Extending Your Season with a Greenhouse
Updated: May 28
At our latitude it’s not unusual to see snow on the ground six months of the year – or more. Having such a compressed growing season presents many challenges. Using a greenhouse can help by extending the season and offering protection from extreme temperatures and weather-related damage. But greenhouses can also benefit our friends who live outside of the snowbelt.
Greenhouses can guard against birds and mammals, and reduce exposure to pests, weeds, and disease. They can also make it possible to produce a wider range of plants, bringing exotic items like citrus trees, melons, and eggplants further north than would otherwise be possible.
When calculating the pros and cons of a greenhouse, the first thing that often comes to mind is cost. The good news is that greenhouses can be as varied in size and expense as pickup trucks.
Thinking about extending your season with a greenhouse? Here are a few things to consider about the structure, placement, heating, and ventilation.
Types of Greenhouses
Greenhouses can start as simply as a cold frame or a hotbed, constructed very inexpensively with scrap lumber and clear plastic. Window-mounted greenshouses provide a third small-scale option. While any of these can extend your growing season on a very modest budget, let’s focus on larger alternatives that you can actually step into. Here the first consideration is whether the greenhouse will attach to an existing structure or stand on its own.
The Attached Greenhouse
An attached greenhouse shares a wall with an existing structure, often a house. This saves some construction costs and offers accessibility to the water, power, and heat from the house. It also minimizes the walk to the greenhouse on snowy days.
There are two types, the lean-to and the even-span. The even-span design makes larger builds possible. Regardless of which you choose, the shared side of an attached greenhouse cannot exceed the height and width of the attached structure, so there are built-in size limitations. Plus your plants can’t get sunlight from the shared wall, reducing the total amount of available light.
Finally, if a greenhouse shares heating and ventilation with the attached structure, your ability to control these items separately is likely limited, too.
The Free-standing Greenhouse
Because a free-standing greenhouse shares no existing walls, it’s inherently more expensive. It’s also much more flexible. The only limits on size, location, heat, and ventilation are your property lines, budget, and any applicable building codes.
Among the popular designs, most fall into three roof designs; triangular, gutter-connected, and curved.
These traditional-looking greenhouses include the gable, even-span, and A-frame designs. They offer maximum sunlight and lots of headroom (particularly in the center). However, they typically use glass floor to ceiling, which requires more significant (and expensive) framing.
Gutter-connected structures include ridge & furrow and sawtooth designs. Similar in many ways to triangular builds, they essentially connect multiple roofs at the gutter and are more cost effective over large areas. The tradeoffs inherent in these designs include reduced ventilation and sunlight.
Curved roof greenhouses are typically constructed by draping a polyethylene film over a hoop frame (made of plastic or metal). Popular designs include quonset, hoop, gothic, and tunnel. Some offer roll-up sidewalls or end walls that can be raised for ventilation.
Curved roof greenhouses are less expensive to build than more substantial triangular structures, and don’t have the ventilation and sunlight issues of gutter-connected roofs.
Plans for a wide variety of attached and free-standing designs are available online, often at no charge. Check out these free plans from University of Georgia Extension.
Covering and Frame Materials
Covering options include glass, polycarbonate, and plastic films, each with pros and cons in terms of price, durability, transparency and heat/moisture retention.
Glass remains transparent as it ages, retains heat and moisture, but is easy to break, expensive, and requires more extensive infrastructure.
Polycarbonate can vary greatly in quality and price; more expensive double-wall construction can retain heat better than glass, cheap offerings can yellow with age.
Plastic film options offer the transmissivity of glass at a fraction of the price, but will eventually have to be replaced (some sooner than others).
Frame options can include wood, plastic, aluminum, and steel, again with varying qualities and prices. Wood, for example, is easy to work with but becomes susceptible to rot in the moist confines of a greenhouse.
Whatever size greenhouse you choose, chances are you’ll fill it up sooner than you think, so plan accordingly. You may choose a bigger footprint than you think you need, or to choose a design that facilitates expansion. Don’t forget to account for walkways and aisles which can take up a third of your space.
Especially if you’re tall, keep in mind that some designs only have ample headroom at the peak of the roof. Consider the consequences of both stooping and the need for heating and ventilation when considering any design.
Greenhouse Placement and Location
In the northern hemisphere, the preferred placement for an attached greenhouse is on the south or southeastern side of the house. The eastern side would be your next choice; the northern side would be your last.
For free-standing greenhouses, latitude plays a factor in optimal placement. In northern latitudes (above 40°N) using an east/west orientation can result in more sun in the winter months. South of 40°N, a north/south orientation is preferred. Optimum placement may differ depending on the type of greenhouse used however, so keep an eye open for that.
Typically you’d like your greenhouse to be in a spot that provides direct sunlight at least six hours a day, although afternoon shade from deciduous trees that lose leaves in the winter, can have advantages.
Other orientation (and door placement) considerations include drainage (depending on how level the site is), prevailing winds, and access for the items you’ll haul in and out as well as water and power.
Depending on what you’re growing you’ll want to determine whether you’re simply trying to keep the minimum temperature over 40°F, or if your target is closer to 60°F. Also be sure to factor how much colder than that it gets in your neck of the woods.
Free and low-cost heating options include the passive solar benefits of the greenhouse itself, composting, and techniques that can help retain daytime heat. These can range from sealing up cracks and adding various kinds of insulation to thermal mass options like building a cinder block wall or installing 55-gallon drums of water.
There are all kinds of things you can burn to heat up your greenhouse, too. Some options, like wood, are very labor-intensive. Many combustible fuels produce dangerous carbon monoxide. All require safe venting.
Other heating options include everything from space heaters and fans to forced air furnaces and steam systems.
Shade and Ventilation
Of course greenhouses can get too hot – and humid – in any climate. Shading can reduce light and heat in the greenhouse, while proper venting can let excess heat and humidity escape.
In addition to taking advantage of the seasonality of deciduous tree shade, there are two categories of active shading; compound or curtains.
Shade compound, or whitewash, is applied like paint onto the greenhouse covering. You control the amount of shade provided by diluting the application. Pick a dry day to apply compound as a rainfall within 24 hours can remove the coating. Otherwise it should remain in place until you wash it off in the fall. Of course the amount of shading remains fixed in the meantime.
In contrast, shade curtains offer maximum flexibility as they can be opened and closed at will. They can be placed inside or outside the greenhouse. Placing them outdoors increases their effectiveness, but can reduce their lifespan. Not only can they reduce sunlight at high noon, they can also be used to retain energy at night.
On the downside, they are much more expensive than shade compound, and can compete for space with lighting, power, and ventilation.
Even in northern winters a greenhouse can get too warm on a clear, sunny day. Proper ventilation is a must.
Hand operated vents are inexpensive, and can make the difference, but need to be constantly monitored and maintained. Automated options offer better control and require far less vigilance but add significant costs.
Still interested? Check out some of the sites below to take your greenhouse research to the next level. Good luck and stay warm!
Frame Consideration Horitech Greenhouse
Greenhouse Growing: Tips for Basic Greenhouse Cultivation Mother Earth News
Greenhouse Management Online University of Arkansas, Fayetteville
Greenhouse Site Selection NewFarm.org
Greenhouses: Heating, Cooling and Ventilation University of Georgia Extension
Hobby Greenhouses University of Georgia Extension
Keep Cool with Greenhouse Shading GreenhouseGrower.com
Orientation and Structural Considerations HighTunnels.org
What are the Different Types of Greenhouse Structures? GrowingGreenHouse.com
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