Worried about frost damage to your plants? Build an Underground Greenhouse

Would you pay $300 for a hole in the ground covered with polythene sheets? Lots of people do, apparently. The idea started in the cold mountainous regions of South America, to allow people to grow plants all year round.

These underground greenhouses are called Walipini, an Indian word for ‘place of warmth’. They are popular with the green movement because they are 100% natural.

undergroundgreenhouseA combination of just two simple geophysical phenomena — the ‘thermal constant’ and the ‘flywheel effect’ provide a stable temperature and enough light to encourage plant growth throughout the year. Passive solar heating is trapped and released during the night.

For those willing to give it ago it isn’t too difficult to make. It is, simply put, a rectangular hole 6-8 feet deep, covered with two layers of sheet plastic and surrounded by earthen walls (see picture, above).

The science bit

Heat generated by the earth’s molten core heats the entire planet. You need dig only four feet down to encounter this geothermal energy. Because at that depth temperature across the planet is always between 50 and 60 degrees Fahrenheit—regardless of the temperature at the surface. This is the so-called ‘thermal constant’. It is this phenomenon that accounts for the Walipini’s effectiveness. Its power derives from the thermal constant.

Its earth walls absorb heat, acting like batteries. When heat is stored in in this way — built up during the day and released at night — this cycle is referred to as the ‘flywheel effect’.

Walipini are designed to absorb most sunlight during the three coldest months of the winter. It is critical that they store enough heat so that on the coldest nights of the year plants are not damaged by frost.

A double layer of plastic sheeting — effectively double glazing — is used on the roof. And just as it does in our own homes, this insulates the interior, slowing the escape of heat at night. The air space between the plastic sheeting should be 3/4” to 4” thick for maximum benefit.

Water danger

When choosing a site for your new underground greenhouse it is critical to take the effects of local water into account. Two dangers are posed. That water penetrates the earth walls, causing them to collapse. And that water seeps up from the water table, ruining the roots of your plants.

You should choose an area where its lowest point is at least 5’ above the water table. When ground walls are ramped add a layer of water-proof clay or plastic sheeting between 6” and 1 foot under the surface. These should be slanted away to drainage ditches. The drainage ditches should be carefully constructed to carry surplus water away from the area where your earth-walled greenhouse is situated.

The Walipini is a great example of the resourcefulness of humankind when faced with environmental challenges. That their construction requires no damage to the eco-systems in which they are placed is commendable in a world where much damage is wrought in the pursuit of other forms of energy.

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