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. Water is a good thermal mass. The absorbed heat from the air warms up the water during the day and gives it back during the night. In our GeoDome 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 4′ x 8′ 5/8″ sheets of plywood. The sheets are held together on the short edges and the water pressed 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 one metric ton of mass 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.
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. This spring we exchanged the black plastic with 2 layers of 6 mil vapor barrier plastic (see first picture, it looks better, eh?). Note: to line the tank with special pond liner would have been a better option.
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.
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. Our water tank was 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.