the propane heater thing works great as long as you have the space and the right kind of set up. I've done it, but it can be a pain in the ass, because you really have to time it right and check it to make sure you don't burn anything. it sucks, but it is worth it for a good harvest. you can also heat water up throughout the night and early morning, not boiling, but warm, and lightly water the roots in 2 hour increments... that only works if you don't have extreme cold though, because the water can freeze if it's too cold... you'll also be up all night, and can only do it so much so as to not drown the roots... i'm in the same situation, and got a new "mr.heater", but I forgot the damn canisters... hopefully it doesn't get too cold tonight and I can pick some up tomorrow... it's already 34 here... 25 is where it gets really dicy... these cold snaps suck... I get em every year, usually even earlier than this year, and it's always a challenge... I grow mainly sativas, and many aren't done until late November into December, which makes this time of year especially difficult for an outside grow... i'm south enough to get the light needed, but high altitude enough to where it gets super cold early... but it's wonderful... and i'm high... and apparently very talkative... sorry.