Correct, an unheated, stand alone garage will get just as cold as the outdoor temp. especially during long cold periods. Problem with storing potted material in them is the raising of tempatures in the early spring as well as higher temperatures during the night. The building will have higher average temperstures.
With early bud break trees like most prunus, they will break dormency way ahead of their counterparts planted outdoors.
Jerry
[NAFEX] Storing in unheated garages.,
Jwlehman, 01/16/2005