Greenhouses

A greenhouse can open a new world of enjoyment for the seasonal gardener who otherwise has to put away his gardening tools each fall. It can also increase the pleasure of the indoor plant grower who needs better light and better control of temperature and humidity.

Rising fuel costs, the relative high cost of the structure, and the needed environmental equipment limit the number of people who can invest in a hobby greenhouse. Although food can be grown in the off-season, the expense generally will be greater than buying the product in the supermarket.

A small greenhouse can be designed and built by an individual or can be purchased from one of the many manufacturers. Some shapes and styles look quite different from the conventional and show that one's imagination is the limit. They can be attached to the house or freestanding, away from any building.

The home greenhouse can be attached to the home where space is limited or where it blends well into the architecture and landscaping. The attached greenhouse is conveniently accessible and water, electricity, and heating facilities may be shared with the house. Heat loss is less since one side is shared with the house and not exposed to the weather. The amount of sunlight may be less because of shading if the greenhouse is on the east or west wall of the house.

Choice of location for your greenhouse may be limited but there are several things to consider. Of primary importance is available sunlight. A southern or southeastern exposure would be first choice for good winter lighting when young seedlings are growing. The east side is second choice because it gives exposure to the morning sun. The north side of a building is the least desirable location unless only shade-loving plants are grown.

The greenhouse can be partially shaded in summer by deciduous trees, but should not be shaded in winter. Keep in mind that the winter sun is lower in the sky and shadows of evergreens and buildings are much longer.

The building site should be well-drained with the greenhouse floor built up and the surrounding ground sloped from the site to carry rainwater away. Water, fuel for heating, and electricity should be available nearby so utility connections can be made. Check local electrical and plumbing codes, and get qualified people to do the work. Shelter from wind helps reduce heat loss.

Size of the greenhouse will often be limited by cost and available space. If cost is a major factor, consider a coldframe or hotbed instead. The temperature is more difficult to control in a very small greenhouse and heat loss is high compared to a larger house. A 6- by 6-foot greenhouse could better be replaced by one or two 3- by 6-foot hotbeds.

Greenhouse size depends on the amount of growing space you feel you need and can maintain. Also bear in mind that many people soon find they run out of space. Space may be needed in the greenhouse for working and for storing soil mix, chemicals, tools, and other items. However, heating the work and storage space will be expensive. Estimate the amount of use throughout the year in justifying an investment.

Width of the greenhouse is fixed once construction is completed. But with prior planning, the length can be increased in the future to give more growing space. Benches generally are 2 to 4 feet wide for easy working from one side, and 2 1/2 to 3 feet high. Aisles would be a minimum of 2 feet wide, or 2 1/2 to 3 feet wide if a cart, wheelchair, or wheelbarrow is used.

Leave at least 6 inches between each sidewall and the bench to permit good air circulation. Do not crowd yourself on headroom either. Gable roof houses should have at least 5 1/2 feet eave heights and a minimum of 8 feet at the ridge.

A minimum greenhouse size is about 8 by 10 feet for a freestanding house. A lean-to greenhouse is limited to widths of 7 to 12 feet by the roof slope. Roof slope for glass greenhouses should be 6 inches of rise per foot of width, and 7 to 8 1/2 inches of rise per foot for fiberglass and polyethylene (film plastic) houses.

The steep slope is to permit moisture condensing on the inside of the roof to run off instead of dripping onto the plants, promoting disease. It also allows snow to slide off to let light in.

Ideally, a greenhouse has a minimum framework needed to support a translucent cover so the maximum amount of light is received. The supporting frame can be made of wood, aluminum, plastic, and black or galvanized iron. The cover may be glass, rigid plastic or fiberglass, or plastic film. Each material has advantages and disadvantages which should be considered for the type greenhouse planned.

Glass greenhouses are long lasting; require a good foundation and strong, rigid frame; and have high initial cost. Maintenance is reduced but heat loss is increased if a metal frame is used instead of wood. Fiberglass has a variable service life depending on the quality product used.

----2'-3' 1 1/2-3'

bench width,- aisle width Top, freestanding, slant-leg greenhouse. Above, suggested minimum dimensions for a greenhouse. Benches should be placed far enough from side wall to allow adequate ventilation.

Film plastic has a service life of one or two years and can be used on quite temporary frames. Annually or every second year, commercial people replace polyethylene on good frames which last many years. This spreads out the cost of the cover over the life of the greenhouse.

Regardless of the cover used, be sure it is a high quality material made for greenhouses.

A recent development is the use of two layers of film plastic spaced 3/4 of an inch to 4 inches apart to reduce heat loss by 30 percent. The two layers are often installed together and then separated by a small fan.

Polyethylene houses lose heat by radiation more rapidly than glass and fiberglass and therefore require more heat. Polyethylene houses are tighter than glass houses so less heat is lost by air infiltration or leakage.

The small greenhouse requires heating and ventilating equipment. Part of the winter heat is received from the sun, and on mild sunny days you may have to ventilate to reduce the temperatures. However, at night no heat is received from the sun and heating units are required to maintain the temperature.

Heat can be distributed in the greenhouse in three ways: forced hot air; natural convection from small space heaters, hot water or steam pipes; and direct radiation. Any of the common fuels may be used.

A good quality heater has a thermostat and is vented to the outside, except for electrical units. Unvented heaters can discharge gases harmful to plants and deadly to humans into the greenhouse, and should never be used.

The amount of heat required for a greenhouse depends on the maximum temperature difference between outside and inside, the surface area of the greenhouse, the quality or tightness of construction, and the wind.

A simple formula for determining heat loss is (surface area of transparent greenhouse cover) times (maximum temperature difference to be maintained) times (heat loss factor for the covering and wind) equals (heat loss, in BTU/hr.) To convert to kilowatts for electric heaters, divide by 3,413. Heater output should equal the heat loss calculated.

Heat loss factors are given in the table. Maximum heat loss is normally about an hour before sunrise.

Ventilation is an exchange of air inside the greenhouse with outside air. Ventilation is needed for cooling, to reduce high humidity, and to replenish carbon dioxide. The air exchange can be achieved by opening vents and doors or by using fans.

Exhaust fans in combination with inlet louvers are a good ventilation system used by many greenhouse operators. Fans can be wired to a thermostat for automatic operation, and sized to seasonal requirements of the greenhouse. A two-speed fan is desirable to permit spring and fall ventilation at half the summer rate. The exhaust fan and inlet louver should be placed at opposite ends of the greenhouse.