It really depends on what you're growing, and when. I used to grow vegetable plants for one of the farmer's co-ops while I was in TN, zone 7. I only needed heaters for about a week or two, at the end of February or so, and only for a few hours each night. Much more troublesome was venting the GH when it was warm - in March the sun was bright enough to bring the temps up to 100ยบ during the day, then drop into the 40's at night. Having roll up sides (which were very easy to construct - this was a hoop house, BTW) were a real life saver, as long as you were home at the right times. If you're in zone 8, I suspect you might have even warmer temps.
Ken, if I understand correctly, you're using your GH for tender plants that aren't going dormant? Orchids and such?
