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Electric warming mats might be an option, with thermostat, and fleece over the top when its particulary cold. At worst, only the leaf tips will get frosted.(Most mail order places do them)
I refuse to even consider any heat...it's too far from the house to the greenhouse to lay a cable for any reasonable amount of money. My strategy in Winter is to keep everything as dry as I dare. Anything that doesn't survive is replaced once or twice but after that I give up. At least now I do have everything plunged in sand which helps. Louvres and doors stay open all year, cold wind being preferred to Botrytis.
I've an 8' x 7'6" greenhouse which I'm trying to keep frost-free (it's full of Nerines, some pelargoniums & echeverias); I've clipped bubble-wrap to all the struts, so the thing is more-or-less insulated on the roof & all sides; I don't have a thermostat, only a max-min thermometer & the Met Office forecasts & I've been using an electric fan heater when the temperature is -3 degrees C or below (down to -10 last night), a paraffin heater if it's -1 or -2 degrees C; electric fan heater is horribly expensive, about £6 per day when it has been running all day, paraffin heater costs about 1/3 of that to run. I might try investing in a second paraffin heater, I'd really like to avoid using electricity, it's so expensive & not an efficient use of energy.Quote from: PeterT on Today at 07:52:26 PMI don't have any heat on, the glass round the sides is only to keep cats away from the sand. I have found a roof alone controls the moisture level which allows me to regulate the growth patterns of the plants and If I get it right Undecided many supposedly tender plants will survive freezing. Much of this green house is oncos so maximum air flow.A clever Idea to store warm air but it would require a lot of storage to provide enough warmth to mitigate a frosty night.I read of a tecnique of temperature control where steel drums of water are put in a green house. During the day the glass traps heat and the water warms up, at night the water acts as a radiator delaying the night frost. When the sun heats the green house the next day the drums of cold water slow down the temperature rise. The overall effect is to moderate the temperature fluctuationsThink next year, I'll certainly get a thermostat as well ~ save me from leaping out of bed at 4 am in a panic to traipse down the garden in my jimjams & wellies just to make sure that the heating is functioning ok.