I suspect that you won't get a lot more growth season, though it depends on what you are growing (e.g., cold season crops vs. tomatoes). I just used a greenhouse heating calculator to get a rough idea of what you can expect as far as heating requirements, and it looks like, if it is zero outside, the heater going at full blast will keep the inside at about 35F. Obviously, when it is warmer outside, it will be roughly that much warmer inside the greenhouse, but do you really want to be using a huge amount of energy to keep the greenhouse at a temperature that will allow you to actively grow? I live in a much milder climate than you, and I also use a 1500W space heater, but my goal is to protect sensitive plants, not to try to grow tomatoes that will cost me $20 apiece due to energy costs. I'm not trying to discourage you...just sharing my experience: I had high hopes my first winter until I realized what was practical and affordable.

I agree with above poster. It depends on what you want to grow. I grew spinach all winter here in NH with a simply unheated high tunnel. Eliot Coleman has some good books out about growing in high tunnels. Keep in mind that in addition to heat, plants expect sunlight. During the darkest months, there is little growth. Good luck!

Thanks so much folks! I use low tunnels that typically extend my growing season by 6 weeks in the fall ( to Dec 1). Being new to GH growing, I wasn't sure what (if any) advantage the GH would offer over my low tunnels as far as season extension goes.....

Sounds like I can cross tomatoes off my list :( I'll still give the cool season veggies a try......

I have Eliot Coleman's,"The Winter Harvest Handbook". It has a ton of good info! He's Inspired me to try some of these season extending techniques..........