conservation Website
www.boxturtlesite.info