The Passive Solar Greenhouse

Our most regular contributor reports on the perfect greenhouse.
by Don Tollefson, 599 California Avenue, Venice, California 90291, USA
Chamaerops No.32 Autumn 1998

Introduction: For most palm enthusiasts the primary objective is to attain a successful, outdoor, palm collection. One that looks as good on New Year's Day as it does any other day of the year. Since most palm species were not biologically intended to grow in a temperate climate, the first requirement in the development of such a collection is to grow the palm to the "optimum size" before planting it outside in the ground. Optimum, outdoor, planting size for most palms is as a substantially rootbound, five gallon. This size provides adequate mass to endure the cold of that first, and most difficult, upcoming winter. Any size smaller is a disservice to the collector, the collection and the palm.

The greenhouse advantage: Nothing facilitates growing palms quickly to the optimum, outdoor planting size like a greenhouse. Nothing! Estimates are, that in a temperate climate, a greenhouse can produce an optimum sized palm approximately eight to ten times faster then the same palm grown outdoors. This is not to mention the fact that most species capable of growing in a temperate climate cannot survive outdoors as young palms. With an effective greenhouse a grower can expect to grow a palm from a newly sprouted seedling to a robust, rootbound, five gallon size in about three years. Why then, are greenhouses so conspicuously absent from the arsenals of temperate climate palm enthusiasts? Primarily, because in a temperate climate, greenhouses are so extremely expensive to heat at night during the winter, that most palm enthusiasts are reluctant to have one.

Levelling the Playing Field: A greenhouse, effective for cold climate palm growing, can be unnecessary to heat at night or maintained with only a small amount of back up heat on cloudy days. How? By collecting maximum, passive solar heat per window size during the day and by providing (and containing) maximum, passive heat during the night. Always remember, the test comes during the coldest period of the winter. This requires the following:

A. Southerly exposure: Southerly exposure is the starting point in collecting maximum, daytime, solar heat. It's a simple formula. Southerly exposure is excellent. Easterly and westerly exposure is marginal. Northerly exposure is disastrous! It is during winter that greenhouses most need heat and during the winter most sunlight arrives from the south. Approximate south (in the northern hemisphere) is the direction of the sun at noon. To determine "the south", locate the sun at "solar noon". Solar noon is halfway in time between sunrise and sunset. Consult a local newspaper for these times. True south can also be located with a compass, but remember to account for the approximate 20 degrees difference between true south and magnetic south. The wide side (longest side) of the passive solar greenhouse must face south.

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