The Greenhouse Effect

One offshoot of this decade's passion for gardening is a rise in the number of residential greenhouses.

The current passion for gardening and the concurrent growth in disposable income have bred, pardon the pun, a new hybrid: an interest in building residential greenhouses. While the absolute numbers aren't enormous—just one in 2,000 homes in America has a greenhouse versus one in five in England—business is blooming. Katherine Hickson, CEO of Private Garden Greenhouse Systems, considered by many the premier greenhouse manufacturer in the United States, says the company's residential business has increased 230 percent in the past two years alone. "Baby boomers have more disposable income, and gardening is their largest hobby," she says from her office in Hampden, Massachusetts.

Wayne Boddie, the southern U.S. representative of Amdega-Machin, a leading British bespoke conservatory manufacturer, concurs. "We've seen a rise in the number of clients who want to use our conservatories as greenhouses, especially in the last two years," he says. "Five years ago, we had maybe eight or ten clients per year in the United States who wanted to do this, now we have about fifty per year. People have more time and more money, and they want to raise plants."

Right now, industry observers agree, the trend is limited to avid gardeners, most of them at or near retirement age. "They're often successful businesspeople whose time-management skills are excellent," says Hickson. "Some are retired and have time for a new vocation. They look forward to harvesting their own fruits. Of course, it's the most expensive fruit they'll ever eat." Boddie says Amdega-Machin's clients range from "twentysomethings to elderly millionaires," and that most are CEOs and entrepreneurs.

Case in point: Harry Gates, a landscape architect, recently installed an 8.5 by 12.5-foot greenhouse from Private Garden Greenhouse Systems on his property in Washington, D.C. He plans to grow English boxwood and American holly, his horticultural passions, and orchids and vegetables. "Gardening has been my hobby all my life. I've always wanted a greenhouse," he says. "Now I'm a young sixty, and I'm out there every morning and first thing in the evening. You have to be a good gardener to run a greenhouse."

Just how good—and how ambitious—a gardener you are is the litmus test for deciding between building a greenhouse or a conservatory, a very different structure. Strictly speaking, a greenhouse is designed for a wet environment and the propagation of plants; a conservatory is designed for displaying plants. "Conservatories are a display house where you want to put your finished products," says Dale Rahn of Q-Com, a company specializing in greenhouse environmental control systems. "You maintain plants there; your goal isn't to make them grow more. In a greenhouse you propagate plants—you push a bloom." Joe Hickson, president of Private Garden Greenhouse Systems, says: "Everybody says their conservatory can be used as a greenhouse, but the conservatory doesn't lend itself to a moist environment. Conservatories are really designed to look good."

"Conservatories have less glass than greenhouses, and the structural pieces are stronger because you have to deal with more thermal qualities," says Peter Cook, a Long Island-based architect who owns a greenhouse and has designed greenhouses for others. "In a conservatory you don't want windows fogging up, and you want to cut some sun out so you don't get too hot. A greenhouse is airier, lighter than a conservatory. It's also pretty primitive."

Mark Ward, who specializes in finding and restoring antique greenhouses from the 1920s through '40s, then relocating them, cites one client who didn't understand the difference. "She had a room that was designed to be a conservatory/sunroom," he says. "But she had been using it as a greenhouse, watering the plants with a hose. Now it's falling apart, the Sheetrock is literally coming off the walls. If you want to run around with a hose, the structure has got to be built like a bathroom."

Finding a greenhouse manufacturer isn't difficult—one glance at the back pages of gardening magazines or a surf on the Internet will reveal dozens. Finding one that can build a structure to complement the design of your home is another matter. "There are quite a few national greenhouse companies on the commercial market that will sell to anybody, and lots of little regional ones," says Rahn. "But most of these are hobby greenhouses. There's a very small number of companies designing beautiful structures."

"Quality greenhouses are expensive," says Cook, who believes the high prices are due to import costs. "A lot of them are made in England," he says. "The structure itself isn't expensive to make, it's the shipping, handling, installation." His own 14 by 20-foot Hartley greenhouse, from Private Garden Greenhouse Systems, cost $60,000, "and that was only for the greenhouse structure itself," Cook says. "Then you have to add on the cost of all the interior elements, as well as the foundation and kneewalls, which are usually twenty-four to thirty inches high and have to be ready when the greenhouse installers get there." Says Joe Hickson: "We tell people to estimate an extra fifty percent of the cost of the greenhouse for those things."

