Build Your Own Underground Greenhouse

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The idea of an underground greenhouse suggests a bleak future where growing plants above ground is no longer feasible – right? Maybe not. Underground gardening can extend growing seasons by a month or two, or even allow for year-round planting if you’re up for putting in some real effort. Read on and find out how it’s done.

There are various options for underground gardens and all make use of the fact that soil has thermal mass. In other words, it can absorb heat and slowly release it. This prevents temperature fluctuations because the excess heat of the day is absorbed, and released overnight. This process can even work in winter by using a south facing window to capture the heat of the sun.

The Pit Greenhouse

An underground or pit greenhouse is buried six to eight feet into the ground and covered with a glazed roof. In the Bolivian mountains, farmers use them to protect crops from the high altitudes and harsh weather. These walipini as they’re called (meaning place of warmth) are a lifeline to farming communities, allowing them a reliable source of income to provide for their families.

inside of an underground greenhouse

A traditional Bolivian walipini

A normal greenhouse wouldn’t last long under the strong winds and high UV rays of the Bolivian mountains. But these buildings are perfectly suited to their environment and blend seamlessly into the landscape.

So why build an underground greenhouse in your garden?  For one thing, running costs are lower because the soil’s thermal mass reduces the need for heating in the winter. Also, building costs are significantly less than an actual greenhouse. Of course, there’s a bit of effort in the initial construction, and some important points must be considered before you head out the back door, spade in hand.

Don’t Forget Your Shovel…

First things first – as with all building projects, location is key. If you’re in the northern hemisphere, the roof needs a decent slope to make the most of the winter sun. Otherwise the roof’s angle will be too low for the sun to reach your plants for much of the winter.

Checking out the angle of the winter solstice sun will help you work out where to position your roof. Then you can raise the north side of the roof to create a south-facing slope. However, it’s not advisable to build into a hillside because downward pressure from the soil will make supporting the walls more difficult.

underground greenhouse

This pit greenhouse is entered via an adjoining shed and fits nicely into the surrounding landscape

The depth of the water table is the second important consideration. You’ll need to build at least five feet above this – so considering the height of the greenhouse, water table depth should be eleven to thirteen feet at least. Water table depth can be difficult to ascertain as it can fluctuate from year to year. In Ireland you can contact Geological Survey Ireland for help and advice.

It’s also a good idea to check for gas pipes or electricity cables before you dig. You can contact Gas Network Ireland’s Dial Before You Dig service on 1850 427747. For the ESB Network’s service call 1850 928960.

Drainage & Ventilation

Drainage is crucial. You should fill the bottom foot or two of the greenhouse with large stones, then smaller stones, and finally topsoil. The floor should be sloped slightly from the centre to either side. At each side of the building, a gravel filled sump (with a removable lid to allow for manual removal of excess water) will allow for internal drainage.

External drainage is also important, so the berm (the raised bank of earth supporting the north side of the roof) must have a steep enough incline to move water away quickly. The berm may also be covered with a layer of plastic sheeting to help the process along. This rainwater can even be collected for use inside the underground greenhouse.

Finally, you’ll need to support your walls and allow for ventilation. Traditional walipini use rammed earth for support; a gentle backward incline from bottom to top to discourages crumbling earth from caving inward.

Rammed earth is a traditional building method in which a damp mixture of earth is compressed into a frame or mould to form a solid wall. However, it’s best for soils with less than 15% clay and higher amounts of sand or gravel. Ireland’s clay-rich soils make cob construction a more suitable method for supporting the walls. There are plenty of video tutorials on YouTube that will take you through either of these methods.

The Earth-Sheltered Greenhouse

If the Walipini-style building is too big for your garden, the earth-sheltered greenhouse, designed by Mike Oehler, may be a more suitable option. Oehler was an American author and designer of sustainable alternative housing who died in 2016. He wrote the book on underground gardens, literally!

Motivated by a desire for fresh greens (which grew for just three months of the year in the cold Idaho climate where he lived), he eventually published The Earth Sheltered Solar Greenhouse Book which is full of plans, advice and wisdom gleaned from his own adventures in gardening.

diagram of an earth-sheltered greenhouse

As with the pit greenhouse, Mike’s design includes a south-facing sloped roof to make the most of the sun on shorter autumn and spring days. At three feet  wide, the growing bed is comfortably accessed from the walking platform. The cold sink below the platform allows the coldest air to spill down below the plants where it is warmed by the earth.

Mike recommends using upright posts (buried ends wrapped in plastic), lumber shoring behind the posts and a protective layer of polyethylene sheeting between the lumber and earthen backfill. The posts need to be buried one and a half to two feet down so that they resist lateral pressure from the soil. The roof beams support the upright posts at their top ends.

Air vents at the front of the greenhouse between the window and the soil, and in the back of the greenhouse just above the earth berm, will allow for ventilation. As with the pit greenhouse, you can also collect rainwater from the roof.

The Growing Pit

If your garden is small, another option is a growing pit. This is a hole about two to three feet deep, filled first with a layer of manure or compost, and then with topsoil, leaving a space just below ground level for growing. The compost layer below the topsoil will keep your plants warm.

The pit should be covered with glazing and packed around the edges with soil or straw as insulation; an old window or sheet of perspex will do the job. On sunny spring days the window can be propped up at an angle to catch the winter sunlight, remaining closed for colder days and frosty nights. Your plants will benefit from a greenhouse effect with minimum cost and effort.

plant covered in frost

An underground garden will protect your plants from spring frosts and snow

In his quest to extend his growing season, Mike Oehler discovered that farmers used these pits in years gone by. He tried it out himself prior to designing his earth-sheltered greenhouse, and found it reasonably successful. It’s a relatively simple method that will extend your growing season by a month and
save you from much anxious weather forecast checking and wondering if you’ve planted too early.

So, there you go – three methods for building an underground greenhouse, all of which will increase your growing season and output. And if you’re looking for something weird and wonderful to plant, check out our guide to fascinating fruit and veg.

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