Posted: 04/04/2013 at 14:31
I have found a cheap free method to heat a greenhouse and it's so simple.
cut the grass. Stuff clippings into a compost or coal bag put bag(s) in the greenhouse.
you might want to just pile them up initially to get them brewing.
Grass as it rots generates heat, once your into the first few inches the heat is massive. google "jean pain method"
This is a bit extreme but this guy is actually generating hot water from compost, basically running about a 100 foot of pipe through his compost pile.
I guess once the pile cools off it can become part of your regular compost heap or used as a mulch or soil enricher...