A GREENHOUSE IS a splendid antidote to a Canadian winter.
It's a little oasis, a piece of summer, or at least a temperate zone where
plants can flourish, sheltered from the cold. A greenhouse is the ticket,
whether you plan to raise exotic orchids year-round near Whitehorse, or grow
cool-season vegetables for Easter dinner around the Niagara escarpment. But
you'd need two different greenhouses to accomplish these purposes. It's
important to choose the right greenhouse and plan carefully to meet your needs.
Winters are notoriously harsh in Canada, but summers can be,
too, especially for a delicate flowering plant trapped in an eight-by-ten-foot
glass bubble. In its native home, it is warmed by the moderating effects of a
nearby ocean, and cooled by its breezes. The glare of the July sun overhead at
noon in Regina will be enough to cause the plant's buds to drop - or worse, if
the stomates (pores) don't close fast enough.
Kyle Exner, manager of B.C. Greenhouse Builders, fabricates
greenhouses in British Columbia's lower mainland and ships them across North
America. He says it may take longer to select the proper greenhouse than to
build it. "You need to ask yourself some critical questions early on. Do I see
myself tending tropical plants in December, or starting my bedding plants in
The cost of a greenhouse will depend partly on its purpose
and partly on how much time the gardener plans to spend in the structure, Exner
says, adding, "Most of the greenhouses we build are 12 by 16 feet, or smaller.
Most are of energy-efficient materials, and most will be a permanent addition
to a yard."
In B.C.'s lower mainland or along the shores of Lake
Ontario, south of Hamilton, a four-season greenhouse is possible, thanks to a
single layer of glass, or a six-millimetre sheet of light-weight,
shatter-resistant polycarbonate. A heat-cured plastic, it is best known as the
substrate for compact disks, and in plastic eyeglass lenses. It is the material
of choice for building the modern hobby greenhouse.
Exner says he still sells a lot of single-pane glass for
greenhouses in southern B.C. and on Vancouver Island, but most of his customers
opt for more than one layer of glass, to reduce winter heating costs. And the
majority of his customers choose polycarbonate.
Twin-wall polycarbonate takes advantage of air spaces
sandwiched in large cells between two sheets of plastic. In colder climates
north of Toronto, in Quebec, in most of the Maritimes and in the British
Columbia interior, this will create a greenhouse capable of fending of all but
the harshest conditions.
On the Prairies, in northern B.C. and any other region where
the mercury drops below -20 C for more than a day or two each year, it is
advisable for greenhouses to have triple-wall polycarbonate - especially if one
wants to cultivate orchids or fruit-bearing plants. "In those climates," Exner
says, "you can over-winter begonias and fuchsias with only a small electric
heater and a twin-wall glass or polycarbonate greenhouse. With a larger heater,
you can grow cold-season vegetables all season. If you want tomatoes in
January, then 16-millimetre, triple-wall material is for you."
The cloudy quality of polycarbonate reduces the need to
cover the roof with a cloth to keep out the burning rays of the sun during the
summer months. For a similar price but a clear view, twin-walled tempered glass
Those new to greenhouses should pay particular attention to
the roofing material. "Your roof is where your major heat loss takes place,"
Exner says. Typically, it's also where solar gain needs to be reduced. His
company offers a roof made of triple-wall polycarbonate. This material admits
only 75 percent of the light that strikes it; with glass, it's nearly 100
percent. With glass, the owner gets a view, moderate heat efficiency and a
showpiece for the property.
Suitable heat and other utilities for the greenhouse must
also be carefully planned. Electricity is critical, and running water and
natural gas are useful. Fans control dreaded condensation, and lighting
convinces plants that the warm weather is a product of the longer days they
would enjoy if they were rooted closer to the equator.
Having invested in the building itself and the plants
inside, a greenhouse hobbyist will do well to select adequate fans for the
structure. A hardware store propeller fan will meet most growers' needs, but to
ensure mildew and moisture control, a horizontal airflow fan is necessary.
Mounted near the ceiling, this type of fan encourages plant growth by
increasing carbon dioxide availability and promoting uniform temperature from
floor to ceiling.
Adding a season or two to the gardening year means adding
some daylight. There are several ways to accomplish this, Exner says. One can
add specialized, full-spectrum fluorescent bulbs to a light fixture. Another
approach is to purchase one warm-colour temperature fluorescent bulb (yellow)
and one cold-colour temperature fluorescent (blue) to use in the same lighting
fixture. This is an economical way to create a full-spectrum light: each bulb
costs about $4, whereas each full-spectrum bulb costs $30 and up. In either
case, an inexpensive shop light fixture to hold the bulbs costs about $18.
"If you are growing high-value exotic plants, you will be
choosing high-pressure sodium or metal halide lighting," Exner says.
