There is all different types of heating availble - depends on how much you want to spend and what utilities you have to hand.
I cant tell you how long to use for as I dont live in your neck of the woods so it would be different.
here is some good advice about different types of greenhouse heating.
http://www.rhs.org.uk/advice/profile...se_heaters.asp
you might also want to consider making a 'hot bed' using a bale of straw or a raised bed using manure (as the material breaks up it releases heat) - I have tried this and have found it very effective
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