Winters in Centralia used to be so cold that Eric Lawman probably never imagined that he’d one day be tending a solar greenhouse and growing things all through the winter here in central Missouri. But that is what he finds now as part of his work at the MU Bradford Experimental Farm on Range Line Road, just north of Highway AC.
After the recent second warmest summer in the U.S. on record, some gardeners are also thinking of cool-season gardening as an alternative to bugs, drought and day-upon-day of temperatures well above the 85 degrees when plants stop growing and producing.
As my good friend Wolfgang said to me in Germany last summer of their well-above-normal temperatures — yes, there too — “Something is happening.”
Scientific opinion is now essentially uniform that this “something” is global warming, and even our best efforts will not unwind the dynamics now in motion for at least several decades. Why then not take advantage of what a warmer climate will offer us here and consider cool season, and even winter gardening, perhaps in a passive solar greenhouse of the kind Eric took me through at Bradford Farm?
The Bradford greenhouse uses only the sun as its heat source. The greenhouse is a duplicate of one first built at the UM system’s Southwest Research Center in Mt. Vernon, where it has successfully been in use for a dozen years, producing flowers and vegetables from November through April. The interior north wall of the south-facing greenhouse is lined with black 50 gallon plastic drums containing 1,100 gallons of water. During a winter day, whatever heat available from the sun warms the greenhouse, and this heat, together with direct solar radiation, in turns warms the water in these containers.
Overnight, the heat in the water is released and keeps the greenhouse and plants in it above freezing. That is the theory of it.
I drove out to Bradford Farm a week ago to see how the greenhouse had fared over glacial January and February. I was told that the greenhouse had remained above freezing and all its plants were doing well. So the thing works in practice.
The Bradford Greenhouse is 12 feet wide by 24 feet long with the ridge of the roof about 11 feet high. The south-facing sloped roof is clear-glazed to encourage the entry of all available sunlight and is about 12 feet in height and rests upon a short vertical wall, also clear-glazed.
The north wall and roof are both about 7 feet in length, six inches in width and stuffed with R-19 insulation, as are the east and west walls; all insulated surfaces are faced on the interior with white plastic of the kind used for shower enclosures. An insulated metal door allows entry and exit.
One technical design feature is that the south wall/roof consists of two layers of heavy clear plastic through which a small blower pumps air that separates the two layers, again by six inches, providing insulation on that side of the greenhouse while allowing solar radiation to enter. Alternate solar greenhouse designs use double- or triple-walled greenhouse polycarbonate glazing that needs no airflow through it.
Both water and electricity are available in the Bradford greenhouse.
It turns out that the most critical consideration in greenhouse function is ventilation, and the Bradford greenhouse uses a fairly large, thermostatically actuated fan to pull fresh exterior air through the greenhouse whenever interior temperatures exceed 85 degrees, as can easily happen on a sunny day in the depth of winter.
If you find the idea of being able to work with green plants in bright sunlight on the coldest of days appealing, you will want to learn more about solar greenhouse design. Much can be found on the Web but you may want to start with the UM Extension publication by Andrew Thomas and Richard Crawford, “Performance of an Energy-efficient, Solar-heated Greenhouse in Southwest Missouri,” reprinted from Small Farm Today in March 2003.
The Bradford research farm can also supply you with a materials list and estimated cost for its greenhouse. I’m thinking of building one myself and am exploring slight design alternatives that can make it entirely “off the grid.”
Dennis Sentilles, MU professor emeritus of mathematics, is a Missouri Master Gardener, a member of Katy Trail Slow Food International and has a love for working outdoors and eating simply and well every day. He can be reached at sentillesd@missouri.edu.