The Owner Built Home & Homestead
(Page 2 of 7)
July/August 1972
By Ken Kern
Much of my understanding and appreciation of greenhouse functions grew out of a brief 1957 visit with F.W. Went, then director of the Earhart Plant Research Laboratory in Pasadena. Through lengthy and painstaking experiments, Went found optimal temperature and humidity requirements. Tomatoes, for instance, require an optimal daytime temperature of 80 to 90 degrees Fahrenheit, nighttime 65 degrees F. The optimal daytime humidity was found to be 50 to 80%, nighttime 95%. Went's findings proved important to the furtherance of plant growth knowledge — it also pointed to some obvious inefficiencies of conventional greenhouse design.
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The "greenhouse effect" is an expression which applies to a building having excessive radiation buildup. As one would suspect, greenhouses are troubled with "greenhouse effect" . . . so is our atmosphere. Atmospheric vapor filters shortwave solar radiation (ultraviolet). Water vapor, however, is transparent to visible light, which warms the earth and re-radiates longwave (infrared) rays back to the atmosphere. Some of this infrared heat is absorbed by the atmosphere, and some is reflected back to earth. The earth's atmosphere acts like glass in a greenhouse: opaque to longwave but transparent to shortwave radiation. In a greenhouse situation this effect works in much the same fashion: the ground and vegetation inside are heated by the transmission of ultraviolet rays from the sun. These contents then give off heat in the form of infrared radiation. Window glass, however, will not allow these longwave radiations to escape so they are retained (to the actual detriment of the vegetation inside).
Heating from strong radiation reduces the nighttime humidity of the air when a high water-saturation is especially needed. Artificial heating also tends to lower the relative humidity. Went overcame these obstacles in his experimental greenhouses by employing elaborate, highly sophisticated, artificial conditioning devices. These methods are of course not available — nor even desirable — for homestead greenhouse production. A better home-grown solution is to design a greenhouse structure that provides optimum growing conditions.
A little greenhouse research reveals the fact that, although Washington and Jefferson both had greenhouses, the oldest reported forcing structure in the U.S. was not a greenhouse as we know it today. It was, rather, a pit covered with glass on the south side, and earth insulation on the north. This so-called pit greenhouse was built into the side of a Waltham, Massachusetts hill about 1800. I found that the pit greenhouse is practically unknown among horticultural circles, yet it proves to be a far more sensible, economical, and efficient forcing structure.
In principle the sun pit is an "unheated" greenhouse. That is, it relies entirely upon solar and ground heat rather than auxiliary furnaces — which are always required in conventional greenhouses.
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