Milly, I think an electric fan heater is probably best. You can set it to "frost free" or any other temp you like. 5 degrees is a growing temp and will keep your plants growing . The fan will help with air circulation and can be set in summer without heat to help keep temps down and the humidity under control.
But have a think about it Milly.
What is it you are going to keep the frost off in winter - and is it worth the cost.
I think most of us grow without heat.

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From each according to his ability, to each according to his needs.