An Underground Greenhouse Is the Secret to Year-Round Gardening—Here's What to Know
An underground greenhouse is the perfect place to overwinter your spring and summer plants.
As fall creeps in and winter quickly follows, many gardeners must put their passion on hold until spring. Underground greenhouses can prolong the growing season, protect plants vulnerable to freezing temperatures, and provide an excellent way to enjoy gardening year-round. To learn more about these unique structures, we spoke to two gardening experts who shared everything you need to know about underground greenhouses.
Meet the Expert
Debbie Merriam, executive director of the Mary M.B. Wakefield Arboretum
Danny Kovach, technical design expert at BC Greenhouse Builders Limited
What Is an Underground Greenhouse?
An underground greenhouse, often called a walipini, is a sunken outdoor structure designed to keep plants alive year-round by levering the insulation properties of the surrounding earth. These structures vary by location, budget, and intended use, but they’re typically built a few feet into the ground or on the side of a hill, wall, or landmass.
“It’s like having a root cellar,” says Debbie Merriam, executive director of the Mary M.B. Wakefield Arboretum. “Because it's below ground, the temperature stays much more constant. It doesn't get really cold, and it doesn't get very hot." Since they maintain consistent temperatures, underground greenhouses are a great option for gardeners who want to start plants in the cooler months and move them into the garden when the climate is more suitable.
Benefits of an Underground Greenhouse
Adding an underground greenhouse to your property offers many benefits. It allows you to overwinter plants, extend the growing season, and get a head start on planting for next spring.
Overwinter Plants
Even a simple, inexpensive underground greenhouse can keep many plants alive during winter. This makes it an excellent option if you want to overwinter spring and summer plants but can’t or don’t want to bring them inside.
Depending on where you live, underground greenhouses don't typically dip below 35 degrees Fahrenheit. Many plants, especially woody ones like trees and shrubs, can tolerate those cold but not freezing temperatures, says Merriam.
Extend the Growing Season
Cold-season crops, like root vegetables, brassicas (cauliflower, broccoli, etc.), and leafy greens, may get an extended growth period when moved into an underground greenhouse, even if the outdoor temperature gets below freezing.
If your underground greenhouse includes a heating element or thermal materials, or if you live in a warmer climate, you may also be able to grow other crops, such as berries, tomatoes, beans, etc. Mushrooms can also thrive in damp, dark recesses.
Start Plants for Spring and Summer
Another use for underground greenhouses is starting plants that may not be suitable for the cool temperatures of late winter and early spring. “When it's too cold to start plants outside. You could start them in your pit house and know that they would be safe,” says Merriam, highlighting common crops like tomatoes and lettuces.
Once the weather conditions improve, you can bring the plants outdoors in containers or transplant them into the ground.
Energy Efficient
Though underground gardens can have a heating or thermal element, it's not required. By leveraging the surrounding earth, solar energy, and some well-chosen supplies, you can easily maintain an underground garden with little reliance on man-made heating components.
Budget-Friendly
Though you can build an elaborate underground greenhouse, it's not necessary. A basic underground garden can be a simple structure made from items you can find at your local home improvement store.
An underground garden can be built with a few polycarbonate panels, wood 2x4s, and vellum or another material that lets light in. Structures built using these tools are very practical for homeowners, Merriam says.
Related: How to Winter Sow—Plus, 6 Expert-Approved Seeds to Plant During the Cold-Weather Months
What to Consider When Building an Underground Greenhouse
Underground greenhouses can be simple, inexpensive structures made with common building materials or complex units that allow for passive and off-the-grid gardening. Regardless of your structure, there are a few key factors to consider when building one.
Location
Location is key when digging an underground greenhouse. Exploring the dynamics of any given location and understanding its relationship with the earth around it is essential to success, says Danny Kovach, technical design expert at BC Greenhouse Builders Limited.
Observe the environment where you're going to put the greenhouse through all the seasons that you plan to use it, he says. Take note of everything from the sun's habits and tree coverage (and how it changes over the year) to the winds that pass through and the way water drains around the location.
Gardeners should also be mindful of electrical and irrigation lines before digging to avoid potentially unsafe or costly issues, says Merriam.
Tip
One of the most advantageous ways of building a greenhouse and incorporating it into a lifestyle is doing a home-attached greenhouse, where one or two sides of the greenhouse walls are attached to your home or a heated space, says Kovach.
Greenhouse Depth
To determine how deep to dig your greenhouse, Kovach recommends that gardeners find out what their local frost depth is. You can do this by contacting your local building department or simply searching online for the frost depth in your area.
Once you determine the frost depth in your location, Kovach suggests doubling it to determine how far to dig. For instance, the frost depth where he lives is 18 inches below ground, so he would recommend digging to 36 inches. This ensures that the ground (and your plants) won’t freeze during extremely low temperatures.
Foundation Material
The quality of your foundation will dictate the success of your underground greenhouse. “Some sort of retaining wall inside your structure is going to be necessary,” says Merriam, adding that timbers and cement blocks are viable options.
Kovach cites water and drainage as a major concern for the foundation and encourages builders to consider their water table and drainage before they choose a location, especially if they’re close to adjacent structures with roofs.
Ventilation
Depending on how you build your greenhouse, ventilation may be an issue worth considering, as it can help regulate humidity, discourage mold growth, and prevent gas pooling. If you walk into the greenhouse and most or all of your body is below ground, the potential for gas pooling is enhanced. More shallow greenhouses or those with loosely built structures and natural ventilation won’t present the same level of concern as deep or tightly sealed underground gardens, says Kovach.
In a recessed area with rotting foods and decomposition, gasses can displace oxygen, says Kovach. Radon, a radioactive gas produced from the natural decay of uranium found in rocks and soils, may also be a concern, depending on your location. Adding a ventilation component will ensure your underground greenhouse functions safely.
Access to Sunlight
Merriam recommends building an underground greenhouse so that the south-facing side receives sunlight, with the backside closed. This keeps the greenhouse warmer in cooler months.
It's also beneficial to add a reflective surface to the inner north-facing roof. “If you can make the underside of the roof reflective, you're going to get more photosynthesis bouncing off the underside of that roof down onto your plants,” Kovach says.
You can further leverage the heat produced by the sun by painting certain surfaces black to absorb the heat. Not just any surface, though. “You want to paint the surfaces that have higher material heat capacity, so stone, concrete, water barrels, etc.," says Kovach.
What to Grow in an Underground Greenhouse
How you use your underground greenhouse and what you grow in it depends on how your underground greenhouse is constructed and where you live. Potential crops for underground greenhouses may include the following, but it’s best to examine your specific space and the environmental conditions of potential plants.
Cold-tolerant crops, such as spinach, lettuce, and arugula.
Root vegetables, such as carrots and turnips.
Herbs, such as chives, parsley, basil, cilantro, and lavender.
Fruits, such as strawberries, small citrus trees, and blueberries.
Seedlings for a variety of plants.
How to Maintain an Underground Greenhouse
Underground greenhouse maintenance often depends on the structure itself, but these tips can help ensure a clean environment and increase the longevity of both the structure and the plants inside.
Regularly remove any leaves or debris that, if left on the ground, would allow for mold growth or disease spread. Clean the outside of the greenhouse to remove potential mold. Merriam recommends using a biodegradable soap and brush to keep panels clean.
Check draining and ventilation systems to ensure they are functioning properly. Identify and fix any holes or leaks as they occur to prevent further damage.
