Greenhouses offer an effective method to grow vegetation throughout the year. Heating them in cold climates is essential to maintaining plant growth, but can become expensive. There are cheap ways to generate heat for the greenhouse using various methods of passive heat-gathering. Many of these can be used in conjunction with one another.
Greenhouses can be placed in the ground in the form of a dugout pit. The walls of the greenhouse are compacted dirt, and the roof is made from heat-gathering material such as solar panels, framed plastic sheeting or glazed windows. The dirt acts as a natural insulator, gathering and holding the heat generated by the roof and the earth itself. An alternative method to a full pit is a greenhouse dug into the side of a hill, in which three walls are compacted dirt and the roof and entrance wall are made from other material. This method keeps greenhouses 10 degrees warmer on a consistent basis, according to the Michigan Tech School of Forest Resources and Environmental Science.
There are two efficient methods of heating greenhouses cheaply with water. These can be used in tandem or alone. The first uses a 55-gallon metal drum painted black. Multiple barrels are lined up around the inside of the greenhouse walls or used as walls and filled with water. The sun heats up the metal of the drum, and the metal transfers heat to the water. The water holds the heat as long as the metal is hot; when the metal begins to cool during the night, the water releases the heat into the greenhouse.
The second method uses pipes buried below the floor of the greenhouse. Hot water is run through the pipes. The metal of the pipes acts in the same way as the metal drums to collect the heat from the sun. The water releases the heat into the floor.
Facing the greenhouse in the right direction can help heat it up with minimal cost after the initial setup. A south-facing greenhouse with a sloped roof traps the most amount of sunlight.
Thermal heat collection can be achieved using a variety of recycled materials, like walls built using stacked tires and compacted dirt. Dirt fills the spaces in and around the tires. The rubber of the tires collects the heat, while the dirt works the same way as it does in the pit method. Plastic bottles embedded into walls are another method of thermal heating using recycled material. The plastic is painted black to attract heat, which it redirects into the greenhouse.
Building walls and floors out of dark brick or patio stone helps collect and hold heat until temperatures decline, cooling off the material and releasing the heat into the greenhouse. Floors built from tightly packed gravel, or concrete with a tile layer overlayed upon it, work in a similar fashion.