To have a greenhouse in colder climates is very valuable. It helps to extend the growing season and gives the plants a lot more heat. Although on some days there is more heat than the plants can use, the greenhouse gets way too hot.
A good way to regulate the temperature in a greenhouse is by using a thermal mass, and water has a very high specific heat capacity. The absorbed heat from the air warms up the water during the day and gives it back during the night. In our Geodesic Dome greenhouse we installed a 280 gallon water tank.
To use water as a thermal muss has also the advantage of serving as plant watering water. Plants grow better when watered with stagnant warm water. Learn more at How-to water your vegetable garden.
The Greenhouse Water Tank Structure
We built the tank out of two 4X8 5/8″ sheets of plywood. The sheets are held together on the short edges and the water then presses the sheets outwards forming a fish shape. The tank is buried one foot in the ground, this way we did not have to worry about 2200 pounds bursting the tank. The top long edge of the plywood is being held by a metal band. There is a lot of water to hold.
For our second Geodesic Dome greenhouse I built a different shape as you see on the picture below, which worked much better and seems to be more stable.
Here is how you build one for yourself: You will need two 4X8 5/8″ sheets of plywood, two 2X10 4ft long boards. Then you bevel the long edge of the boards at a 13° that makes them a trapezoid when viewing from the end.
Attach the short edge of the plywood to the long edge of the boards using screws with large OD washers or better reinforcing the plywood edge with a metal strip. For my tank I used recycled aluminum windows frames.
Since our second greenhouse is a bit smaller, I dug the hole for the water tank about 4 ft deep for couple reasons. For one, to increase the volume because the amount of water is important to cool and heat the greenhouse with a radiator. For two, the ground at 4 ft has a lower temperature than at the surface, and since the lowest 2 ft of the lining is below the plywood and has direct contact to the ground that will add to the specific heat capacity of the tank.
The Greenhouse Water Tank Lining
We lined the tank first with tarp and then 4 layers of 4 mil black plastic. It lasted for two growing seasons. The reason why it ripped was partly because of the ice that built up in the winter. Our frost comes so suddenly that one year we hadn’t emptied the tank on time and ice damaged the plastic. The second year we were on time, but there is always a bit of water left.
Then we exchanged the black plastic with 2 layers of 6 mil vapor barrier plastic. Note: to line the tank with special pond liner would have been a better option but you would have to have it in one piece.
The Greenhouse Water Tank Biological Filter
For filtration we built a simple biological filter. We took a five gallon bucket with a lid, drilled a hole in the lid and on the site of the bucket near the bottom. The opening in the lid we connected to the pump and the one at the bottom functioned as an inlet so the water could be circulated through the bucket. We filled the bucket with sponges. Soon bacteria built up in the bucked on the sponges and started to clean the water biologically. Note: You only need a filter if you grow plants or fish in the tank.
We could have raised some fish in the water tank but we did not. Instead we grew some water plants in there and used the water for watering the plants in the greenhouse. Our water tank is also a base for an active heating and cooling system through a radiator. Read more Here.
If you want to build a Geodesic Dome too, we share a step by step How to build a GeoDome Greenhouse.
A Geodesic Greenhouse Book: Building and Using a Solar-Heated Geodesic Greenhouse.