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I'm completely new to greenhouses and would like to get just a general idea of what kind of temperatures I could expect in a non-heated greenhouse during the dead of winter (outdoor temps averaging +20 to +30 F) and in early spring. For those who use basic passive solar technologies (e.g., south-facing black water barrels), what kind of temperature boost are you getting?
 

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I'm in Zone 5a. Winter temps tend to hover in the 20s and 30s, but we usually get at least a week or two of -10 to + 10F weather.
 

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Plastic covered? Polycarbonate?

It all depends on the amount of sunlight you get. One day of 20 degrees with overcast skies isn't the same as a 20 degree day with sunny skies. Technically, I'm zone 6b. I'm tucked up against the lake in SW MI. I don't find the climate zones help that much. I'm supposed to be warmer than you, BUT, I'd love to have 20-30's all winter with just a few days down into the 10's.

However, I can keep lettuce, spinach, onions, and beets alive all winter. I try to get it up to harvest size before mid-Nov. Then I cover with frost blankets and harvest as needed. The lettuce still grows through Jan, but it's very, very slow. My greenhouse is plastic covered- 1 layer. I don't have enough sunlight to grow much else that my family will eat.
 

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We are zone 7 and have two layers of plastic and water stored for heat sink. When it hit 3 degrees this past year, we lost most things. That has never happened before. The lettuce sort of came back but looked pitiful. We transplanted the green onions in the garden and the cabbage recovered somewhat. This year I have chickens in half of the green house to see if that improves the temperature. We'll see. Most years, we manage to keep hearty greens, cabbage, onions, and sprouting broccoli alive.
 

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It varies a lot based on the amount of sunlight--and night temperatures relative to the size of your heat sink. At our old place in 6B, we had a greenhouse with no heat sink that would typically stay a few degrees warmer than the outside world--probably only 5 to 10 at night, but during a sunny day it could be huge. I remember sitting out there on a sunny 5 degree day in my t shirt because it was 70 in the greenhouse.

In general, it's not so much the average temperature that matters as the number of times that it gets really cold in there. Most cold weather plants could take one trip below 10 to 15 degrees, but even the hardiest kales would usually give up after two trips below 5 degrees.

We were only there two years, but both years in late February, we had a massive aphid outbreak. We had to rip out everything, flame weed, and bring in the spring transplants. It still bothers me that I can't figure out what caused it or how to fight it other than by utter destruction. The plants were too fragile for insecticidal soaps, and it was too cold for the beneficial insects.
 
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