How to Build an Underground Greenhouse
Growing plants year-round in colder or inconsistent climates can be challenging, but an underground greenhouse—or walipini—offers a sustainable, efficient solution. By harnessing the Earth’s natural insulation, these greenhouses maintain stable temperatures, shield plants from extreme weather, and extend the growing season. This guide provides a step-by-step approach to building and maintaining your underground greenhouse.
1. Advantages of an Underground Greenhouse
- Thermal Stability: The Earth’s natural insulation keeps temperatures more consistent, protecting plants from sudden fluctuations. A few feet below the surface, the ground temperature remains stable year-round.
- Protection from Elements: Underground greenhouses are shielded from strong winds, heavy snow, and hail, reducing potential damage to plants and the structure.
- Extended Growing Season: A stable microclimate allows for cultivation even during harsh winters or extreme summer heat.
- Energy Efficiency: Thanks to the Earth’s insulation, underground greenhouses require less energy for heating or cooling compared to above-ground structures.
- Sustainability: With reduced energy needs, these greenhouses offer an eco-friendly solution to year-round gardening.
2. Choosing the Right Location
- Sunlight Access: Pick a spot with maximum sunlight. In the Northern Hemisphere, choose a south-facing location, while in the Southern Hemisphere, opt for a north-facing area.
- Good Drainage: Prevent water pooling by selecting a well-drained site. If necessary, use gravel beds or raised planting areas to improve drainage.
- Accessibility: Ensure the greenhouse is easily accessible throughout the year, especially during snow or heavy rain.
3. Building the Underground Greenhouse
a. Excavation
- Depth: Dig 8-10 feet deep to take full advantage of the Earth’s insulating properties. The depth may vary based on your region’s frost line.
- Dimensions: A common size is 10×20 feet, but customize based on your gardening needs and available space.
b. Walls
- Materials: Use durable materials such as bricks, stones, or earthbags. For added strength, reinforce walls with concrete.
- Insulation: Insulate the northern wall to conserve heat. Options include straw bales, foam boards, or reflective thermal materials.
c. Roofing
- Materials: Use UV-resistant greenhouse plastic, polycarbonate sheets, or glass for the roof.
- Design: A slanted roof maximizes sunlight exposure and allows rain and snow to run off efficiently.
d. Flooring
- Gravel: A gravel floor improves drainage and prevents waterlogging.
- Optional Upgrades: Concrete or natural soil can also be used, depending on personal preference and drainage needs.
e. Ventilation
- Roof Vents: Include roof vents or windows to release hot air and regulate humidity.
- Doors: Install doors at both ends of the greenhouse to promote airflow and ensure easy access.
4. Maintaining the Underground Greenhouse
- Temperature Monitoring: Regularly check and maintain the temperature and humidity inside the greenhouse. Install a thermostat for precise monitoring.
- Pest Control: Inspect for pests regularly, as the enclosed space can attract certain insects or rodents. Use natural pest deterrents like companion planting or diatomaceous earth.
- Watering: Adjust watering schedules based on plant needs and greenhouse humidity. Overwatering can lead to drainage issues, so monitor soil moisture carefully.
5. Additional Tips for Success
- Thermal Mass: Place barrels of water inside the greenhouse. These barrels absorb heat during the day and release it at night, helping to stabilize temperatures.
- Raised Beds: Raised planting beds improve drainage and reduce the risk of root rot.
- Reflective Surfaces: Use reflective materials on the walls to maximize light distribution inside the greenhouse.
- Composting: Incorporate composting systems within the greenhouse for heat generation and nutrient supply.
Conclusion
An underground greenhouse is a powerful tool for sustainable, year-round gardening. While the initial construction requires effort and investment, the benefits of fresh produce and extended growing seasons outweigh the costs. Regular maintenance, temperature monitoring, and creative additions like reflective walls or thermal mass barrels ensure optimal growing conditions. With a well-constructed underground greenhouse, you can enjoy homegrown vegetables, herbs, and flowers regardless of the weather outside.