Greenhouse in a Swimming Pool - Winter Harvest

 Salad greens, chives and braising greens thrive in this winter growing space, a greenhouse converted from an old swimming pool. Outside temperatures dipped down as low as 3F (-17C); yet this space remains productive without any additional heat. It is heated by the sun during the day, and stays relatively warm (around 32F (0C)) on coldest of nights. Plants are happy and harvest is in full swing.

This unheated growing space was created in an old swimming pool that has fallen into disrepair. Old swimming pools are hard fix; this one was built in the 1950-s and doubled up as water cistern for irrigating fields and pastures. It is made with poured-in-place cement and a 4'5" (1.3 meter) earth embankment on three sides - and making anything functional out of it was really an interesting project. Being a hole in the ground, it collects water (the old drain does not function); and also functions as a default catchment for autumn leaves. If left neglected, it could fill up with fallen leaves and branches within 2-3 seasons and become a problem, as happened with a number of abandoned pools in the area. It is simply unpleasant as it spells danger for children and animals.

Photo above is taken around 5pm in January - with low winter sun shining from the west (left side on the photo). Oriented long axis North-South, this old pool made a perfect shell for a sunken greenhouse. One half of the pool was covered with a roof; a South-facing wall completed the new growing space. Of course, when dealing with anything sunken, solar exposure (or lack of thereof) and water-harvesting virtues are to be considered. In other words it is likely that there will be some lack of sunlight from the shade cast by the surrounding walls; and water will collect in the lowest spot (i.e. under your feet) after each rain. Here in NM the rainfall is around 9" per annum, and though there are very heavy rains in the summer that flood the pool/greenhouse to about 3", such rains are followed by hot dry days and water simply evaporates without much damage. The floor in this (and most pools) is slightly uneven and most water does not even enter the greenhouse (a simple wooden threshold holds water back!). The old drain is not functioning, yet no problem so far after 10 years of use.

Some years we use fallen leaves that collect on the bottom of the pool and push them against north wall where they act as a sponge for extra water and decompose without any assistance into wonderful rich soil food - which, as you may guess, is then used in the greenhouse itself.

 

This greenhouse has another great advantage - being sunken into the earth it has a substantial thermomass, which means it is significantly more immune to temperature swings than a typical freestanding glass/plastic greenhouse. Less temperature swings means a longer growing season on both ends of the year - there is slower cooling off in the winter (plants stay cozy longer and grow better) and slower warming up in the spring (plants don't bolt so fast and they don't get infested with bugs due to unfavorable growing conditions). In general, when thinking about a greenhouse, it is best to choose an attached (to a house or another building) construction. In this case, the earthen banks serve as thick insulation from the elements, an attachment of sorts. 

This particular greenhouse is a part of an active poultry/goat yard. This is why you can see some wood logs placed on the roof of the greenhouse (photo above) - this is to discourage goats from using the roof as their playground (yes, logs work). There is nothing more worrisome than an active goat family on the roof of the building while you are inside and under them!

 

Seven steps down, across the pool, through the door - welcome to the greenhouse. Since this is a permaculture greenhouse, it serves more than one function (Each element serves multiple functions in permaculture design; element being any object, thing, component or participant of the design). Besides growing greens, it is used as milk parlor for two dairy goats. To produce high quality raw milk, it is very important to have a clean, contained, dust free environment that is removed from the animal barn. The goats get milked in the greenhouse, which is easy to keep relatively clean (broom + occasional shop vac), and which stays considerably more comfortable and warm in the winter (try milking when your hands are freezing!!!) and cozy in the summer (no flies, no eager chickens interested in goat feed). On the photo above, Nubian goat Rosa is on the milk stand, eating her meal before being milked. Foreground shows blue barrels filled with water for additional thermomass (I started to paint one of them white to increase light reflection, but paint smell made me stop resulting in an ugly paintjob. I think it will remain that way for ever now.). Sun makes the water in those barrels warm (relatively speaking) and then the heat is radiated at night to keep the space yet a little warmer. 

Another element to notice is insulation on the roof and back wall. Heat raises. It exits through the roof, in every building that has insufficient insulation. That is bad news for buildings made with glass and plastic roofs, such as greenhouses! One way to deal with this is to realize that in the winter the sun is quite low, most of it shines on the plants through the South glass wall; not as much through the roof. Therefore, if Northern half of the greenhouse is fully insulated, top and sides, there is no noticeable loss of light! Southern half of the space has partial insulation, and partial clear glazing - with heat losses occurring here for sure, but only via about 20% of total roof space. Take another look at the picture above to see how sun reaches directly to the far (Northern) wall; with the photo taken around 5pm in late January (short days, weaker sun).

In the summer the greenhouse gets a rest, as far as growing goes - the milk parlor use continues. Now we are experimenting with figs, artichokes and leeks and will probably keep it green year round. August - May is the main growing season for this space, with winter greens abundant and healthy as no bugs can survive the summer resting period with no greens to munch on.

Last word on keeping it warm in bitter cold. Soil and water barrels are placed on top of 2" insulation, so they don't "bleed" their heat into the surrounding cement. On coldest nights row covers are placed over greens to give them an additional cushion. Adding more water barrels or a large pond would be a good next step, as water stores heat and adds to the ambiance of any greenhouse. Happy growing!