There is growing interest in horticulture, and greenhouse
owners have many high-end lighting choices available at the flick of a switch,
says Patrick Vivian, of Sunlight Supply Inc. in Vancouver, Washington. The
lighting manufacturer builds and distributes lighting systems for commercial
growers and hobbyists. The technology once available only to industry is being
adapted for much smaller operations.
"There has never been so much choice in the market," Vivian
says. There are small, high-pressure sodium systems for a few hundred dollars.
By contrast, the top-of-the-line lighting strategy uses a combination of light
from red-spectrum sodium bulbs and from blue-spectrum high-output T5
After determining how much greenhouse you want, the next decision
is where to place it, Exner says. Attaching it to the home is the most
efficient, inexpensive option. Builders agree sharing utilities and a common
wall can save $2,000 to $4,000 in construction costs on the average greenhouse.
A stand-alone structure will require buried utilities trenched in from the
Greenhouses require firm foundations. If glass is used for
the walls, a concrete base is mandatory to prevent shifting and breakage. At a
minimum, a four-by-four or four-by-six cedar grade beam is needed over a packed
clay and gravel base. New pressure-treated woods should be avoided because they
contain copper, a substance that will corrode aluminum, a common component of
In softer soils or where soil is well frozen in winter,
concrete piles will be needed to anchor and support the structure.
Many greenhouse owners opt for a concrete pad and short pony
walls to add height. This provides an easily washed and maintained floor
surface as well as a sturdy foundation. In addition, concrete is easily
sterilized in the event of a plant disease infection. "If it gets into your
dirt floor, it is a real problem to get rid of," says Exner.
Most hobby greenhouses arrive as do-it-yourself kits, but
contractors will assemble them for a fee. If a contractor is providing all the
labour and when benches, lighting and utilities are included, a buyer can
expect to double the cost of the greenhouse, Exner says.
"You can reduce that dramatically depending on how much of
the work you do yourself. A lot of country homeowners like to do that sort of
stuff on their own, so they can add a good deal of sweat equity to their
However, a greenhouse owner doesn't want to experience
excessive sweating, especially in summer. Most greenhouses have roof ventilation
systems and automatic pressure cylinders that open these vents when the
temperature reaches anything above a simmer and close them as the room cools.
Additional wall vents and fans can be added, especially in larger greenhouses,
where heat buildup can occur swiftly, even on a mild February day.
When the sun's heat is inadequate, supplementary heat is
required. "If you are planning a greenhouse and will be bringing in water and
electricity and natural gas is handy, put it in," Exner advises. "You will
spend a lot more up front and get it all back before you know it, when you
don't have to run that electric heater you plug in day and night."
One way of moderating the temperature is to sink large
plastic barrels filled with water, crushed rock or both below the floor of the
greenhouse. These heat sinks will take in energy during the day and release it
Properly situating the greenhouse will also reduce the
amount of light and heat required to operate it. A southern exposure is best,
although east and west will also work. A greenhouse close to the house will
reduce utility runs and facilitate trips to the washroom.
If the hobby sticks, an expansion may be desired, so the
location of the greenhouse should allow for one end to be extended. Most greenhouse
systems provide for additional panels to be added, with the end wall moved out
to accommodate an addition.
||Double-wall glass, walls and roof, or double-wall
|Triple-wall or multi-cell polycarbonate, walls and
roof. This is the most efficient combination of all. For the best view with
better efficiency, install double-wall glass with triple-wall polycarbonate roof.
Garden hose water supply for three seasons and a water bucket in the winter. Electrical for light-duty fans and small power tools on an extension cord.
||Water and electricity underground from the house for in-greenhouse use throughout the year. A small sub-panel supplying power for lighting, fans and heater during the winter.
||Water and electrical as above, plus natural gas for heat. Natural gas costs more initially, but saves money over electricity in the long run.
||Compressed gravel over clay, suitable only for smaller polycarbonate greenhouses.
||Cedar 4 x 4 grade beam over compressed gravel. Concrete piles in soils prone to freezing and shifting.
||Concrete floor and foundation, piles as necessary. Concrete removes the risk of disease persisting in the floor soil.
||Plastic. Prone to bending, it can be very durable otherwise. Easy and light to assemble and modify. Doesn't accept paint well. Lacks strength for hanging baskets, fans and other equipment.
||Wood. Strong, excellent construction material. Heavy, and unless it's pressure treated or cedar, it is prone to rotting.
||Aluminum. Light and strong. Cost is similar to wood. Works with proprietary building systems. Available in many colours. Very strong for mounting interior hanging accessories.
A high-quality, entry level eight-foot by 12-foot
polycarbonate greenhouse, framed in aluminum, costs $3,500 and up. Double-wall
polycarbonate will add several hundred dollars to the price. A high-end,
triple-wall polycarbonate or double wall glass with a polycarbonate roof costs
Annual operating costs could range from several hundred
dollars to $1,500 or more. This takes into account geographic and weather
variables, preferred crops, the size of the greenhouse, the type of panes
chosen for the structure, and the type of heating used.