Building an underground greenhouse, also known as a “walipini” or “pit greenhouse,” provides an efficient and sustainable solution to grow plants year-round, even in regions with cold winters. This innovative structure leverages the Earth’s natural insulating properties to create a stable environment, ideal for extending the growing season and cultivating plants in cooler climates. Here’s an in-depth guide on how to build and maintain your own underground greenhouse for year-round gardening.
1. Advantages of an Underground Greenhouse
Thermal Stability:
One of the most significant benefits of an underground greenhouse is its ability to maintain consistent temperatures. The ground beneath the surface remains at a relatively stable temperature, typically between 45°F (7°C) and 60°F (15°C), depending on your location. This insulation minimizes the need for additional heating during the winter months, helping to protect plants from freezing temperatures and allowing for year-round growth.
Protection from the Elements:
Underground greenhouses are shielded from harsh weather conditions such as high winds, snowstorms, and heavy rain. Because the structure is built into the ground, it is also less prone to the wear and tear that traditional above-ground greenhouses experience from storms, strong winds, and extreme temperatures.
Extended Growing Season:
The Earth’s natural thermal properties create a microclimate inside the greenhouse that allows plants to grow beyond their typical growing season. The warmth provided by the earth’s mass enables plants to thrive in colder months and ensures a more reliable harvest. This feature is particularly beneficial for gardeners in regions with short growing seasons.
Sustainability:
An underground greenhouse is an energy-efficient option for growing plants year-round. The natural insulation reduces the need for artificial heating, lowering the environmental impact of your greenhouse and cutting energy costs. It is also more resilient to power outages since it relies on passive heating from the Earth rather than electrical systems.
2. Choosing the Right Location for Your Underground Greenhouse
Maximize Sunlight Exposure:
The success of an underground greenhouse relies heavily on adequate sunlight. Select a location that receives direct sunlight throughout most of the day. Ideally, this should be a south-facing site in the Northern Hemisphere or a north-facing location in the Southern Hemisphere. The positioning of the greenhouse is key to ensuring maximum sunlight intake, which will help your plants grow efficiently.
Consider Water Drainage:
Good drainage is essential when building an underground greenhouse. Poor drainage can lead to water accumulation, which can damage plant roots and promote mold growth. Choose a site with naturally good drainage, or you can add a layer of gravel around the foundation to facilitate water runoff. Raised beds can also improve drainage inside the greenhouse.
Accessibility:
It’s crucial to select a location that is easily accessible throughout the year. While underground greenhouses offer temperature regulation, they can still become difficult to access during heavy snowfalls or wet conditions. Consider how you will navigate the site year-round and ensure that the location does not become challenging to reach in inclement weather.
3. Building the Underground Greenhouse: A Step-by-Step Guide
a. Excavation:
- Depth of Excavation:
Digging the right depth is essential for taking advantage of the Earth’s natural insulation. A depth of 8-10 feet is typically recommended, as this provides optimal thermal stability. In areas with extremely cold climates, you may want to dig deeper, as this will protect your greenhouse from frost and freezing temperatures. - Size of the Greenhouse:
The size of the greenhouse will depend on your available space and how many plants you want to grow. A standard size for an underground greenhouse might be around 10×20 feet or 12×24 feet, but you can adjust the dimensions to suit your needs. Larger greenhouses will require more excavation and materials but will allow for more diverse crops.
b. Constructing the Walls:
- Material Choices:
When constructing the walls, you can use a variety of materials, such as bricks, cinder blocks, stones, or earthbags. These materials provide strength and insulation. If using earthbags, you will need to fill the bags with earth or sand and stack them like bricks, ensuring that the walls are secure and stable. - Insulation:
The northern wall of your greenhouse should be insulated to maximize heat retention. You can use straw bales, foam insulation, or other natural insulating materials to keep the temperature inside the greenhouse stable. This will help reduce the amount of external energy needed to maintain warmth.
c. Roofing:
- Choosing Roofing Materials:
The roof is a critical element of the greenhouse, as it allows sunlight to enter while protecting plants from the elements. You can use polycarbonate panels, UV-resistant greenhouse plastic, or glass for the roofing material. Polycarbonate panels offer good insulation and durability, making them a great choice. - Roof Design:
The roof should be slanted to capture as much sunlight as possible while allowing rain and snow to runoff. The slant also encourages proper air circulation, helping to regulate humidity and temperature inside.
d. Flooring:
- Gravel Flooring:
A gravel floor is often the best option for an underground greenhouse, as it allows for excellent drainage. Gravel helps prevent water from pooling around the roots of your plants, reducing the risk of waterlogging or root rot. It also provides a stable surface for walking and working. - Other Flooring Options:
You may also consider a concrete floor or even leave the floor natural with soil. While a concrete floor will provide a more durable surface, it may require additional drainage solutions to ensure water doesn’t pool.
e. Ventilation:
- Roof Vents:
Install vents or windows at the high points of the slanted roof to allow hot air to escape. This will help regulate the temperature and prevent overheating in the summer months. - End Vents:
In addition to roof vents, installing a door or vents at both ends of the greenhouse promotes cross-ventilation. This ensures that fresh air enters and stale air exits, keeping the environment within the greenhouse healthy for plant growth.
4. Maintaining the Underground Greenhouse
Temperature and Humidity Control:
Regularly monitor the temperature and humidity inside the greenhouse. Since underground greenhouses are naturally more insulated, they require less manual temperature control, but it’s still essential to check for any fluctuations. You can use a thermostat to maintain the right environment for your plants.
Pest Management:
An underground greenhouse, while well-protected from the weather, can still attract pests. To minimize pests, employ natural methods such as companion planting or organic sprays. Regularly inspect the plants for signs of pests and diseases.
Watering System:
Be mindful of the watering schedule inside the underground greenhouse. The consistent moisture in the soil can sometimes lead to overwatering if not carefully monitored. Ensure that the drainage system works well, and use raised beds if necessary to prevent waterlogging.
5. Additional Tips for Optimizing Your Underground Greenhouse
Water Barrels:
Placing water barrels inside the greenhouse can help stabilize the internal temperature. Water absorbs heat during the day and slowly releases it at night, helping to regulate temperature fluctuations.
Reflective Materials:
To maximize the sunlight that enters the greenhouse, use reflective materials such as white plastic or mylar to line the walls. This will help distribute light evenly throughout the space, ensuring that all plants receive adequate sunlight.
Raised Beds:
Use raised beds inside your underground greenhouse to provide better soil drainage and prevent root rot. Raised beds also help you control the soil quality, ensuring that plants get the nutrients they need for healthy growth.
Conclusion:
Building an underground greenhouse is an investment in sustainability, energy efficiency, and year-round food production. By leveraging the Earth’s natural thermal properties, these greenhouses offer a stable environment for growing plants, allowing you to extend the growing season and cultivate a wide variety of crops. While the construction requires planning, effort, and materials, the rewards—fresh produce throughout the year and reduced energy consumption—make it a worthwhile project for any serious gardener. With proper design, maintenance, and care, your underground greenhouse can become a vital resource for growing food and providing a continuous harvest, regardless of the weather outside.