How to Heat a Hoop Greenhouse

A hoop house, usually covered by polyethylene, can be heated various ways.

A hoop greenhouse is a popular greenhouse design for amateur gardeners because it is easy to build and the raw materials can be purchased inexpensively. Hoop greenhouses allow gardeners in northern climates to extend their growing season, and gardeners in southern climates to grow vegetables all winter long. Although sunlight will raise both the temperature of the air and humidity inside the hoop greenhouse during the day, gardeners often have to provide heat at night. There are a several strategies to warm a greenhouse without resorting to a radiator or an electric or gas heater.

Things You'll Need

• Shovel
• Wood chips
• Manure
• Black water barrels
• 3/4 inch plywood boards
• Planting flats

Instructions

1. Place two stacked rows of black or dark-colored plastic rain barrels along the north wall of your greenhouse. Make sure the south wall is largely unobstructed and will allow passive solar energy to reach the barrels in winter. Fill the lower level of barrels with water and securely fasten their lids before adding and filling the second level. Use a heavy board or thick plywood cut to size between the two layers, if desired, for structural stability. As demonstrated by the University of Missouri's passive solar greenhouse, the barrels will soak up solar radiation and heat the water during daylight hours. When nighttime temperatures fall that stored heat will be released by the water, warming the greenhouse air at night.

2. Dig three trenches in rows along your hoop house, to try a variation of the "compost heater" method. Each trench should be 3 feet wide and 3 feet deep. Leave a foot and a half of space between each trench.

3. Fill the center trench halfway full of wood chips. Bury fresh (uncomposted) manure in the wood chips throughout the trench. The ratio of manure to wood chips should be 2 parts wood chips and 1 part manure. As the manure and wood chips break down, they will provide heat to the greenhouse. Cover the trench with 3/4 inch plywood to walk over.

4. Remove the boards and shovel the manure and wood chips from trench to trench every few days. This will cause the organic material to mix and heat up as it decomposes. When the organic material is fully decomposed into compost, you can use it in your garden or potting soil. Once the organic material no longer heats up, start a new compost pile in one of the empty trenches.

5. Combine the two methods, for two sources of greenhouse heat.

Tips & Warnings

• Monitor hoop house temperatures with a thermometer. Open a greenhouse door to vent the heat once the greenhouse reaches temperatures too hot for the plants inside. Cool season vegetables such as lettuce will suffer if temperatures climb too high.

References

• Texas A&M University Extension: Greenhouse Heating Requirements
• Ohio State University Extension: Greenhouse Condensation Control Bottom Heating and Between-row Heating
• University of Florida IFAS Extension: Greenhouse Heating Checklist
• University of Colorado Extension: Hobby Greenhouses: Heating & Cooling

Resources

• Centre for Alternative Technology: Heating a Greenhouse
• University of Missouri Agricultural Experiment Station: An Energy-Efficient Solar-Heated Greenhouse
• Mother Earth News: Mother's Compost Heater Update II