Building the Perfect Greenhouse

From modest window units to prefab glass houses, here's how to select the right greenhouse for your growing needs.

Credit: Christina Symons

Glass greenhouse

Greenhouses create a year-round growing season

From modest window units to prefab glass houses, here’s how to select the right greenhouse for your growing needs.

For many gardeners, having a greenhouse would be a dream come true. Imagine extending the growing season by several months a year, or consider the economy of starting your own plants from seed or by propagation. There are numerous sound reasons for having a greenhouse, but perhaps the most meaningful is the opportunity a greenhouse affords to watch plants sprout and thrive. In my opinion, anyone serious about growing plants can enjoy the benefits of gardening under glass.

A greenhouse, quite simply, is a space that provides a controlled environment for plants. The structure itself can range from a modest backyard unit to a luxurious solarium constructed as part of a house. Greenhouses can be made of glass, rigid plastic or plastic sheeting, and with metal or wooden frames. You can follow build-from-scratch plans, choose from various styles of prefabricated greenhouse kits, or opt to have a unit professionally built. Greenhouses can be simple or complex, small or large, modestly priced or outrageously expensive. The challenge is to select a model that’s appropriate for your requirements, price range and available space.

Before selecting the greenhouse, first consider the most suitable site. A southern exposure for the sides of the building is ideal; a west- or east-facing exposure would be the next best choice. The ideal spot should be flat and relatively well-drained.

Next, evaluate the direction and force of the prevailing wind and avoid situating the greenhouse in a blustery area. While healthy plants require good air ventilation and do benefit from moderate outdoor breezes that move the air inside and outside the greenhouse, continuous chilly winds will significantly lower the temperature inside the structure, stress the plants and drive your heating bill sky-high.

Everyone knows that a greenhouse should not be built under trees, however sometimes we forget about the dangers posed by those large trees in the distance, whether they are on our own property or the neighbours’. If a tree should fall during a bad storm, would it or any of its flying branches hit the spot you have in mind for a greenhouse?

Finally, analyze the distance and difficulty of installing water and electricity to the site. Power is required for lights, heater, heating cables (for germinating seeds) and a fan. Water is a must. Utilities are normally extended from the house to the greenhouse, so consider the best route. Will the installation of electrical wires and water lines (laid in an underground trench) have to contend with such obstacles as a paved driveway, swimming pool or patio? How about retaining walls, favorite trees and shrubs, or rocky out-crops? And, of course, the further the distance, the more expensive the installation.

Obviously, there are many factors to consider when choosing a greenhouse site. You may have to be creative – a corner of a deck or patio might well be the only location on your property that meets the requirements. One of my friends opted to build a greenhouse on a second-floor balcony rather than disturb her small yard landscaped with treasured perennials and shrubs.

Choose with Care

Choosing the perfect greenhouse can take a lot of thought and research. The first step is to determine the quantity and type of plants you want to grow. If you are content to have a few potted plants and start a small number of annuals or vegetables from seed, a window greenhouse might be all the space you require. Window greenhouses are just that – a glass box extending out 35 centimetres or so from an existing window space. This provides a wide, sunny sill on which to grow plants. It is also an economical way to get started – prefabricated window units can be installed into existing window openings and are heated by the warmth of the room.

If a window greenhouse is too small for your needs, consider a lean-to structure attached to an outside wall of a house or other building, consisting of one long, south-facing glass wall and two glass end walls, one containing a door. This allows the gardener to position benches for plants in an L-shaped configuration and to use the solid wall of the building as a potting area, with storage and shelving. Not only is this a handy layout, it is also significantly less expensive to supply power and water to this style of unit than it is to a free-standing unit, as you can simply extend utilities through the wall of your house, instead of trenching pipes and wires to a freestanding unit.

One very expensive option is to have a sunroom or solarium built as part of your home. This may be the best choice for the horticulturist who grows tropical or other tender plants that require steady, warm temperatures. As an added bonus, your greenhouse will become part of your living space, providing constant enjoyment. Imagine the pleasure of sipping afternoon tea in your very own lush paradise!

However, it’s usually a free-standing structure that comes to mind when one thinks of a greenhouse. I am lucky enough to have two: the first is a “temporary” greenhouse that my husband Joe constructed from lumber and plastic sheeting 12 years ago. This structure has no water or power so we use it as a “blight-free” zone in which to grow tomatoes during the summer. Come fall we fill it with pots of hardy perennials, shrubs and vines that were started from seed or propagation in the spring but are still too small to plant outside. The plants survive quite nicely with the help of some Reemay sheeting tossed over the pots during cold snaps. The plastic shell has been replaced several times, which is a bit of a nuisance, but not that big a job.

