Winter in Montana can be a very cold experience. Karl and I were in San Diego, Calif. in Nov 2014 and the first cold snap of the season came unexpectedly. It was an unexpected -17º F cold snap and we were in San Diego, there was nothing we could do. We had the inner greenhouse up and a heater in the greenhouse. But the power had gone off and no one was home to restart the heater. We just KNEW everything in the greenhouse was frozen solid. We were amazed when we checked the greenhouse when we returned – NOTHING had died! The tomato tops had wilted back, but that was the worst of the damage.
So how did this happen and what did we have set up?
Walipini Style Greenhouse
First it is important to understand the type of greenhouse we are talking about. We now have two greenhouses (2021) – but for this post we are talking about our Walipini or pit greenhouse. It is in the ground where the soil stays a moderate temperature – usually between 40-60º F. In our case our pit greenhouse back wall is 11 feet tall. About 2 feet is above ground. The front of the greenhouse is not underground. It is like a daylight basement. We do lose some heat because we are not totally underground and outer shell is not air tight.
Winter is an interesting time. The coldest days are usually the days of full sun. Which heats up the greenhouse. The temperature swings are hard on plants. We cover/uncover the vents and monitor temps and humidity during the day. At night we cover all vents and make sure the greenhouse is sealed up.
The heat source for a Walipini style greenhouse is the earth. Our goal is to keep the heat from the earth in the greenhouse. To help keep that heat in we put up a inner greenhouse and keep it as air tight as possible. We also keep the plants against the back wall as much as possible. We built a pvc frame inside the greenhouse and put a layer of plastic on it. this inner greenhouse does not have to withstand Montana’s weather. All it needs to do is be as air tight as possible.
Now, in addition to the back wall we work to help retain heat using several 55 gallon drums of water. The barrels are dark green to absorb the heat but do not let light in which stops algae growth. The water barrels have an additional benefit. We do not have running water in the greenhouse so we use the water from one of the tanks during the winter. We are always amazed at how little the plants need water in a closed system.
The image ‘zones’ are used in our Arduino Greenhouse Monitoring System.