Wonder if you could use this......

[boggybranch]

....to heat a greenhouse.

http://www.motherearthnews.com/Do-It-Yourself/2002-10-01/Build-Your-Own-Wood-Fired-Earth-Oven.aspx

 

[vfem]

You know what... I saw a huge 100 gallon water container/tank with a solar heater on it somewhere. Perfect for a greenhouse, plus it has a hose attached to water your plants with the warm water! I wish I could find it again. I think it was on craigslist!