At the end of November I wrote about how I heat the greenhouse. We had a couple of nights back then where temps hovered around freezing. Heating a Greenhouse in a Coastal South Climate.
We had a real test last night. Predicted low was 18 or 20 degrees, depending on your source.
By 10pm we were looking at a 7 degree difference in temperatures outside and in the greenhouse. I went out and fine-tuned the little thermostats on the two heaters until there was a wider difference and rising.
During the night we got up and looked at the remote temps. At 2:40am it was 26 degrees outside and 38 in the greenhouse. The next time I got up I forgot to write down the time but I think it was around 5am. We were looking at 38 degrees in the greenhouse and 23.3 outside. The eventual low was 23 outside and the greenhouse held.
When the sun came up, the greenhouse commenced to rise. By 10am I unplugged the heaters so the settings would be constant tonight. When the sun starts down in the west I will plug them back in.
Is there a contingency plan if the electricity fails? There's a kerosene heater. We have generators.
Many of the plants in the greenhouse, like hyacinth bulbs I'm forcing can take short bursts of freezing temperatures without permanent damage. Some things, like cuttings that have rooted in water are expendable if worst came to worst -- there are always more. I might have to choose which of the 5 Epiphyllums would get to come into the house so I would have a stock plant and the same with multiple Schlumbergeras. A plus for us is that freezing temperatures are short-lived here when the sun comes up.
A final note about the heaters. Why are there two? Why do we run them on low? There are two so they can run on low rather than running one on high. Our belief is that prolonged running on high can possibly burn out the wiring. In the event that one should fail for some other reason, there's one left that could run on high overnight.
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