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Front Page » March 5, 2009 » Focus on homes and gardens » Planning a greenhouse
Published 2,408 days ago

Planning a greenhouse

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Some gardeners swear by them, others swear at them.

Greenhouses can be a useful tool in the gardening hobby, but they are also some work to take care of.

For the fair weather gardener, the spring and summer bring a lot of fun and work. Some may like that duty of watering, weeding and feeding plants in the good weather, but taking care of a greenhouse is a year around endeavor if it is to be done right.

But let's say that a committed gardener is interested in taking care of vegetable plants or flowers all year round. Let's say they are dedicated to starting plants early and having them be beautiful before their neighbors barely get their yards off the ground in the early spring.

Where does one start with planning a greenhouse? Well it begins with planning.

Careful planning is important before a home greenhouse project is started. Building a greenhouse does not need to be expensive or time-consuming. The final choice of the type of greenhouse will depend on the growing space desired, home architecture, available sites, and costs.

In the end, however, the greenhouse must provide the proper environment for growing plants.

The first consideration is location. The greenhouse should be situated where it will get maximum sunlight.

The first choice of location is the south or southeast side of a building or shade trees. Sunlight all day is best, but morning sunlight on the east side is sufficient for plants. Morning sunlight is most desirable because it allows the plant's food production process to begin early; thus growth is maximized.

An east side location captures the most November to February sunlight.

The next best sites are southwest and west of major structures, where plants receive sunlight later in the day.

North of major structures is the least desirable location and is good only for plants that require little light.

Trees can effectively shade the greenhouse from the intense late afternoon summer sun; however, they should not shade the greenhouse in the morning. Deciduous trees in particular also allow maximum exposure to the winter sun because they shed their leaves in the fall. Evergreen trees that have foliage year round should not be located where they will shade the greenhouse because they will block the less intense winter sun.

Anyone planning a greenhouse should aim to maximize winter sun exposure, particularly if the greenhouse is used all year. Remember that the sun is lower in the southern sky in winter causing long shadows to be cast by buildings and evergreen trees.

Good drainage is another requirement for the site. When necessary, build the greenhouse above the surrounding ground so rain water and irrigation water will drain away. Other site considerations include the light requirements of the plants to be grown; locations of sources of heat, water, and electricity; and shelter from winter wind. Access to the greenhouse should be convenient for both people and utilities. A workplace for potting plants and a storage area for supplies should be nearby.

In combination with these factors one also has to consider what kind of greenhouse they plan on constructing.

A home greenhouse can be attached to a house or garage, or it can be a freestanding structure. The chosen site and personal preference can dictate the choices to be considered. An attached greenhouse can be a half greenhouse, a full-size structure, or an extended window structure. There are advantages and disadvantages to each type. Here are some of the types to look at.

•Lean-to. A lean-to greenhouse is a half greenhouse, split along the peak of the roof, or ridge line, Lean-tos are useful where space is limited to a width of approximately seven to 12 feet, and they are the least expensive structures. The ridge of the lean-to is attached to a building using one side and an existing doorway, if available. Lean-tos are close to available electricity, water and heat.

The disadvantages include some limitations on space, sunlight, ventilation, and temperature control. The height of the supporting wall limits the potential size of the lean-to. The wider the lean-to, the higher the supporting wall must be. Temperature control is more difficult because the wall that the greenhouse is built on may collect the sun's heat while the translucent cover of the greenhouse may lose heat rapidly. The lean-to should face the best direction for adequate sun exposure.

Finally, consider the location of windows and doors on the supporting structure and remember that snow, ice, or heavy rain might slide off the roof or the house onto the structure.

•Even-span. An even-span is a full-size structure that has one gable end attached to another building. It is usually the largest and most costly option, but it provides more usable space and can be lengthened. The even-span has a better shape than a lean-to for air circulation to maintain uniform temperatures during the winter heating season. An even-span can accommodate two to three benches for growing crops.

•Window-mounted. A window-mounted greenhouse can be attached on the south or east side of a house. This glass enclosure gives space for conveniently growing a few plants at relatively low cost. The special window extends outward from the house a foot or so and can contain two or three shelves.

•Freestanding greenhouses are separate structures; they can be set apart from other buildings to get more sun and can be made as large or small as desired. A separate heating system is needed, and electricity and water must be installed.

The lowest cost per square foot of growing space is generally available in a freestanding or even-span greenhouse that is 17 to 18 feet wide. It can house a central bench, two side benches, and two walkways. The ratio of cost to the usable growing space is good.

When deciding on the type of structure, be sure to plan for adequate bench space, storage space, and room for future expansion. Large greenhouses are easier to manage because temperatures in small greenhouses fluctuate more rapidly. Small greenhouses have a large exposed area through which heat is lost or gained, and the air volume inside is relatively small; therefore, the air temperature changes quickly in a small greenhouse. Suggested minimum sizes are 6 feet wide by 12 feet long for an even-span or freestanding greenhouse.

One piece greenhouse. Several companies produce one piece greenhouses manufactured out of composite materials. These greenhouses are shipped from the factor or distributor and are installed by digging a trench the size of the base which is then set down into the trench and buried for an anchor.

These houses are usually small, although regular sizes can go up to 30 feet long or more with custom orders. Small single piece greenhouses can be seven by eight feet with a seven foot ceiling.