Our second greenhouse is a traditional free-standing glass unit. The temperature in this greenhouse is maintained a few degrees above freezing with a small, thermostatically controlled electric heater, which guarantees that my pots and baskets of fuchsia, geranium, petunia, datura, chrysanthemum and other tender perennials will survive the winter unscathed. From March onward the greenhouse is bursting at the seams with hanging baskets, newly seeded vegetables and bedding annuals. Because we have a number of large gardens and enjoy growing our own plants, our greenhouses have paid for themselves several times over.

A Big Investment

Make no mistake – a greenhouse is a big investment in time, effort and cost. As you do your research, remember that the price of that perfect little aluminum and glass prefabricated greenhouse is just the base figure.

First add GST, PST and delivery charges. Next, your greenhouse needs a foundation and perhaps some drainage tile if the site tends to get soggy. What about the cost of a tile, wood or gravel floor? Then consider adding power and water. In addition to materials, you may also require some expert assistance in setting these things up. In addition, don’t overlook the possibility that a permit might be required in your municipality. And, finally, think about those extras – thermostatically controlled vent and fan systems, heating coils, fluorescent lights, benches, sink and a potting/storage area.

Does this sound a bit daunting? Certainly, the initial cost and work of constructing a greenhouse can be onerous, but remember, this is an investment. A well–built unit will provide years of trouble-free usage.

Your greenhouse will be a busy place, needing a lot of attention. Be prepared for a constant cycle of daily watering, seeding, transplanting, dividing, repotting, deadheading and general upkeep, month after month. For some people this might be a burden, but for those of us who enjoy plants, it’s a labour of love!

Plants for a Cool Greenhouse (8°–15°C)

Abutilon (flowering maple)
Calceolaria (slipper flower)
Calendula (pot marigold)
Camellia (camellia)
Chrysanthemum (chrysanthemum)
Cyclamen (cyclamen)
Datura (angel’s trumpet)
Dianthus (carnation)
Fuchsia (fuchsia)
Pelargonium (geranium)
Primula (primrose)
Salpiglossis (painted tongue)
Freesia (freesia)
Hyacinthus (hyacinth)
Narcissus (narcissus)
Nemesia (nemesia)
Oxalis (sorrel)
Schizanthus (butterfly flower)
Vegetables: green onions, lettuce, parsley, peas, radishes, spinach

Plants for a Warm Greenhouse (16°–20°C)

Begonia rex (rhizomatous begonia)
Bougainvillea spectabilis (bougainvillea vine)
Caladium bicolor ‘Candidum’ (angel wings)
Ficus benjamina (fig)
Passiflora caerulea (blue passion flower vine)
Saintpaulia ionantha (African violet)
• Solenostemon scutellaroides (coleus)

Greenhouse Tools and Supplies

• Plastic garbage can for storing soil
• Bamboo or cedar stakes
• Knife, scissors, trowel and dibble
• Pruning shears
• Rubber hose with fine nozzle
• Plant labels and masking tape
• Scoop for soil
• Soft twine and twist ties
• Clear plastic bags for propagating plants
• Water pail and spray bottle
• Whisk broom and dust pan
• Thermometer
• Assorted pots
• Gardening books
• Small watering can with long thin spout that can reach into the edge of containers
• Radio to entertain you while you work!

Keeping Your Greenhouse Healthy

• Keep your greenhouse clean
• Give the whole unit, including benches, a yearly scrub with warm, soapy water and a stiff brush.
• Clear away all dead foliage and debris from soil surfaces and benches. Discard immediately.
• Use sterilized potting soil. Do not reuse old soil – add it to one of the outdoor borders.
• Clean all tools after use and don’t forget that pots and containers must be squeaky clean, too.
• Watch out for trouble. Inspect all plants for insects and/or diseases regularly. Isolate and, if necessary, discard infested plants.
• Plants are like people – they need a breath of fresh air. Give your greenhouse plenty of ventilation. Open the door as often as possible.
• Water with care. Potted plants require less water during the winter but can dry out very quickly in the summer.
• Remember: stressed plants are more prone to disease and insect attacks.

Sunshine Coast resident Vonnie Kovacic is a strictly organic gardener who has developed a passion for starting plants from seed.