How to Build a Pit Greenhouse on Your Homestead

Learn how to build a pit greenhouse with this excerpt from "The Owner-Built Homestead."

| July/August 1972

There's an ancient Persian proverb that says, "When you understand how to do a thing, the doing is easy; if you find it difficult you do not understand it." There are of course numerous homestead activities where a basic understanding can make the difference—not only between making a thing simple or difficult, but between a gratifying success or disheartening failure. And nowhere on the homestead is this dichotomy more evident than when one attempts to modify plant environment by the use of a forcing structure.

Some types of plant shelter are simple and easily understood; a shade or windbreak screen, an arbor or even a cold frame are rational structures requiring minimum knowledge to construct and manage. But it's a different ball game when a homesteader attempts to modify plant environment in a greenhouse situation.

A greenhouse is something more than a sun trap and a light trap for the benefit of plant growth; it's complexity lies in the fact that plant forcing, itself, is a highly complicated affair. In a greenhouse there exists a so-called trinity of plant ecology, which necessitates a balance between light (heat), moving air, and controlled humidity. Temperature, first of all, affects plant growth because it directly influences such internal processes as photosynthesis (food manufacture). Plant growth also requires respiration — which is energy generated by the breaking down of foods manufactured by the plant. Now to illustrate how this trinity principle works: during the day, sunlight promotes plant growth through photosynthesis; plants absorb light energy to reduce carbon dioxide in the air to sugar. High daytime temperatures require high relative humidity and high soil moisture to balance the increased water loss through the plant. We see here that the plant environment includes not only the vegetative — above ground - considerations such as temperature, humidity, radiation, air movement and gas content of the air. There is also the root environment to consider: root temperature, soil moisture, plant nutrients, and soil structure. And there is yet another complication: not only do different plants require different environments, but the nighttime factors are different from the daytime. At night photosynthesis stops and reactions associated with reproduction occur. A low temperature at night produces growth, flowers and fruit.

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.

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.

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