User blog comment:Timurovich/Farming in Winter?/@comment-24522773-20140526004913

Items don't have their own temperatures. Not even mobs. It would be a complete waste of programming/game size/resources. The player has temperature, the ambient has temperature. And food, left in certain places, spoils slower. But not even food is required to have a temperature of its own to program that feature easy.

You can test it easy though, create a new world with permanent winter and spawn/build a couple of farms and fireplaces to see how they can't grow anything or be fertilized.