The Greenhouse as an Ecosystem
In our mini-manual on environmental management inside the solar greenhouse, an expert from the New Alchemy Institute reveals the secret of gardening under glass.
November/December 1984
By Colleen Armstrong
In the early 1970s, when energy costs first began their upward spiral, the accelerating fuel prices drove many commercial glasshouse vegetable producers right out of business. This inflationary upswing also started, in a modest way, a new trend in American gardening.
With the price tag on winter tomatoes and other warm-weather crops on the rise, many people equipped their homes with attached greenhouses, which ranged from expensive, custom-built models to low-budget, low-tech structures composed of plastic-covered frames.
We've now become accustomed to seeing such food- and flower-producing add-ons everywhere. There's no longer any reason to limit a passion for vegetables and flowers to the outdoors. Even those gardeners new to greenhouse management can be successful at indoor horticulture by following four basic rules.
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Know Your Greenhouse Environment
Each attached greenhouse has its own environment, which is created by its location, design, construction, glazing, thermal mass and interior layout. (To expand your knowledge of all these factors, I recommend you consult books 2, 3 and 9 in the "Suggested Reading List" that accompanies this article.)
A solar greenhouse, more than any other type, is sensitive to its surrounding environment. In many ways, this type of greenhouse is analogous to a living plant cell: The sun is its primary energy source, and its glazing acts as a membrane between the inner and outer world, allowing an exchange of heat, light and air. A greenhouse can, in fact, become a miniature ecosystem if the adept gardener can manage the interaction between abiotic factors (such as wind, snow, oxygen, carbon dioxide) and the biological community.
At New Alchemy Institute, we call our greenhouses "bioshelters," because our biotic residents include more than just green plants. The bioshelter grower manages soil that's alive with tiny microorganisms and other animals that break down organic matter into humus. Plant-feeding, plant-pollinating and plant-predaceous insects stake out their own niches. Most of the flora is edible, but a small percentage of space is reserved for herbs, flowers and woody plants. Even the heat storage acts as a substrate for living organisms, since we use transparent water columns laden with green algae and stocked with phytoplankton-feeding fish, or bank huge masses of compost against the north wall. Any grower can become proficient at this type of biological management when treating the greenhouse as an ecosystem. The key is to let nature be your guide.
As the seasons blend one into another, milder versions of the world outside unfold inside the greenhouse. Since each structure is unique, keeping a record of the pilot year is the best way to learn about your under-glass environment. At the very least, enter minimum and maximum air and soil temperatures (6 inches underground) for each week. Other information such as plant vigor, future harvest dates, or insect and disease problems will guide you as you make your seed selection.
Light, temperature, humidity, carbon dioxide, ventilation, water and soil compose the atmosphere within a greenhouse and are interrelated. For example, as the temperature rises, the relative humidity drops. Likewise, under the best lighting conditions, low levels of carbon dioxide will restrict the rate of photosynthesis, limiting plant growth. You must be aware of such interactions and be prepared for some give-and-take in the environmental management of your indoor garden.
LIGHT. All plants require light for photosynthesis, and this essential gardening ingredient can be analyzed in two ways: by quantity and by quality. Quantity is scientifically measured in footcandles (FC). One footcandle is a unit of illumination equal to the direct illum nation on a surface one foot from a standardized source called an international candle. You can, however, obtain a "thumbnail" measure of the amount of light striking a surface in your greenhouse by using a light meter in a 35mm camera.
The quality of light is defined descriptively—bright or direct, diffuse, partial, or shady. Within the greenhouse, diferent locations afford varying light levels (see Fig. 1). The greenhouse's southern section receives plenty of direct sunlight, whereas areas along a northern wall may be shaded. Light beams passing through a translucent glazing (such as fiberglass) scatter, resulting in diffused light without shadows. In front of the greenhouse, deciduous or evergreen trees may obstruct incoming light, creating the same effect. This arrangement is helpful during warm summer months. Partial light occurs when a plant receives only part of a day's sunlight.
Maximizing light and minimizing condensation and shading will improve plant growth during the winter months when light levels are lowest. In greenhouses with partial or shaded light conditions, the gardener's best strategy is to match the right plant with the available light. It's nonsense, for example, to attempt to grow tomatoes in a dimly lit greenhouse in January. Plants that are victims of low light conditions show slow growth, elongated stems, yellow lower leaves, weak and floppy leaves, and phototropism (plants' bending drastically toward the light source).
The amount of light a greenhouse receives will depend on its location, design, and type of glazing. Two or three layers of glazing material will reduce heat loss but will also reduce light levels. Greenhouses with insulated, opaque walls should use as much reflective light as possible. Plenty of white paint on these walls will bounce the light back into the structure.
Supplementary light or artificial light can be used at your own discretion. I'd suggest using this kind of lighting when starting warmweather transplants such as tomatoes, European cucumbers, or peppers early in the year. Whatever the day length at that time, boost it up to 16 hours of light.
In the warmer months, it's equally important to shade the greenhouse, reducing both the amount and intensity of light entering it. Gardeners often overlook this critical step in climatic management. You can apply a commercially available liquid shading compound to glass at various concentrations for the coverage you desire. Eventually, the rain will wash it off: Such compounds are usually nonabrasive, nontoxic, and easy to apply with a spray tank, but shade cloth may be better suited for plastic or polyvinyl glazing. Greenhouse supply companies (see "Greenhouse Tools and Supplies") offer cloth that provides different amounts of shade. For a biological screen, plant vining crops—grapes, melons, cucumbers, and runner beans, for example—in or outside the greenhouse along its southern wall. When the time comes for brighter light, simply pull the crop out. But make sure that you choose vining perennials or shade trees that will be mature during the seasons when the greenhouse requires shading.
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