A greenhouse creates a warm indoor environment that protects tender plants from outdoor conditions when the temperatures are too cold for plants to survive. A greenhouse is, by nature, poorly insulated against cold weather. Although the structures are naturally warmed through solar radiation during daylight hours, at night greenhouses can become very cold. Central-heating systems for greenhouses are one way to warm the structures, but there are several ways to heat a greenhouse that are much less expensive.
Position your greenhouse at the south side of a stone, block or brick building. The stone will absorb solar radiation during the day and radiate heat into your greenhouse at night.
Spray-paint rain barrels black. Place the barrels inside your greenhouse and fill them with water from a garden hose. The black barrels will create a heat sink by absorbing solar radiation during the day that will heat the water in each barrel. At night, the warm water will release this heat to warm the air around the barrels.
Create a tent inside your greenhouse with bubble wrap. The air pockets in each bubble have an insulating effect that helps keep the air under the tent warmer.
Dig a pit in the center of your greenhouse with a shovel for composting refuse. Compost piles can heat up to 150 degrees, which can raise the ambient temperature of the greenhouse. Excavate a compost pit that's 3 cubic feet in size, then fill it with alternating six-inch layers of sawdust and manure. Soak the compost with a garden hose until it is as damp as a wrung-out sponge. Cover the compost pit with a 3/4-inch sheet of plywood so you can walk over it. Check the compost with a thermometer that has a probe. Stir the compost with a shovel any time the temperature drops below 120 degrees.