The first step would be to accurately assess greenhouse requirements. Would a cold frame, hot bed, or window greenhouse meet the requirements, or is more growing space required? (It seems that regardless of the size selected, the structure will be too small eventually.) Larger greenhouses (over 300 sq. ft.) cost less per square foot of usable space than smaller ones, but cost more initially and cost more to operate. Greenhouses are available in a large variety of shapes. Select the style that will harmonize with other features on the proposed site.
The greenhouse may be a freestanding structure, which generally will provide more growing space and flexibility to be placed anywhere on the property, or a lean-to structure attached to a wall of the home, garage, etc. Lean-to models are frequently less desirable because there are limitations on where they can be placed around the home. Preferred exposures are east and south. Regardless of the basic design, however, the greenhouse must receive a minimum of five to six hours of unobstructed light during the winter months.
Attached Lean-To
A lean-to greenhouse is built against a building, using the existing structure for one or more of its sides. It is usually secured to a house, but may be attached to other buildings (figure 1). The greenhouse is limited to single or double-row plant benches with a total width of 7 to 12 feet. It can be as long as the building to which it is attached. The advantage of the lean-to greenhouse is that it usually is close to available electricity, water, and heat. The lean-to has the following disadvantages:
· Limited space
· Limited light
· Limited ventilation and temperature control
The least expensive type is a window mounted greenhouse. It allows space to grow a few plants at relatively low cost for heating and cooling. This reach-in greenhouse is available in many standard sizes, either in single units or tandem arrangements for large windows. Only simple tools are needed to remove the regular window from the frame and fasten a prefabricated window greenhouse in its place.
Freestanding
The freestanding greenhouse is a separate structure and consists of sidewalls, end walls, and gable roof (figure 2). It is assembled similar to an even-span, except that a freestanding greenhouse is set apart from other buildings, allowing full exposure to the sun. It can be made as large or small as desired.
The freestanding greenhouse is more easily adapted to the builder’s ideas of location, size and shape than attached greenhouses. It also provides more light, but requires more heat at night because of the additional exposed surface area. Separate heating and cooling systems will be necessary.
Locating the Greenhouse
After deciding on the type of greenhouse desired, its placement will need to be determined. An ideal site for a greenhouse would be well drained, nearly level, and with full exposure to sunlight. It would slope slightly to the south and have a windbreak on the side of the prevailing wind.
The first choice for a greenhouse site should be on the south or southeast side of the house in a sunny location. The east side is the second best location, since it will capture the most November to December sunlight. The next best locations are the southwest and west, and the north side is the least desirable location.
A greenhouse can be placed where it will be partly shaded during the summer when a reduction in light is desirable. Be sure the greenhouse isn’t shaded too heavily, and take into account the possibility of falling limbs that can damage the greenhouse. Some plants will grow in a greenhouse in any location. However, types of plants which can be grown will be limited if the greenhouse is not placed in the best possible location.
Fiberglass Greenhouses
Fiberglass is lightweight, strong, and practically hailproof. Corrugated panels or flat fiberglass are available. Poor grades of fiberglass will discolor, which reduces light transmission. A good grade, on the other hand, may make a fiberglass greenhouse as expensive to build as a glass house. If fiberglass is used, select the clearest grade. Do not use colored fiberglass.
Plastic Greenhouses
Plastic greenhouses are increasing in popularity because:
· Construction cost per square foot is generally one-sixth to one-tenth the cost of glass greenhouses.
· Plastic greenhouses can be heated as satisfactorily as glass greenhouses.
· Plants grown under plastic have the same quality as those grown under glass.
· Plastic greenhouses are sometimes considered temporary structures and usually carry a low assessment rate for tax purposes, or may not be taxed at all.
Plastic greenhouses are sometimes can be made of polyethylene (PE), polyvinyl chloride (PVC), copolymers of these materials, or other readily available clear films. Plastic coverings will have to be replaced more frequently than other glazings.
Greenhouse Management
Heating
Many types of heaters and heating systems are satisfactory for greenhouses. Consider the initial cost, operating expenses, and available fuel. Greenhouses can be heated efficiently with coal, electricity, gas, or oil. A space heater, a forced-air heater, a hot-water or steam system, or electric radiant heaters can be used. Radiant heat lamps over plants can be combined with soil heating cables under plants.
Ventilation
Even during cold weather, a greenhouse can get too warm on bright, sunny days. Ventilation equipment, therefore, should be built into the greenhouse to control temperatures and humidity in all seasons. When using hand-operated roof vents, frequent temperature checks will be required. As outdoor weather changes, sashes must be opened and closed manually to keep plants from getting too hot or too cold.
Automatic ventilation eliminates manual work and is the best way to cool a greenhouse. If your greenhouse has roof vents, for example, a thermostat and electric motor will open and close the vents. Temperature sensitive hydraulic pistons can be used to open and close small side and roof vents. Since they offer automatic ventilation without relying on electricity, proper ventilation will be maintained, even during electrical outages. Fresh outside air is brought in through the side vents via convection as the warm air flows out through the roof vents. In addition to cooling the greenhouse, the change of air improves growing conditions. Responding to this air transfer, the thermostat will turn off and on to keep desired temperatures for plants. Circulating and exhausting fans provide good ventilation and are needed in both large and small greenhouses.
Shading the Greenhouse
When protection from the sun is needed, use roll-up screens of wood or aluminum, polypropylene shade cloth, vinyl plastic shading, or paint-on materials. Roll-up screens are available with pulleys and rot-resistant nylon ropes. These screens can be easily adjusted from outside the greenhouse, as weather and sunlight vary.
Polypropylene shade cloth can be purchased in custom sizes, in black or green, and in 30 to 100 percent light transmission. These fabrics can be secured with rope or hooked onto the greenhouse by grommets woven into the binding.