Just curious here, if they were in a protected greenhouse - from the harsh cold, snow, wind etc., why heat it? Of course if you have other plants that need a bit warmer temperature I could understand it. Otherwise I would leave it unheated, and water once a month. My family has a fig tree on Long Island - over 80 years old, outside, and hasn't been covered in quite a few years. I think this past winter it has suffered some damage, I'll have to ask my family, otherwise it does pretty well.