At Amdega-Machin, conservatories start at about $50,000 for a freestanding structure ($20,000 for an attached one) and go up to about $150,000, the average price being $60,000 to $80,000. "We're like a Ferrari, or a Lamborghini," Boddie states. "You're getting a product that's custom-designed, handmade, and imported—not a kit; there's no manual that comes with it. They're made in our factory based on the client's specs, then disassembled and shipped from England. We then send our full-time team of installers to construct them on the client's property." Amdega-Machin's fees include the ventilation system, fans (on certain models), insect screens, and shades. Boddie estimates a $60,000 greenhouse will require an additional $15,000 to $20,000 for flooring, benching (the tables that hold the pots), heating, cooling, lighting, watering, and climate-control systems. Boddie always recommends that clients hire a local contracting firm to put in the extras rather than have Amdega-Machin do it. "It is much cheaper," he explains.

One thing that greatly increases the cost is not building to a standard size. "There are two choices when it comes to the structure," says Cook. "You can go with stock components, but that limits the dimensions. Or you can custom-design, either by altering the stock dimensions, as you can with Private Garden Greenhouse Systems, or by starting from scratch with an architect. When you alter preexisting dimensions you can change the price from $40,000 to $80,000 just by adding a foot or two on either side." Ultimately, the cost depends upon the plants you want to grow and the technical support that they require. However, there are some standard technical elements common to all greenhouses. Here's what you need to know.


The Hicksons advise brick or stone with a tile path down the middle, plus gravel with drains. "The brick, stone, and tile are for the area you walk on," says Katherine Hickson. "Under the plants we recommend gravel, which is traditional, with eight to ten inches of sand below it for drainage." Gates chose gravel because "it's very nice aesthetically, and it's good for reflecting light and retaining heat." Boddie concurs: "Gravel is the best for drainage, and it's easier to take care of than tile. But it's hard to roll a wheelbarrow on gravel because the wheels sink in." (Like the Hicksons, Boddie suggests laying a pathway of stone or tile with gravel on both sides.) One client of Amdega-Machin, he says, put a three-foot perimeter of gravel with drains in her 20 by 30-foot greenhouse so she could water right over the floor. She installed slate tile in the rest "so she'd be able to roll in her five-hundred-pound pots, and could have a table and chairs for card games with her friends."


Here the big controversy is single- versus double-glazed (also called double-layered or insulated) glass. According to the Hicksons, single-layered is better for a greenhouse environment. "Double-layered glass is too tight for growing," says Katherine. "It allows too much moisture retention, which promotes plant disease."

Boddie disagrees: "Single-layered glass provides no insulation," he says, "and you have a lot of condensation. The glass looks like you've sprayed the panes with a hose. Amdega-Machin uses double-layered glass, and none of our conservatories have a condensation problem. We even have one client that's a big florist shop in Philadelphia. They have fountains, and they spray plants in there all the time. There's no condensation buildup at all." Says Ward: "Single-glazed glass retains less heat, and it results in more condensation. For plants that like cool temperatures, using single-glazed glass makes more sense. But for tropical plants, double-glazed is more practical."

Double-glazed glass, Boddie adds, lets in less light, but the difference here is minimal. "It's like the difference between 99.6 and 99.8 percent," he explains. Sometimes Amdega-Machin will tint glass, which reduces sunlight by up to 10 percent, or use glass with a low-emissivity, or low-e, coating, which reduces the amount of heat loss and blocks UV rays without greatly lessening the amount of sunlight let in. "But those are mainly for our clients who are using the structure as more of a living space, not as a greenhouse," he notes.

They all agree, however, that whether single- or double-glazed, the glass should be tempered, which means that after being cut to the required size, the panes are reheated to 1,200 degrees Fahrenheit, then their surfaces are cooled rapidly. "Tempering the glass prevents it from shattering into shards," Cook says. (In fact, tempered glass is four times stronger than ordinary window glass.) "You can bang on it with a hammer and it won't break," says Ward.


"If you don't have an adequate ventilation system, the greenhouse could get to a hundred eighty-five or two hundred degrees, depending on the solar gain," says Rahn. "I've seen greenhouses in which PVC plastic pipes have melted into puddles and metal has bent on its own."

