Once we finished the roof there wasn’t much more to do before some seeds and starts (And in this cover photo you see the first green to be a part of this pit greenhouse!) could go into dirt, but there were a few pit garden finishing touches that were needed. The pit garden concept is that walls being below grade maintain a constant temperature year round. They can’t though counteract 0 degree Montana winter days, so we want to capture, and maintain as much heat as possible from the sun to keep the plants happy.
To capture the heat we need a heat sink. Water is an excellent storage medium for heat energy, and our goal was to utilize 5 55 gallon drums to store water in. These would get direct solar heating during the day, and slowly cool giving off this heat at night. I was unable to find drums that were either clean from chemicals, or reasonably priced, so I used trash bins instead.
8 32 gallon bins, plus two 44 gallon bins give us 344 gallons of water storage. This 344 gallons contains a lot of heat energy. Lets say that we can heat the water to 80 degrees on a bright sunny day, and then it cools off to 50 degrees overnight. Using the equation E = (1 Btu/lbmoF) (30 oF) (344 US gallons) (8.3 lbm/US gallon) we get nearly 86000 Btu’s of energy released into the pit during that cooling. The 8.3lb is the specific weight of water. Additionally we added spigots to a couple of barrels so that we could use the heated water for irrigation. Check it out here.
Once you’ve stored the heat from the day the last thing you want to do is let it escape! We’re using an insulated tarp that is meant to assist in curing concrete in cold temps to cover the poly roof.
I put it on each night as the sun drops below the horizon and then roll it back up each morning when the solar panels start registering good solar energy. I’ve also cut some 1″ rigid insulation left over from the fill part of the project to put up into the front windows.
Once all the insulation is in place, I started a renewed caulking effort utilizing a non-contact infrared thermometer to point me to spots where we were getting cold air through our defenses.
The barrels used as a heat sink take up a lot of prime square footage in the pit greenhouse. I didn’t want to throw it away, so I built some grow beds above them. I had seen fellow homesteaders in Montana use a cut up pipe to grow smaller plants such as lettuce. 6″ sewer pipe with a bit taken out of round is still very rigid, and should have enough dirt depth to grow these smaller plants. Some quick framing over the barrels and it starts to take shape.
I also built a couple raised beds that we’re using for Tomatoes, Peppers, etc. We’re going to plant the rest of the garden right into the top soil fill we dropped in a few weeks ago making up the floor. For the raised beds and the pipes though we decided to go with high end potting soil from our local organic gardening store. They had on sale and recommended Roots Organics Soil. (That link takes you to Amazon, but please don’t buy it there, our local price was 1/4 of what Amazon is charging!)
You’ll notice in the above photo we’ve also added a grow light to our pit garden. This light helps us out three ways. First, during these short winter days it’ll give the plants enough extra light to think it’s growing season. Secondly the heat this thing gives off will take the edge off those coldest nights. Finally it really helps keep the humidity down which has been a bit of an issue for us. We’re using a 1000 Watt High Pressure Sodium bulb from Hortilux, a 1000 Watt electronic ballast to match, and a hood to direct the light down. So far very pleased with the light this gives off.
We can’t wait to get some fresh produce out of this! Very strange to plant seeds in December….
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