Imagine: the trees are still bare, the odd snowflake is still flying, but your seedlings could already be reaching their way skyward — if you had a greenhouse.
Sooner or later, here in the land of the all-too-short growing season, every market gardener starts scribbling figures on the back of a napkin. How much would a greenhouse cost? How much extra revenue could it generate?
No question, a greenhouse can be a smart investment. But choosing the right one means making a bewildering number of decisions: shape, size, covering, location, ventilation and a slew of optional features.
For help navigating the main options, we turned to Norm Eygenraam, owner of MultiShelterSolutions in Palmerston, Ontario. The industry veteran, who has been selling greenhouses for four decades, starts with two simple questions. How are you going to use your greenhouse? And what kind of conditions will it have to stand up to?
Norm Eygenraam, owner of Multi Shelter Solutions in Palmerston, Ontario, has been selling greenhouses for four decades. He asks prospective buyers two questions: How are you going to use your greenhouse? And what kind of conditions will it have to stand up to?
Because different crops have different requirements, think about the plants you’ll grow in your greenhouse. How much space will they need? How much heat? How much light?
Next, assess the kind of wind conditions your greenhouse will face. Eygenraam points out the similarities between the profile of a greenhouse and the profile of an airplane wing. “The problem is,” he says, “you want the opposite result. When you’re sitting in an airplane, you want it to get off the ground. When you’re standing in your greenhouse, you want it to stay.”
Snow load is another consideration. Underestimate it and you run the risk of your greenhouse crumpling like a concertina after a January blizzard. And if your region gets lots of hail, you’ll want to take that into account as well.
Now you’re ready to start assessing options and — inevitably — making trade-offs. “You’re not going to get everything you’re after,” Eygenraam warns. Start early, ask lots of questions and prepare to prioritize and reprioritize your needs until you find the right solution.
Size
When you’re estimating how much square footage you’ll need, give yourself room to expand. After all, it’s a lot cheaper to build a bigger greenhouse now than to add space after the fact.
Now think dimensions: width, length and height. “Most of the time, going narrower and longer is cheaper than going wider and shorter,” explains Eygenraam. “The cost of the two ends is the same whether you’re building 30 feet long or 300 feet long.”
Height is a trickier question. A taller structure costs more to heat because greenhouses lose heat in direct proportion to the surface area that’s exposed to the outside. It’s also more likely to catch the wind.
On the other hand, a taller greenhouse will shed snow more efficiently. As well, it has more air volume inside, so the temperature won’t fluctuate as much and you’ll get more air circulation, creating a healthier environment for your plants.
Shape
Google “greenhouse” and you’ll find just about every shape you can imagine. However, there are basically three tried-and-true choices. Quonsets are the simplest and cheapest option, and they don’t require completely even ground. However, their semi-circular profile doesn’t shed snow well.
Like a Quonset, a gothic arch design also has rounded sides. However, the walls come together to form a peak at the top, which sheds snow much better. This structure also gives you more flexibility in terms of width and height.
An A-frame has vertical side walls and a roof angled at 35 to 40 degrees. It needs a heavier structure to create rigidity and support the weight of the covers. A-frames are usually limited to pre-engineered sizes.
Covering
Glass is the traditional choice, of course, and extremely durable. It also transmits light exceptionally well. As a rule of thumb, for every one per cent increase in light transmission, you’ll see a one per cent gain in crop production.
However, glass will cost you $4 to $5 per square foot. It doesn’t hold heat and it breaks easily, unless you fork over even more money for tempered glass. While glass can be a smart choice if you’re growing crops that require high levels of light, cut flowers, for instance, most farmers today opt for either polycarbonate or polyethylene.
Polyethylene is flexible plastic: a thicker version of the same material used to make plastic bags. At 12 to 17 cents a square foot, this is by far your cheapest option, notwithstanding its limited lifespan.
“Even if you replace your plastic every three years rather than four years, which of course starts to add up, it’s still going to be quite a few years before that plastic structure costs you more than the glass house,” Eygenraam says.
He suggests doing a “finger test” each fall on the sunny side of your greenhouse. If you can easily poke your finger through the plastic, he says, you’ll want to replace the covering before winter hits. Keep an eye on light levels as well: polyethylene yellows as it gets older.
If you’re looking for good heat retention in your greenhouse, consider a double layer of polyethylene with air in between. You’ll reduce your heat loss 30 per cent, minimize condensation and extend the life span of your plastic at the same time.
