The Owner Built Home & Homestead

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THE OWNER-BUILT HOMESTEAD, CHAPTER 9
THE PIT GREENHOUSE 

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.