Check out the Greenhouses forum here.

Dave

Here is a link that might be useful: Greenhouse forum

I have a 10 X 12 greenhouse in Southern NJ and have a 220 electric heater which does a great job keeping it warm, even in the coldest of evennings.

I use wood-fueled, forced-air furnace in a separate building.

I also use wood stoves but within the structure as it reduces costs considerably. Used wood stoves are cheapest at springtime auctions and flue pipe can be wire-rigged to stand alone with just a post for support. In 12 years of using woodstoves I never had a problem with external "Sparks" but I do use a box fan inside to blow air over the stoves and use circulation fans to keep all inside air at the same temp.
Propane space heaters are a good backup heat source or for marginal nights when you only need a few degrees of added heat. I just can't afford electric heat or solar panels and relying on water tanks as heat sink is wishful when you want to keep significantly above freezing.

I would like to find out more about this. We have two greenhouses, a small 6 x 8", which is what I started out with, and one we put up last February which is a 16' x 24'. I can't afford to heat electrically, but I'd like to find other options so it can be used during the winter.

If you have the money, I think the best option is one of the new external wood stoves that heat water, which is then recirculated through the building you want to heat. They look like a metal shed in your backyard. They're not cheap, and there is a lot of trenching involved in burying the cable, but for about $10,000 you can have some very nice heat. If you used it to heat your house, too, it could pay for itself over enough time. It's nice to have the mess and smell of the wood smoke outside of your structure, instead of within, and from a physics perspective, it is a lot more efficient to heat water than to heat air, like I do. I just didn't have the $10k :(

Heating a greenhouse discussions from the Greenhouse forum - the FAQs too.

Dave

Here is a link that might be useful: Heating a greenhouse discussions from the Greenhouse forum - the FAQs too