cold frame -- unheated green house

In the raised bed threads, some people indicated they extended their growing season by a couple of weeks at either end. We bought a house with an unheated green house last year and I kept track of the temperature. Generally speaking the lows in the green house were the same (-5 to -10 C) as the outside temperature. I did have water in the green house (~40 litres) to act as a heat sink.
The transition from winter to spring here is abrupt and variable. Days warm up to 15 C but the overnight lows are well below freezing.
Do the people who have success extending the season live in milder climates with more gradual springs?
Are there better techniques than using water as heat sink?
Thanks
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Try running a drop light out there with a 80 to 100 watt light bulb and closing everything up tight. This keeps mine from freezing for the most part.
Some composting in there will heat it too.
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My tomato plant is still producing and budding in Tulsa Oklahoma area. All I did was put up a frame around it and wrap the frame with clear plastic.
Then I put a 100 watt bulb on a well house thermostat so that it comes on at 40 F, and a florescent grow light on a timer to keep my days at 12 hours.
We have had a few freezing nights already, and everything else is pretty much dead, but the tomatoes are still producing and even budding out.
Something you may want to try is;
Use an old hot water tank hooked to a wood stove so that you can cheaply heat up the room and the heat sink water when it is colder.
Make sure that the green house is closed up tight, with a good door seal.
Use 100 watt bulbs on well house thermostats in clamp on brackets to set around next to the plants to help keep them warm.
Divide up the green house by making little green houses around the plants and use the bulb inside that.
Make your heat sink out of full 55 gallon drums. Bigger holds the heat longer then smaller.
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I heat my greenhouse in the winter with electric heaters. I found heat sinks worthless.
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I found there was a period of some weeks (2-5, depending on the season) where unheated hoop house protected plants into the fall, and gave me at least 4 weeks jump in the spring.
If you have an actual greenhouse, ie, glass/plexi, I suspect there might be a leak somewhere. I routinely had air temps in the 40s in the dead of winter (z5) when the sun was out, and I KNOW my doors were not airlocked/air tight.
This is NOT to imply you should be able to grow warm season crops with no heat a-toll.. But cool weather crops should be no problem for you. If you go that route, you want to make sure you do not overheat the GH.. keep the highs around 39-40 max.
Good luck!
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According to my "real property report" I have a portable shed/ greenhouse. That said, one side is glassed in with sliding storms on each end. The green house and attached shed are normal garage style construction and on a cinder block foundation wall. The green house has slatted (2*4) floor over dirt. I can heat it, two infra-red heat lamps (I really don't want to know how much it would cost to use these) and an oil filled heater I bought for the house. There is a fan that blows through a 3 in hole, I don't think it has a heating element.
I don't want to heat it because then it would be cheaper to buy the plants I need/want: 6 tomatoes, 24- 48 bedding plants, 20-30 geraniums. Not that I ever pay full retail :)
We just bought the house last year and I'm learning to use what I have. Also, I have a cold frame (strawberry patch) and a supply of storm windows.
Thank you all for your replies.
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