Cannabis

So how big is your enclosure?

I once tried one of those little plastic greenhouse things at high elevation and for heat I placed a group of terra cotta pots stacked like little ovens with big candles in them, three gave off enough heat to keep it 42F inside during the chilliest hours. A 1" wide 6" tall candle, like they have in those camping kits will do, just get it going before going to bed. Also, I had a couple lightweight bunting style blankets that I put on the top of it to help hold in the heat, removed them during the day.

If your room is small, that could be enough to hold them into chilly months, even a small space heater would do.