Thermal Mass & The Above Ground Pond

Above Ground Pond Thermal Mass in the Greenhouse
Above Ground Pond Thermal Mass in the Greenhouse

Water is a wonderful source of thermal mass in any building. Greenhouses are no exception. Every Growing Dome® kit comes with an Above Ground Pond that keeps the geodesic Growing Dome warm in the winter and cool in the summer. Each pond is appropriately sized for every Growing Dome. This creates an optimal environment in the greenhouse for year round growth. Strategically located inside the Growing Dome on the north wall, the pond resides directly under the Reflectix® insulation. The Reflectix aids in keeping the pond warm in the winter and cool in the summer. The pond acts as a thermal battery bank in our greenhouse kits. It is an oval shape in the 15′ – 26′ Growing Dome sizes and round in the 33′ – 42′ Growing Domes.

Many Uses of the Above Ground Pond

Greenhouse Pond Painted by Artist
The Growing Dome Pond as a Canvas

Unique among greenhouse designs, the pond is not just a source of thermal mass. It fosters a beautiful environment for both aquatic plants and fish.  The pond also works well as the main fish tank in Aquaponic systems. Plants growing in the raised beds benefit from the humidity provided by the pond. Fish waste is converted into nitrogen with the help of two types of naturally occurring bacteria, so go ahead and give your plants a drink of pond water next time they are thirsty. The pond needs to be painted a dark color to absorb heat effectively, so it is a great place get creative.

Common Floating Plants for the Pond

Floating Plant Islands in the Greenhouse Pond
Floating Plant Islands in the 33′ Growing Dome Greenhouse

Duckweed, Azolla, Water Lettuce, Water Hyacinth, Parrot Feather, Water Lily, and Frogbit are the most common floating water plants for the Above Ground Pond in the Growing Dome greenhouse. Floating islands work well for plants that need a substrate or media to grow into, such as gravel. Growing Spaces recommends covering 65 – 80% of the pond surface with floating water plants, floating plant islands, or shading the surface of the pond to prevent algae blooms. The floating plants listed here do not need a floating island. For more information, see our water garden links and resources document or our aquatic plants article.

Common Fish for the Pond

Goldfish in the Above Ground Pond
Goldfish in the Above Ground Pond

Floating plants in the pond rely on the fish for their nutrients. The freshwater species of fish listed below are the most common types of fish raised in the Growing Dome pond. Local pet stores or fisheries are the best place to get freshwater fish, but they may also be purchased online and shipped to your door overnight.

  • Goldfish
  • Koi
  • Yellow Perch
  • Freshwater Sunfish, such as Bluegill
  • Catfish
  • Bass
  • Tilapia

Thermal Mass Details

First of all, why do we choose water? It is non-toxic, cheap, readily available, allows the gardener to create a pond ecosystem, adds humidity, and it’s physical composition is perfect for retaining heat. Good thermal mass provides thermal inertia. Its thermal conductivity is high, it is moderately dense, it has a high specific heat, high heat capacity, and high thermal storage capacity. Water is a better source of thermal mass than brick, stone, or concrete. The Above Ground Pond is appropriately sized for each Growing Dome. As a general rule of thumb, a passive solar greenhouse should contain about 2 – 4 gallons of water per square foot for optimal solar gain. For example, the 22′ Growing Dome is a 350 square foot greenhouse and the pond will hold about 900 gallons of water, which equals about 2.6 gallons of water per square foot.

Growing Dome Greenhouse Passive Solar Design