A typical greenhouse ventilation system is composed of vents, windows, and often fans. The Hicksons say that the best choice is to use continuous roof vents, which extend from gable to gable, plus side-wall vents. "We're talking natural convection," she says. A common option is thermal-piston vents, which open and close automatically in response to temperature changes.

The other main alternative is electronic vents, which are programmed to open and close at certain times. "You program the system to do specific things," says Rahn, "such as close the vents when the temperature drops below fifty-five degrees or open them one hundred percent when the wind is under twenty-five miles per hour. You can even program it to open them five degrees if the wind is at a certain speed and it's raining. It's very sophisticated." However, he adds, that's not to say you have to be a weather expert to do it. "You just put in the numbers and we do the math," he says. "But you do have to be familiar with plant growth and know ideal temperature and humidity levels."

As for side windows, Boddie usually recommends awning-style, which open at an angle, rather than casement-style. "Awning-style windows are typical of historic conservatories," he explains. "They're hinged at the top and push out from the bottom. They're better when it rains because less rain can enter." Boddie also advises placing the windows as close to the ground as possible, where the air is cooler. To keep out insects, screens in the windows are necessary, with the best designed to keep out even microscopic ones. (The Hicksons' choice: Echo Net from Ludvig Svensson.)

In some locales, a micromisting system (55-degree droplets of tapwater sprayed into the air) is necessary as well as vents and fans to keep greenhouses cool. "It's essential in dry, hot climates," say the Hicksons, "where a greenhouse can become brutally hot in summer, even with the vents." Boddie agrees. "It supplies moisture where the heating system might dry the air out. Micromisting works best in arid climates."

Air Circulation

It's essential to keep the air inside greenhouses moving to prevent condensation buildup, which can promote leaf disease. The Hicksons recommend Horizontal Air Flow, or HAF, fans. Says Katherine: "HAF fans circulate the air to maintain an even temperature range." The alternative, she says, is an air-flow system designed to exhaust hot air. "But they are ugly and overpowering in terms of velocity and sound." Boddie, however, thinks HAF fans are a bit noisy: "Our Machin-style system, which includes fans near the roof that draw the air out of the room, is quieter. HAF fans detract from the beauty of the structure if they are at waist or head height. Sometimes they're necessary, especially if you have a very long room."


The greenhouse manufacturer usually provides the shading system, which amounts to material hung parallel to the side wall and roof of the greenhouse. "Each type of material has a different shade coefficient and different heat-retention level," says Joe Hickson. "The material is frequently strung on a monofilament wire or hung from it on a roll system, like blinds." The best fabric, he says, is by Ludvig Svensson and composed of polyester, nylon, and aluminum. Shades in some Amdega-Machin conservatories consist of Kevlar nylon and polyester panels treated with anti-mildew coating. "They reflect out about sixty-five percent of the heat," says Boddie.


It's essential, but how much depends on the plants being cultivated. One thing experts agree on is that forced hot-air heating systems designed for sunrooms are anathema to plants. "It overheats them," says Joe Hickson. "The air blows across the leaves, causing them to dry out." The solution: a radiant hot-water pipe system embedded in the floor, which allows heat to mix with the air before it reaches the plants. "It takes longer to heat this way," explains Hickson, "but the air stays warm longer once the system is shut down."

Boddie agrees, but says that in very cold climates radiant pipe heating isn't practical because it can take as long as eight hours to heat the space. In such places he recommends building a thicker kneewall and running hot-air ducts out of the floor. "That way the air blows horizontally, so it's not hitting the leaves directly," he says.


There are two choices: a hose or an automatic watering system. The type of system to choose depends on what you're growing. For plants that don't require high levels of moisture, such as cacti and certain types of orchids, a drip-irrigation, or "spaghetti tube," system is fine. It employs small tubes to carry water to hooks embedded in the soil near each plant. Plants that need a high-moisture environment, such as African violets and poinsettias, require a "misting system," in which water is sprayed through nozzles at leaf height.


A horticultural term for a table with drains designed to hold plants in pots. The two best benching systems are a "wire-mesh system with expanded metal" and an "ebb-and-flow" system. In the former plants are watered from above and the water drains out. "The good thing is that it breathes," states Joe Hickson. The ebb-and-flow system, in contrast, is made of a hard, grooved plastic membrane. Plants are watered at root level, after which a drain opens. "It can be done automatically by tying it into a computer," says Hickson.


