Greenhouses are heated by the sun during the day, but according to Hobby Greenhouse, up to 80 per cent of all that heat is lost overnight. Electric heaters do a good job of keeping a greenhouse warm, but are expensive to use and could cause in a fire under certain circumstances. There are simple ways to keep your greenhouse warm at all times. Best of all, these techniques are absolutely free.
Water barrel technique
Obtain one or more 247.5 litre (55 gallon) barrels. Factories and warehouses often dispose of these and many happily give them away for free.
Put one or more barrels in any empty spot in the greenhouse. Put them where they will not be in your way when watering or harvesting plants. You need about 9 litres (2 gallons) of water to heat for each 0.09 square metres (1 square foot) of area.
Fill the barrels with water. During the day, heat from the sun will heat the water. As the temperature cools down at night, the heat will emit from the water, keeping the air in the greenhouse warm.
Gather medium to large stones. They can easily be found in wooded areas. Ask friends and neighbours for any stones that may be on their property. Free stones can also be found on classified websites.
Make piles of stones throughout the greenhouse. You need 36.3 kg (80 lb) of stone to generate heat for each 0.09 square metres (1 square foot). If you gather enough large stones, make a short wall along one side of the greenhouse.
Let the stones absorb heat from the sun. Stones retain heat very well and will slowly release it throughout the night, thus keeping the inside of the greenhouse warm.