North American Fruit Explorers mailing list at ibiblio

[Lucky Pittman <lucky.pittman@murraystate.edu>]

Del,
Back in the 'early days' of tree shelters, the manufacturers of the solid 
tubes used to recommend 'raising' them a few inches, along about the 1st of 
October, to create a 'chimney' effect, allowing the plants to go ahead and 
harden off so that they could go into dormancy without having tender 
tissues damaged by normal fall frosts/freezes.  Guess once they went 
dormant, you could slide the tubes back down & shove 'em lightly into the 
ground to (hopefully) exclude voles/mice.
The newer, vented types, with 1/2 inch fenestrations at staggered spacings 
seem to do a better job of allowing the seedlings to 'harden off'  without 
having to raise or open the shelters - and, at least in my hot zone 6, 
prevent young trees from baking during July & August.

Lucky