The rule is simple: If a greenhouse can't get at least six hours of direct sunlight a day it needs artificial lights. The kind depends on the plants being grown. Hickson recommends High-Intensity Discharge lighting, either high-pressure sodium or metal halide lamps. (The two top suppliers, he says, are Sunlight Supply and Hydrofarm.) Phil Orendorff of Hydrofarm says HID lighting is excellent for growing tropical plants, tomatoes, and cuttings and for wintering herbs.

In the past, high-pressure sodium lights were the only type of HID lighting available. They have a narrow spectrum, Orendorff says, which promotes early flowering but "makes everything look red, even the tops of plants." He recommends the firm's Agrosun metal halide bulbs ($250-$399), which, he says, are "truer to sunshine. They include a blue spectrum, great for greening, and the spectrum needed for flowering; and they make plants look nice, with a natural color." Orchid lovers, Orendorff says, "go nuts over Agrosun lamps. It's just like plugging in tropical sunshine." The other alternative, he adds, is fluorescent lighting, used when low light suffices: for instance, for starting seeds and growing plants such as begonias, philodendron, and some orchids, including phalaenopsis.

Environmental Control System

One is necessary if you want an automated greenhouse. "Q-Com's system is one of the best for residential purposes," says Joe Hickson. "It looks at wind direction and speed, moisture levels and temperature, then ties the opening and closing of the vents into it. You can even have timing programmed for watering." Rahn of Q-Com says that the company's best residential system is Grower's Choice, which can be tied into a personal computer and is not difficult to program. (Having Q-Com program it for you costs $775 per day, plus airfare.) The full system—electronics, software, weather station—costs $6,900.

Some argue that this is another instance of allowing technology to run to extremes. Boddie says that most greenhouses can get by on a simple thermostat-controlled heating system and an automatic watering system. (He recommends one from Charley's Greenhouse Supply, based in Washington State, that costs approximately $350.)

Or like Pat Ouderkirk, a landscape designer in Bridgehampton, New York, you can just takes things as they come. "I turn up the thermostat when I'm in my greenhouse and supplement the humidity with humidifiers in winter," she says. "In summer I open all the windows and skylights. The wood moldings are getting ruined by the creeping fig I have climbing up them. But it's my fantasy room, my folly."

Planting The Seed

According to the pros, here are the steps to take when creating a greenhouse.

First, decide what you want to grow in the greenhouse. "It's wise to choose plants in the same temperature group," says Mark Ward of Ward Greenhouses. "If you don't, you have to create different temperature zones." Remember, too, that plants also affect requirements for ceiling height.

Let the interior requirements determine the greenhouse size, not vice versa. "People say, 'I want a thirteen by twenty-foot greenhouse,' " says Katherine Hickson, CEO of Private Garden Greenhouse Systems. "Then they tell us they want a potting shed, sink, boiler, and armchair to fit in it. But now there's no room for the plants. It's never big enough. They never regret going larger."

Hire a contractor to oversee the project. Even with a prefabricated greenhouse, you'll need one for two reasons: to build the foundation, and to coordinate the subcontractors.

Hire an architect too. "It can pay you back two-fold," says architect Peter Cook. "Proper structural planning can save you both time and expense later. If construction lasts one and a half months longer because there are problems with dimensions, you'll pay the installers for that time. And if it takes longer, the site conditions can change. If you look at the fine print in the contracts, there are provisions for that. They have the right to stop work if the weather is bad."

Take your time and spend it on planning. It usually takes 12 to 18 months to go from brochure to finished greenhouse.

Start by looking at company catalogs. "See what's out there, and which company has greenhouses that match your style and budget," says Cook. "Look at the bells and whistles they offer. The more that's available, the less customization you'll need, which means it'll be cheaper in the end."

Decide where to place the greenhouse. Try to position the greenhouse so that it faces east-west, as it will receive 25 percent more sunlight than one that is oriented north-south.

Then make preliminary drawings using the greenhouse manufacturer's parameters. Give the drawings to the greenhouse manufacturer to get its construction drawings and an estimated cost. Once you've gotten the drawings back, show them to a contractor and an architect. "The contractor can help the architect identify what will make the project easier or harder to build," Cook explains.

Start construction in either late spring or summer. "Summer's the best time," says Cook, "because you don't actually use the greenhouse then and the weather is good."

