   Selecting a Greenhouse: Part II - February 16, 2000 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County This week's column is a continuation of our overview of backyard green houses. As promised last week, I will discuss location, construction materials, floors, benches, heating, venting, cooling, and other important factors for greenhouse success. The most critical factor in greenhouse location is available sunlight. This is not usually a limiting factor, but the south side of your house or property is best, Furthermore, the long side of the greenhouse should also face south. In the Verde Valley, too much sun can be a problem. Avoid the west side where the afternoon sun will be too hot. Trees can also shade a greenhouse for part of the day. This can be good or bad depending on your desired crop. Leaves, branches, and limbs can also fall from trees. The site should also be level and well-drained. Easy access to a driveway will make it easier to transfer supplies and plants. Of course it should be accessible to your outdoor garden, water, gas (if needed for heating), and electricity. An effective wind break will decrease the need for winter heating. Finally, consider room for expansion as your hobby grows. Greenhouse frames can be made of wood, metal, or plastic (PVC). Wood is the most common choice for homemade designs. Use a rot resistant wood such as redwood or pressure treated lumber. Metal is most common in prefabricated models. If steel is used, keep a good coat of high quality paint on it to minimize rust. The least expensive material is PVC. It is light, flexible, and easy to work with. Use Schedule 80 PVC if you build your own. The roof pitch should be a 6 inch rise per foot (27 degrees) to prevent water droplets from falling from the roof. Design it so that you can walk comfortably once inside. Lightweight greenhouses must be anchored to prevent them from blowing away. These can be anchored with wooden or steel posts set in concrete at the corners. Larger greenhouses need a concrete or block foundation to prevent settling. Some designs extend these foundations a couple of feet above ground or dig into a slope and build retaining walls. An above ground foundation can improve the appearance of the greenhouse and building materials can be chosen to complement other nearby structures. The floor can be solid concrete (with a drain), stone, brick, or gravel. Walkways can be as little as two or three feet wide in smaller houses. In larger designs, the walkway may need to be large enough for a wheelbarrow, cart, or wheelchair. In an efficient design, 70 to 80 percent of the floor space can be used to grow plants. Benches should be built from sturdy, rot resistant materials. Redwood, pipe, angle iron, or concrete blocks can be used to support them. If solid beds are installed near a sidewall, then leave a six-inch space between the bench and wall for adequate ventilation. Benches accessible from only one side should be no wider than three feet. If they can be accessed from both sides, then five or six feet is acceptable. They should be 30 to 36 inches high (24 to 36 inches for wheelchairs). Your greenhouse can be heated in a variety of ways depending on crops to be grown and location. Solar energy can be captured by putting black containers of water inside the greenhouse. These can be metal drums or several gallon milk jugs painted black. You will need enough of them to capture the day's heat and re-radiate it at night. These can occupy considerable space so plan accordingly when designing. Other common methods of heating are gas furnaces, electric heaters, or even wood burning stoves. Greenhouses attached to homes are often heated by the same system used to heat the main house. All heaters need to be installed safely (i.e. gas heaters must be vented to the outside). A heating and cooling contractor may be consulted to assist in system design. Ventilation is critical at all times of year. Oxygen must be replaced by carbon dioxide, hot air with cool, and moist air with dry. This is often done with roof and side wall vents. Hot air rises pulling cooler outside air into the greenhouse through the side vents. These vents can be controlled manually if you are always near home. Otherwise, temperature sensitive devices are available to do the job automatically. Fans can also be used to pull the hot air out. There are design standards available to calculate fan and vent needs. The simplest way to cool a greenhouse is to reduce the amount of light. This also depends on the light requirements of the plants grown (vegetables, citrus, and flowers need ample sun). Whitewash, shade cloth, or lath are most often used to decrease the light. Evaporative coolers (swamp coolers) are also used for summer cooling. These can be controlled by a thermostat. I hope these two columns have inspired you to build a greenhouse. They can really open up new opportunities for backyard gardeners. Much of the information for these columns came from and Auburn University Extension Publication: ANR-1105, May 1998, by J.R. Kessler, Jr., Extension Horticulturist. Learn about upcoming fruit tree pruning workshops offered by Cooperative Extension at: http://ag.arizona.edu/yavapai/. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@kachina.net and be sure to include your address and phone number. |