How do I heat my greenhouse cheaply?

Propane? Some kind of oil

Cover the surface of the hoop house with clear polyethylene bubble wrap. The clear surface of the bubble wrap will allow sunlight into the greenhouse for extra heat, while air pockets in the bubble wrap will insulate the greenhouse to help hold heat at night.

Place compost bins directly inside your greenhouse. During a hot compost process, a compost pile can warm to 120 to 160 degrees F., which will raise the ambient temperature of a greenhouse.

Make your own solar panels. There are plenty of them on Pinterest and they are quite easy to make. These will certainly add warmth to the greenhouse. Good luck!

Look for "soda can " solar panels. Very cheap, no power needed, just some tools, and some elbow grease.

I can send you a couple of tutorials. I found them on Pinterest. I haven't completed mine yet but my nephew did and his is working great! Do you have an email address I can send them to? I can't get them on here showing you all the instructions. Also keep in mind, you do not have to make these as big as they have made theirs. I am making 3-4 of them but I have a 3 car garage I am trying to get heat into for winter. Your greenhouse is probably smaller so one or two would do nicely. I will wait for you to give your addy.

You can get some barrels and paint black and set in your greenhouse, maybe under potting benches. Fill with water and the sun will help to heat the water and after the sun goes down, the heat from the water will help heat air in the greenhouse.

solar panels

Make a passive solar heater..... I think this is genius.... https://www.youtube.com/watch?v=pHk0PX8pbWY