Ward Greenhouses

Mark Ward, owner of Ward Greenhouses in Concord, Massachusetts, is a cross between an antiques dealer, an engineer, and a greenhouse manufacturer. He specializes in finding, buying, and restoring greenhouses from the 1920s through '40s, then locating new homes for them. "I like them because in many ways they're much nicer than what you can get now," he says, "in terms of the materials, details, and the way pieces are fitted together. It's hard to make an extruded aluminum part not look very, very functional. It's like the difference between buying a wooden boat versus a fiberglass one." In some cases Ward dismantles structures and resells them for $20,000 to $70,000 or more, including installation. In other cases he finds larger structures and subdivides them, such as the 50 by 300-foot greenhouse he found and converted into 20 individual ones.
$ Tel/fax 978-369-1354.

The Hartley Line By Private Garden Greenhouse Systems

We started off in the 1980s mostly refurbishing preexisting greenhouses from the 1800s," says Katherine Hickson, CEO. "But then people started wanting new greenhouses on new properties, only they wanted the classic look with modern technology." Unable to find manufacturers in the United States, the Hicksons turned to England, where they found Hartley, a 118-year-old manufacturer that produces Victorian-style structures with 14 to 16-foot ceilings and period details. "They are really bespoke greenhouses," says Hickson, "one of a kind, and can be made to any size." Prices range from $20,000 to $200,000; the average price is $45,000. 800-421-4527; fax 413-566-8806; Web site:

Amdega-Machin Conservatories

Amdega first opened in Darlington, England, near York, in 1874. It now has six offices across America. In 1991, Amdega acquired Machin, also a leading conservatory manufacturer, known for its French- and Italian-style designs. Amdega-Machin structures are all custom-made, either to plans made by the company's architects and engineers or by the client's. "We coordinate all aspects, start to finish," says Wayne Boddie, the company's southern U.S. representative. "But our primary emphasis is on designing and building the structures, so we have a limited range of accessories, which is why we bring in outside expertise to help with the interior technology." Even the architectural style, he says, is up to the client, although most choose Victorian.

The company strongly believes that a conservatory can function as a greenhouse. The key, says Boddie, is utilizing the right materials and the right paint. The company's Amdega-style conservatories are made of kiln-dried cedar, a durable wood that doesn't warp easily and that contains an acid that is distasteful to insects. As for paint, the company developed a "microporous" paint that Boddie says allows the wood to breathe. Machin-style conservatories are made of aluminum and steel, with the paint pigment baked onto the metal through a process called powder-coating. Average price for both styles: $60,000 to $80,000. $

Oak Leaf Conservatories

Oak Leaf, based in York, England, manufactures custom conservatories of mahogany. The structures include motorized or manual roof vents and manual side windows; the customer is responsible for putting in the foundation, flooring, heating, and other interior technology. "They're frequently used as additional living spaces," says Amy Magner, Oak Leaf's North American representative. "We have one client who is using his as an extension of a private bath. A large portion of the floor area was specially built to hold soil for planting directly into the ground." Oak Leaf Conservatories are "all handcrafted," Magner explains. "Nothing is prefabricated or modularized. And we can match the design, even the moldings, to any home." The cost of the conservatory—which ranges from $60,000 to $500,000—includes installation. 800-360-6283; fax 404-250-6283.

British Conservatories

"Our conservatories are unique in that their frames are made of welded aluminum, which is much more robust than wood," says Jay Jackson, owner of British Conservatories in Temple, Pennsylvania. "On the interior we use a rot-proof polymer composite to insulate the frame. It's the same material that's used on the space shuttle's exterior tank, but we machine it so that it looks like wood." Jackson says that about half his clients use the conservatories as working greenhouses. British Conservatories supplies only the structure, which includes double-layered glass, solid brass hardware, and finials; clients install lighting, heating, benching, and other necessary technology. Conservatories start at $20,000 but cost on average $60,000 to $70,000. 800-566-6360; fax 610-939-9464;
Web site:



Peter Cook, $

Environmental Control Systems

Q-Com $

Lighting Sunlight Supply, Inc. $

Hydrofarm, Inc. $
1 877-610-1600; Fax 877-262-6050;

Suppliescharley's Greenhouse Supply $
1 800-322-4707; Fax 800-233-3078;
