Planning ahead for March, I see greenhouses on Amazon as low as 150 bucks for 26 ft of coverage:
YITAHOME 26’x10’x7’ Greenhouses Heavy Duty Large Greenhouses Walk in Green Houses Tunnel Green House Outdoor Gardening Upgraded Galvanized Steel Ropes Zipper Doors 9 Crossbars Garden White Amazon.com.
Could this type of greenhouse achieve 4-6 f of protection with a 25lb tank mounted propane heater running inside? Thinking about setting it up in mid February and taking down mid May.
You can use a greenhouse heat loss calculator to determine how much heat you’ll need.
Greenhouse Heater Size Calculator from ACF Greenhouses
It won’t take a lot of heat to gain 4-6F. My heater, 150K BTU, will heat my 32x54x16ft greenhouse to 50F warmer than outside. I determined that on a night with a low near zero. With the heater on all night the greenhouse didn’t fall below 50F. The calculator gives a similar answer.
It says 2700 btu would be required to protect 6f during a 24f cold snap for that greenhouse (assuming 4 mil PE). A tank mounted propane heater sells at 15000 btu. This seems like a feasible idea. Thanks fruitnut! I aspire to one day have a more permanent greenhouse like you and grow premium stonefruits to peak ripeness!
I can see that greenhouse crushed under a wet snow in the middle of the night.
We very rarely ever get wet snow with those type of cold snaps, here in western piedmont NC.Last time was the blizzard of 93. With the low r value of the polyethylene,it might melt the snow before it sticks too.
Here in southeast Virginia we get the deepest snows around end of February into early March. I spent many nights getting up every few hours to brush off my greenhouse with a window washer pole. Although its been 3 years since I had to brush. Now that the USDA moved me into 8a, it may never snow again!
A piece of advice. I had 2 friends both new to greenhouse growing with propane heat that failed realize that a sealed tight greenhouse will need some ventilation. Otherwise the heater burns up all the oxygen and cuts off if it has been running hard all night. A very small opening like 2" x 24" can make the difference. Depends on the size of greenhouse. There is a formula for it.
A 1500 watt heater is about 5,000 BTU. That would be a better idea than propane for most situations. No worry about CO and less likely to fail.
That’s what I use for my greenhouse, but the one I have (Dr. Heater) turns on too soon on even the lowest setting, so this year I plugged it into a Thermo Cube TC-3 instead of relying on the built-in thermostat. It’s been doing great so far keeping it above freezing (my only goal), and a lot cheaper than last winter. Here’s what it looked like last winter (the saw pattern overnight is the heater turning on and off):
By contrast, so far this year it has only even turned on a handful of times. Today is our coldest morning yet, and you can see it has a similar saw pattern, just at a 10°F lower threshold:
If you go the electric heater route, I highly recommend a Thermo Cube to save on heating costs, especially if your only goal is to prevent freezing temperatures rather than trying to keep more tropical stuff happy.