While the other kinds of greenhouses can vary in cost based on construction size and materials the average one piece greenhouses run between about $3200 (with shipping included to this area) to nearly $10,000 with custom models.

However rather than look at pre-built greenhouses or kits, many people prefer to build their own. There are many kinds of materials from which to choose when constructing a greenhouse.

First a good selection of commercial greenhouse frames and framing materials are available. The frames are made of wood, galvanized steel, or aluminum. Build-it-yourself greenhouse plans are usually for structures with wood or metal pipe frames. Plastic pipe materials generally are inadequate to meet snow and wind load requirements. Frames can be covered with glass, rigid fiberglass, rigid double-wall plastics, or plastic film. All have advantages and disadvantages. Each of these materials should be considered. It pays to shop around for ideas.

Greenhouse frames range from simple to complex, depending on the imagination of the designer and engineering requirements. The following are several common frames that can be found.

•Quonset. The quonset is a simple and efficient construction with an electrical conduit or galvanized steel pipe frame. The frame is circular and usually covered with plastic sheeting. quonset sidewall height is low, which restricts storage space and headroom.

•Gothic. The gothic frame construction is similar to that of the Quonset but it has a gothic shape. Wooden arches may be used and joined at the ridge. The gothic shape allows more headroom at the sidewall than does the quonset.

•Rigid-frame. The rigid-frame structure has vertical sidewalls and rafters for a clear-span construction. There are no columns or trusses to support the roof. Glued or nailed plywood gussets connect the sidewall supports to the rafters to make one rigid frame. The conventional gable roof and sidewalls allow maximum interior space and air circulation. A good foundation is required to support the lateral load on the sidewalls.

•Post and rafter and A-frame. The post and rafter is a simple construction of an embedded post and rafters, but it requires more wood or metal than some other designs. Strong sidewall posts and deep post embedment are required to withstand outward rafter forces and wind pressures. Like the rigid frame, the post and rafter design allows more space along the sidewalls and efficient air circulation. The A-frame is similar to the post and rafter construction except that a collar beam ties the upper parts of the rafters together.

While the frames support everything, other materials are just as important. The covering used can be of major consequence when building and using a greenhouse. Coverings include long-life glass, fiberglass, rigid double-wall plastics, and film plastics with one to three year life spans. When constructing a greenhouse the type of frame and cover must be matched correctly. Here is a short list.

•Glass. Glass is the traditional covering. It has a pleasing appearance, is inexpensive to maintain, and has a high degree of permanency. An aluminum frame with a glass covering provides a maintenance-free, weather-tight structure that minimizes heat costs and retains humidity. Glass is available in many forms that would be suitable with almost any style or architecture. Tempered glass is frequently used because it is two or three times stronger than regular glass. Small prefabricated glass greenhouses are available for do-it-yourself installation, but most should be built by the manufacturer because they can be difficult to construct.

The disadvantages of glass are that it is easily broken, is initially expensive to build, and requires must better frame construction than fiberglass or plastic. A good foundation is required, and the frames must be strong and must fit well together to support heavy, rigid glass.

•Fiberglass. Fiberglass is lightweight, strong, and practically hail proof. A good grade of fiberglass should be used because poor grades discolor and reduce light penetration. Use only clear, transparent, or translucent grades for greenhouse construction. Tedlar-coated fiberglass lasts up to 20 years. The resin covering the glass fibers will eventually wear off, allowing dirt to be retained by exposed fibers. A new coat of resin is needed after 10 to 15 years. Light penetration is initially as good as glass but can drop off considerably over time with poor grades of fiberglass.

•Double-wall plastic. Rigid double-layer plastic sheets of acrylic or polycarbonate are available to give long-life, heat-saving covers. These covers have two layers of rigid plastic separated by webs. The double-layer material retains more heat, so energy savings of 30 percent are common. The acrylic is a long-life, non yellowing material; the polycarbonate normally yellows faster, but usually is protected by a UV-inhibitor coating on the exposed surface. Both materials carry warranties for 10 years on their light transmission qualities. Both can be used on curved surfaces; the polycarbonate material can be curved the most. As a general rule, each layer reduces light by about 10 percent. About 80 percent of the light filters through double-layer plastic, compared with 90 percent for glass.

•Film-plastic. Film-plastic coverings are available in several grades of quality and several different materials. Generally, these are replaced more frequently than other covers. Structural costs are very low because the frame can be lighter and plastic film is inexpensive. Light transmission of these film-plastic coverings is comparable to glass. The films are made of polyethylene (PE), polyvinyl chloride (PVC), copolymers, and other materials. A utility grade of PE that will last about a year is available at local hardware stores. Commercial greenhouse grade PE has ultraviolet inhibitors in it to protect against ultraviolet rays; it lasts 12 to 18 months. Copolymers last two to three years. New additives have allowed the manufacture of film plastics that block and reflect radiated heat back into the greenhouse, as does glass which helps reduce heating costs. PVC or vinyl film costs two to five times as much as PE but lasts as long as five years. However, it is available only in sheets four to six feet wide. It attracts dust from the air, so it must be washed occasionally.

Depending on the kind of greenhouse one builds, it can last many years and give great service. As with most things, doing something on the cheap is seldom the way to get the best quality, but then gardening should be fun, and not something to continually worry about, especially when it comes to cost.

(Information for this article came from the University of West Virginia and University of Maryland extention programs).

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