I use a soil heating cable in a sand bed and plunge my pots; inside the GHS.

The cheap part is the problem. When I built my greenhouse I provided a heating system that worked well. Within a few years the price of fuel went so high I can not afford to heat it. I have a 3x6 foot bottom heater that works well, but heating the air I can not afford. Al

Well, Al, that's my problem too. Which is why I'm asking the question. I know you are farther north then us so it might be colder at night than here, Days here are usually sunny and warm, but the nights can be below 32ú... which is my problem. I've tried putting out jugs of water to heat up during the day for passive heat, but that's not quite enough. I thought someone else might have tried something passive that worked.

What is your current setup? What is your minimum low outside and your minimum low inside?

:-)). My set up is a bunch of large pots planted with various herbs and vegetables sitting on a concrete slab at the east side of my house. It is somewhat protected now by the house and a stockade fence on the other three sides. My intent is to pull a clear plastic tarp from the eaves to the fence to hold in the heat at night and protect from night-time frosts.

Last winter it got really cold at night and needed something extra in the way of heat. I ran a small heat-unit that used too much electricity, which is why I was hoping for other ideas.

The structure is the problem. It sounds way too leaky. A well made structure will cost less than the fuel costs for heating a poor one. Drafts are a killer. If you have a 1/4" gap along a 10 foot length it is like having a 6"x6" square hole in the wall. I would also consider a double walled system with clear film. Are you just trying to keep it above freezing? Last year that would have meant only like +4F. That should be doable without heating.

Hi westelle,I live in Fresno also just outside the city so it gets even colder in the country ,on some nights we had mid twenties last winter. A hoophouse cold frame would help during sunny days but at night I use several layers of frost blanket protection and on top of that heavy black plastic when below its 32 just over night and remove the plastic in the morning and leave the frost blanket on during cold days. A Mr Heater would help for some of the coldest nights also but the propane or any other heating source would be too costly and unsustainable for a greenhouse over a long period and we can have many freezing nights.I don't use any kind of heat anymore just a Mr Heater when I'm working in my greenhouse.Good luck

Where I live, the outside temps can get down to 10F at times. I have an unheated greenhouse, so at least the plants are protected from wind. But the inside temperature at night is the same as outside. So I cover my tender plants with plastic film, and put a mist nozzle on top and let it run all night. This keeps the plants very close to freezing temperature, which seems to be good enough for them to survive. This works with outside temps down to the low 20's F. If you wanted to do it right, I guess you could turn your greenhouse benches into little hoophouses and rig the mist nozzles above them.

If you don't have a greenhouse, you could cover the plants with plastic and rig a mist nozzle. But you have to make sure that the wind doesn't just blow the mist away. If wind is a problem, you could use a nozzle that makes a larger droplet size. But it will take more water. Or you could move all your plants into the house or crawl space a couple of times a year.