Glass Palaces

My little cottage is surrounded by the brilliant display of fall foliage, trees giving us a last heroic flash before they move into winter’s hibernation – and I’m thinking of growing plants.   So I helped my sister install a small hoop house to extend the growing season a bit.  It felt great to get some dirt under my fingernails, and to use my brain cells on something practical.

Truth be told, I’ve always been a bit fascinated with greenhouses.  There is no finer museum visit than to the old Victorian style conservatories.    I’ll admit that I’m no great botanist, and so can’t really appreciate the uniqueness of the lilies (which seem to be the prize possessions of these glass palaces).   But I have good memories – especially on one winter’s visit to Pittsburgh, where my friend kindly deposited me in the Phipps Conservatory, so I could sit in the desert room and bake some heat into my bones.  

Aside from the propagation of exotic species, greenhouses can serve a very practical function of providing fresh produce, especially greens.   While some vegetables or fruits ship and pack well, such as apples, I always felt that greens are best grown locally, picked fresh and eaten that day.  The good news is that greenhouses can come in all sizes – from the largest greenhouse in the world (the Eden Project in Cornwall) to a simple PVC hoop house.

Regardless of the size, there are a few common denominators.  Greenhouses are essentially passive solar houses – without shading.  A well-designed production facility will normally provide an environment with temperature set points between 55 and 85°F, with humidity levels high enough to reduce water stress and low enough to discourage disease and fungus outbreaks in the crop. So excess heat and moisture needs to be removed  through some type of ventilation system.   For even a moderate sized greenhouse, an exhaust fan tied into a thermostat  can easily automate the process and can also have a timer set to regularly refresh the air.

The other issue is adding heat during the night, since there is essentially no insulation in the walls. The niftiest idea I came across was a subterranean heating and cooling system,  which might be understood as a closed loop geothermal recharge system.  It used the soil under the greenhouse as a type of phase change converter.   A small fan moves the excess daytime temperature and moisture into the ground, where the cooler soil causes the moisture to condense out, thus cooling the air, and returning into the greenhouse as cooler and dryer air.   During this daily cycle, the soil under the greenhouse stores the heat, which then radiates up through the greenhouse at night.    It seems to be a very low maintenance systems, which has been tested in the Rocky Mountains and in Alberta, Canada – some pretty cold places.

I've oftened wondered just how much food we could produce if we converted even a small amount of our garden or side-yard spaces to greenhouses.  Maybe not enough to affect the GNP, but maybe enough to increase our intake of fresh vegetables, and help our kids understand that food actually comes in plant form, and not just from a box.