Velutina

I grew bananas in a greenhouse for years (see my avatar). I tried to keep the minimum temp above 55F. I used about 500 gallons of water as a heat sink, which really helped with heating costs. Another factor is soil temperature if your plants are in the ground. I got around that in zone 6 by having the ground level inside the greenhouse about 2 feet below outside ground level.

What I would do for heating... I had 55 gallon drums filled with water, and a 300 gallon salt water aquarium. I would let the greenhouse get as hot as possible during the day (easily 90-100F during a sunny winter day with outside temps in 40s). The water would heat up to about 85F. During the night, the water temp would go to about 70 before any heaters kicked in. Even with 30F temps outside, the greenhouse could stay above 55F just with the water heat sink. It worked well for me because I used water circulation and fans blowing on the water to control how fast/slow the water temp changed.

something to think about..

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Adam