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Grow any plant, any season, in a greenhouse

By Joel M. Lerner
Saturday, March 6, 2010; E01

The ending of winter and approach of spring intensify cabin fever and turn gardeners' thoughts to the potential that greenhouses -- and their fancier cousins, conservatories -- offer.

A greenhouse or conservatory enables you to grow any plant you want -- winter, spring, summer or fall. Nurture vegetables, flowers, shrubs or trees from seeds or cuttings. Raise exotic flora or overwinter your patio containers planted with bananas, oranges, palms or other tropical plants. Greenhouses will allow you to provide proper climate conditions 365 days a year.

Greenhouses are within the reach of most homeowners, especially if you build one from scratch or buy a kit. They come in a variety of sizes, shapes, colors and materials. There are solariums no larger than bay windows that serve as small greenhouses. You can utilize sunrooms or enclosed swimming pool areas -- any environment that can be controlled will work as a greenhouse.

Gardeners who dream of having a plant conservatory might have to compromise on some of their ideas unless they own a large home with lots of glass and tall ceilings. Conservatories are room-like structures, often custom-designed, that are sometimes extensions of homes and used mainly for exhibiting plants. Beautifully appointed conservatories were a favorite of wealthy Victorians who loved displaying their orchids, tropical palms and other exotics.

Greenhouses, the more utilitarian alternative, can also be custom-designed and -built and are intended to be controlled environments for the propagation and growing of plants.

If you want a traditional greenhouse made of wood and glass, there are companies that specialize in installing and restoring historic greenhouses, which tend to be ornate and expensive.

If your tastes run more economical than elaborate, however, prefabricated units are available. Some of these are designed to attach to a house where a portion of the wall serves as one of the walls for the greenhouse.

The greenhouse part of the structure can have a wood or metal frame. Wood is more expensive and requires additional maintenance; aluminum is cheaper and generally maintenance-free. The "glass" element can be actual glass or a cheaper alternative such as polyethylene, polycarbonate or fiberglass. The panes can be transparent or translucent, depending on the location of the structure and the season.

Glass is preferred, and it should be glazed with a finish to reduce glare and heat gain. Glass also should be tempered. Tempered glass has been heated to 1,200 degrees Fahrenheit and then cooled, which can make it up to four times as strong as regular glass.

Floors require a liberal layer of gravel for drainage and a raised walk made of wood, brick, stone or other fabricated material to provide a dry walking surface. Some greenhouses have concrete or other masonry surfaces for more permanence than wood. Floor drainage should be carefully designed and installed to control irrigation runoff.

The design of floors will depend on how you are planning to water plants. Some greenhouse plants need water only on their roots, while others require moist atmospheres.

There are tubes that will drip water onto planting mediums in whatever way you would like it delivered, spraying or dripping it directly into the soil. Hoses and watering wands offer other choices.

Ventilation is crucial to plant health. It is provided by the exchange of outside air through vents, windows and fans, which can be regulated automatically, depending on outside weather conditions. Greenhouses can get hot. If you've ever left a CD or chocolate bar in your car during the summer, you have experienced the radiant energy the sun can emit through glass.

Air circulation inside the greenhouse is another critical element. Humidity is a necessity for most plants. However, drops of water hanging on surfaces, including plants and tools, can cause too much condensation (regardless of whether vents are open), which may promote plant diseases. Internal circulating fans will mimic breezes and decrease liquid in the air.

You may find it necessary to shade windows during the hottest periods of the day, depending on surrounding trees and other structures. Shades or, on a glass greenhouse, removable paint is typically used.

Because greenhouses are usually intended to protect plants from winter weather, you will need a source of heat and humidity. Regardless of whether you heat with oil, gas or electricity, the drying effect on plants makes it necessary to add humidity to the atmosphere. You can use a humidifier for this. Some people use radiant heat pipes under floors. This can work well because it doesn't dry out the plants as much. However, in very cold climates, the pipes may take too long to heat the air.

Supplemental lighting may also be necessary, depending on the types of plants. You might consider grow lights in a greenhouse redundant, but there are times when you will want them to extend the growth period of a tropical plant so it can fruit or flower when the days are short. Locate greenhouses carefully to maximize the availability of natural light.

Benches and shelves are important for holding pots and keeping everything drained, watered and at a comfortable working height. Homeowners with greenhouses often build their own to get exactly what they want, or they buy pre-made ones through garden centers and companies that sell greenhouses.

There is another -- inexpensive and old-fashioned -- way to protect your plants: using a cold frame. You can make one from an old window and some bricks, blocks or rocks, or buy one made commercially.

Sometimes cold frames, when heated from below using manure that was composting, were referred to as hotbeds. They were used for hundreds of years to propagate and protect tender seedlings and rooted cuttings. Today, the heat can come from electric or other sources, as well as from organic material.

Here are some companies that offer greenhouses, cold frames and supplies:

Turner Greenhouses, http://turnergreenhouses.com, 800-672-4770.

Gardener's Supply, http://gardeners.com, 888-833-1412.

Charley's Greenhouse & Garden, http://charleysgreenhouse.com, 800-322-4707.

Janco Greenhouses, http://jancoinc.com, 443-350-9631.

For more information:

Hobby Greenhouse Association, http://hobbygreenhouse.org.

And, for good examples of classically designed conservatories and greenhouses:

U.S. Botanic Garden, on the National Mall near the Capitol, http://www.usbg.gov, 202-225-8333.

Hillwood Estate, Museum and Gardens, 4155 Linnean Ave. NW, Washington, http://hillwoodmuseum.org, 202-686-5807.

Brookside Gardens, 1800 Glenallan Ave., Wheaton, http://brooksidegardens.org, 301-962-1400.

Joel M. Lerner is president of Environmental Design in Capitol View Park, Md.

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