Another alternative is polycarbonate, a durable, semi-rigid plastic that typically sells for roughly $2 a square foot. A coating on the outside face prevents harmful UV rays from penetrating, while the inside is treated with an anti-condensate. Polycarbonate offers good light transmission and heat retention and typically lasts 10 years or more. It also stands up well to hail, unlike polyethylene or untempered glass.
Support structure
The support structure you choose will depend both on your choice of covering and on your climate. Pre-galvanized steel has a smooth finish that makes it a good match for polyethylene covers, while more elaborate, welded structures typically use hot-dipped galvanized steel.
For greater strength, choose square or rectangular tubing over round tubing. Rectangular tubing gives you more vertical support — an advantage if you live in an area that gets lots of snow. If the biggest threat comes from wind, however, square tubing gives you crucial horizontal support.
You’ll also need to consider spacing. The closer the ribs, the stronger the structure, but the more shadowing you’ll get. To minimize shading without sacrificing strength, opt for stronger ribs: 1x3 inches, for example, rather than 1x2 inches.
If in doubt, err on the side of caution. “One thing I stress over and over is that you don’t give Mother Nature even one inch of wiggle room,” Eygenraam says. Over the course of months and years, weather will gradually work away at even the smallest weakness in your structure.
Nor should you skimp when it comes to securing your greenhouse to the ground. “I can build the Rock of Gibraltar, and if you don’t anchor it properly, the neighbour just ends up with a strong greenhouse,” he says. “There is simply no such thing as too many anchor posts.”
Ventilation
A proper ventilation system ensures one air change every minute to reduce humidity and increase the carbon dioxide exchange rate. Ventilation also helps to reduce over-heating.
If price is your top priority, roll-up sides create very economical ventilation. However, since heat rises, roof vents are more effective. Eygenraam recommends placing a vent near the peak at either end of the greenhouse. The natural chimney effect this generates will create ventilation even on a calm day. Mechanical exhaust fans offer another alternative. These require electricity to run, but by connecting them to a thermostat, you can ensure they switch on only when the temperature gets too high.
Location
It’s tempting to install your greenhouse on a patch of land that isn’t good for much else. But that would be a mistake, says Eygenraam. Strategically locating your structure sidesteps potential problems and sets you up for success.
First, look for a flat, well-drained location. A small degree of slope from end to end is okay, but your site should be level from side to side. Next, make sure you’ll get good sun exposure and minimal shading throughout your growing season.
Don’t think you’re protecting your greenhouse by locating it in the lee of other structures. In fact, you’ll actually expose it to potentially damaging turbulence.
Eygenraam cites the example of a customer who attached his greenhouse to a couple of 6x16 inch barn beams and figured that was plenty since it was sheltered on three sides by other greenhouses. When the wind picked up, so did the greenhouse.
“The wind drove it sideways 60 feet before the leading edge caught something,” Eygenraam recounts. “The other side caught up with it and it sort of folded over.”
Orienting your greenhouse in line with the prevailing wind will increase ventilation and your snow load will be better balanced. If you want to maximize sun exposure, however, consider a north/south orientation.
If you plan to add more greenhouses in the future, Eygenraam recommends leaving at least 12 feet between them to create enough space for snow shedding and room to manoeuver when you’re putting covers on and off.
Finally, make sure you have good access to water, power and the rest of your operations. That moderately priced greenhouse suddenly gets a lot more expensive if you have to fork over $10,000 for a driveway.
Budget
So how much is it all going to cost? Eygenraam quotes anywhere between $2 and $10 dollars a square foot for an entry-level greenhouse. Beyond that, there are simply too many variables to narrow it down further. And no, he can’t offer any general guidelines on where it’s worth spending money and where you can cut costs — it all comes down to what trade-offs make the most sense given your particular needs and constraints.
“Prioritize, prioritize,” he says. “Look at everything as a cost versus return.”
For more information: www.multisheltersolutions.com
You say to•may•to
Terms like hoophouse, high tunnel and greenhouse get tossed around indiscriminately. Yes, they all serve the same basic function, but it’s worth pointing out a few distinctions.
The terms high tunnel, hoophouse and polytunnel usually refer to unheated structures covered in polyethylene that are used to extend the growing season. They may be portable.
A greenhouse is a permanent structure made of double-layer polyethylene, hard plastic or glass. It usually has some form of ventilation and may be heated for year-round use.
Coldframes are small, semi-permanent season extenders, typically only big enough for a flat or two of seedlings.
- Julie Stauffer