The floor needs a liberal layer of gravel for drainage and a wooden, brick or stone raised walkway for a dry walking surface. Finer greenhouses have concrete or other masonry surfaces for more permanence than wood, and floor drains and proper drainage are carefully designed and installed to control irrigation runoff.
The design of the floor will depend to some extent on how you plan to water the plants, and that depends on which type of plants you are growing. Some greenhouse plants need water only on their roots, and some require a moist general atmosphere. There are tubes that will put the water onto the planting medium however you want it delivered, either by spray or dripping it directly into the soil. You could also use a hose with a watering wand.
Some prefabricated units are designed to attach to a house, using it as one of the greenhouse's walls.
(Jason Heath Lerner For The Washington Post)
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Ventilation is crucial to plant health. It is the exchange of outside air through vents, windows and fans. They can be regulated automatically depending on outside wind, rain and, primarily, temperature. It can get hot in a greenhouse. If you have ever left a CD or tape on the dash of a car in summer and come back to find it melted like cheese, you know how hot it can get under glass.
Air circulation is also critical. You want humidity -- molecules of water suspended in the air -- but not large droplets of water hanging on surfaces such as plants and tools inside the greenhouse. Whether vents are open or not, condensation shouldn't form on the windows or plants; it promotes disease. Internal circulating fans will mimic wind and reduce liquid in the air.
Some method of shading the windows during the hottest periods could be required, depending on the surrounding trees and other structures. Shades are often used. A removable paint will serve the same purpose, provided you have a glass greenhouse.
Because greenhouses are meant to protect plants from winter and must be heated and humidified, you will also need a heat source for winter. Whether you heat with oil, gas or electricity, you need to add humidity to the atmosphere. You can use a humidifier in winter. Some people use radiant-heat pipes under the floor to heat a greenhouse, and this can work well, as it doesn't dry out the plants as much. However, in very cold climates, the pipes may take too long to heat the air.
You have a choice when it comes to supplemental lighting. You might think of lights in a greenhouse as a contradiction in terms. But, there are times when you might want them to extend the growth period, so a tropical plant can fruit or flower when days get shorter, or you might want one to supplement low light conditions. It helps if you site the greenhouse carefully to maximize natural light.
Benches are an important tool for holding pots and keeping everything drained, watered and at a comfortable working height. People often build their own plant benches to get exactly what they want, but you can also find benches and all the other furnishings and accessories through the same companies that sell greenhouses.
Of course, there's one very inexpensive and old-fashioned way to protect your plants -- use a cold frame. You can make one from an old window and some bricks, blocks, or rocks. They are available commercially. Sometimes cold frames, when heated from below by composting manure, are referred to as hotbeds. They have been used for hundreds of years to propagate and protect tender seedlings and rooted cuttings. Today the heat can come from electricity or other sources.
Joel M. Lerner is president of Environmental Design in Capitol View Park, Md. E-mail or contact him through his Web site, www.gardenlerner.com.
