Buckeye on Burro Tail Sedum this morning in the GH.
Before you read about how we heat, you need to know these facts:
1. We grow in US zone 8b. Our primary greenhouse concern is not heat but keeping plants cool in the daytime when the sun is shining, even in winter. Our nighttime lows might reach 15º once in a great while but never for prolonged periods. Below-freezing temps quickly rise at daylight. We had two recent freezes with temps that hovered about 31º F.
2. There are sites with greenhouse heaters for sale. We used what we had, ordinary space heaters with a trial period to see if they worked
3. There needs to be a back-up plan. Ours is a kerosene heater. I am thinking about assembling a brick rocket stove as a trial on the greenhouse floor which is brick broken concrete and river stones . Using old chimney bricks it might be a no-cost solution in an emergency.
4. My heaters are on separate breakers in a greenhouse wired for electricity with its own electrical panel. Never try this using extension cords.
5. Orchids grow in the house in a sunny window. I don't put anything in the greenhouse that wants nighttime temps above 50º -- that's for commercial growers or serious hobbyists.
6. Not professional advice, this is how I keep my plants from freezing using what's at hand.
My greenhouse is filled with as much dense thermal mass as I can find and put in there. Thermal mass includes brick, concrete, water and stone. These materials absorb heat and give it up slowly during the night. Note: Thermal mass will not heat your greenhouse, it slows the radiation of heat from the greenhouse when the sun goes down. Eventually the greenhouse is going to be as cold as the air outside.
Concrete blocks are of two types, dense concrete and those lightweight blocks that have air forced into the manufacture. Dense blocks are best. They make nice plinths for big pots, hold up benches and could even be filled with soil and used as planters. As far as I can tell, bricks is bricks, handy for many uses. I use stones as decorative material and do not worry about whether one kind is better.
Water in the greenhouse might take the form of huge tubs for hydroponic growing, not in my league. I have 4 fifty-gallon barrels of water that support a plant table. We tried all kinds of fancy tricks using pumps and black hoses. Those have been abandoned and the barrels just sit, covered in black plastic and a burlap skirt for aesthetics.
Now to the heaters. These are space heaters we already had, regular little space heaters with a fan, a thermostat and two settings, high and low.
Here's the important part: We run these on the lower setting. The difference is typically 600 watts on low and 1500 watts on high. By using two heaters on the low setting we use about the same electricity. Less current pulled through the wires means the heaters may last longer. If one fails, there is still the second putting out some heat rather than a total fail on a cold night.
Why don't I just use a greenhouse thermostat? I would need one for each circuit and they are not cheap.
Otter Tail Power Company in Fergus Falls, Minnesota has a great page on using electric space heaters.
Celery from a Stalk Bottom, still growing.
Bouquet I picked before the freeze.
Linking to Tootsie Time's Flaunt Your Flowers