- The Washington Times - Tuesday, November 15, 2005

A backyard greenhouse allows hobbyist gardeners such as Joan O’Rourke of Silver Spring to garden year-round, grow plants that would not survive cold winters and take in a bit of summer heat and color even when the weather is dreary.

“I can be looking out at everyone outside bundled up in their coats, and I’m warm and toasty inside with my plants,” says Mrs. O’Rourke, a master gardener with the Montgomery County Cooperative Extension in Derwood and a volunteer at Brookside Gardens, a public display garden in Wheaton Regional Park.

Mrs. O’Rourke, a gardener for more than 25 years, uses an A-frame glass greenhouse with more than 200 square feet of space to grow tropical plants, including jasmine, gardenias and her favorite, begonias, for both the flowers and the foliage. The greenhouse is attached to the garage and has an aluminum frame with automatic ridge vents and mesh steel benches to set out her dozens of potted plants.

“Most gardeners really do have some tender plants that they really love. You can bring them indoors, but if you don’t have a greenhouse, they’re not going to do as well,” says Holly Shimizu, executive director of the U.S. Botanic Garden in Southwest.

Greenhouses, which can be free-standing or a lean-to attached to a building, are used to overwinter dormant plants, propagate plants for the spring and grow cuttings. The plants typically are placed on benches, which can be made of plastic, metal, wood or other materials.

“Most temperate plants that grow in this area outdoors won’t do well in a greenhouse,” says Jonathan Kavalier, manager of Merrifield Garden Center in Merrifield. “It’s going to be too much of a tropical environment for them.”

Greenhouses allow gardeners to grow tropicals, subtropicals and hybrids not typically found in garden centers, says Steven Cohan, coordinator of the landscape management program at the University of Maryland in College Park. He adds that seed catalogs provide additional options.

“There’s no limit to what you can grow,” says Mr. Cohan, who holds a doctorate in plant genetics.

Greenhouses typically come in a Quonset hut or A-frame structures. The framework can be made of wood, aluminum or metal with a glass or plastic glazing or covering, which is the ceiling.

Polycarbonate and polyethylene, which are types of plastic coverings, need to be replaced every three to five years, but glass can last for decades, says Tyler Diehl, head of gardens at Ladew Topiary Gardens in Monkton, Md.

“People are getting away from fiberglass. It doesn’t last as long, and it reduces the light,” Mr. Diehl says.

A greenhouse can be assembled from a kit or custom-built, but like any building, it requires some construction know-how and, depending on the locale, may require a permit. The least expensive kit, which can cost several hundred dollars, is for a polyhouse, which consists of poles set into the ground and a polypropylene plastic glazing. The most expensive is glass, which can cost thousands of dollars.

Cold frames, which are low-lying, cellarlike structures that can be opened and closed to control temperature, are used to overwinter and propagate plants without a heating source. Hot frames are similar to cold frames but include heating cables.

Sunglo East Greenhouses in Herndon manufactures and sells a line of free-standing and lean-to greenhouse kits made of acrylic Plexiglas that is double-wall insulated. Another metropolitan-area company, Janco Greenhouses Inc. in Laurel, produces glass greenhouses.

“In this climate, we’re in need of insulation year-round,” says Maryclaire Ramsey, manager of Sunglo East Greenhouses, a mail-order company. “You use the sun during the day to heat your greenhouse, and then the insulating factor is what keeps the heat in from the daylight sun. But eventually, the heat will go out.”

Choosing an environment and the plants that can grow in it is important, especially in smaller greenhouses, says Carol Allen, supervisory horticulturalist at the U.S. Botanic Garden.

“The temperature parameters and the environmental parameters are more difficult to stabilize in a small greenhouse,” Ms. Allen says.

To control temperature, installing a heating system, such as electric, water heat, or propane or natural gas, can keep greenhouses warm during night hours, while ventilation systems and oscillating fans help with cooling and air circulation. A shade cloth, which is made of mesh material, controls sunlight and heat entering the greenhouse. Thermal blankets or screens placed inside the greenhouse trap heat during the night.

During the winter, a good nighttime temperature is between 40 and 45 degrees, enough to overwinter plants but not enough to force growth, says Robert DeFeo, regional horticulturalist with the National Park Service’s National Capital Region in Southwest.

Tropical plants, which are not winter-hardy, generally do best when the temperature is maintained between 45 and 55 degrees during the winter and between 65 and 75 degrees during the summer, Mr. DeFeo says. For propagating plants such as annuals or bedding plants during the winter, temperatures need to be a little warmer than needed for overwintering plants in order to force the seeds to grow, he says. He recommends keeping temperatures at about 65 degrees during the night and 75 degrees during the day, depending on the type of plant.

“If you can’t control your temperature, you can’t control your growth,” Mr. DeFeo says. “You need sufficient environmental controls to maintain a proper growing environment for the plant.”

Another environmental control is watering, which can be achieved by running a hose line out to the greenhouse, as long as it is insulated. Some gardeners may prefer installing an irrigation or misting system, but Ms. Allen recommends hand-watering to give plants individual attention.

In addition to watering, metropolitan-area greenhouse gardeners recommend checking plants daily for dryness, insects, diseases, water spots, dead foliage, leaves that need washing, and any repotting or resoiling needs.

Gardeners should realize, Mr. Cohan says, that plants in a greenhouse need more frequent watering than those outdoors.

Mr. Diehl recommends keeping utilities, the boiler and the potting area in a separate building in a potting shed or a head house, a place for potting plants and doing other dirty work.

“It’s so costly to run a greenhouse that every square inch of that greenhouse needs to be used. You don’t put anything in a greenhouse except plants,” he says.

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