Build a Backyard Greenhouse

Few things brighten a day like the sight and scent of blooming flowers, and nothing makes a meal taste better than fresh vegetables and herbs. If high grocery-store prices make you consider these items luxuries, it may be time to get your hands dirty and grow your own. Even if your climate isn’t conducive to year-round gardening, you can easily extend the growing season by building a greenhouse.

Greenhouses are essentially controlled microclimates that pamper plants even during cold months. With a basic greenhouse, you can sow hardy annuals for spring flowering, protect delicate plants or start seeds early for an outdoor vegetable garden. Many styles and types of greenhouses are available; whether you’re interested in designing your own from scratch or purchasing a ready-to-assemble kit, you’ll find options that suit your budget and skill level.

Glazing materials
When you think of a greenhouse, you might first think of glass. But there are many other glazing options, each with unique properties. Here’s a brief overview of your choices:

  • Glass — Used for centuries in greenhouses, glass has the highest light transmission, and unlike most untreated plastics and fiberglass, it can’t be degraded by sunlight. But besides being the most expensive glazing material, it is heavy and requires substantial, precise framing. It also provides virtually no insulation (unless you can afford double-pane panels) and can be very dangerous if it breaks.

  • Polycarbonate — An extremely durable plastic, polycarbonate is used in most DIY greenhouse kits, but it can also be purchased in sheets for installation in a custom-built framework. It’s almost as transparent as glass, yet it is up to 200 times stronger, weighs much less and will not shatter.

    Polycarbonate is often coated with ultraviolet (UV) blockers that prevent sunlight from degrading and discoloring the material, and it’s available in single-, double- and triple-wall thickness. Single-wall is the least expensive, but it offers less heat retention, light diffusion and strength than double- or triple-wall polycarbonate, both of which deliver better insulation performance thanks to air pockets between the layers.

  • Fiberglass — Even though they’re not transparent, corrugated fiberglass panels allow significant levels of diffuse sunlight to enter a greenhouse. Typically coated to protect against UV damage, fiberglass retains heat better and provides slightly more insulation than glass. It is budget-friendly when it comes to initial costs, but it typically needs to be replaced when the UV coating wears away (usually in about five to seven years).

  • Polyethylene film — According to the National Greenhouse Manufacturers Association (NGMA), polyethylene film is the most popular choice for greenhouse coverings, mostly because of its low cost, light weight and ease of installation. Primarily installed on temporary or hoop-frame greenhouses that are used for seed starting, it comes in sheets that can be applied in single or double layers, depending on the level of insulation you desire, and it works best when installed with airspace between the layers. Poly film typically needs to be replaced every two to four years.

Some greenhouse kits, such as this one from Santa Barbara Greenhouses (above), have aluminum frames for added strength. Others, such as the company’s deluxe model (below), incorporate cedar or redwood framing.

Kit or custom?
From traditional to Victorian to postmodern, and from wood to steel to impact-resistant plastic, greenhouse kits run the gamut in design and materials. The most basic greenhouses are nothing more than an arched framework of tubing (often PVC pipe) covered by sheet plastic — simple, inexpensive (usually no more than a few hundred dollars) and incredibly easy to construct. Of course, strong winds or a hailstorm can destroy them.

To create a more permanent structure, you can use plastic, wood, aluminum and even steel for the framework, along with polycarbonate glazing or glass. But permanence comes at a price: A basic 6 x 10-ft. plastic- or aluminum-frame greenhouse outfitted with two-wall polycarbonate glazing will start at about $1,500. Upgrade to a wood framework or to a more decorative style and you’ll raise the price to a starting point of about $2,500. However, when you consider that a ready-to-assemble greenhouse comes with everything you need for the project — from the frame to the glazing to the hardware — precut and finished, the price seems more reasonable.

If you’re looking for maximum design flexibility, you want a specific look or size or you simply relish the challenge of building a greenhouse yourself, you can design your own. However, you may not save much money compared with buying a kit. I had originally planned to build a custom-designed greenhouse for this article, but after adding up all of the materials, the total expense came to about $3,000. The greenhouse would have been beautiful but it would have cost almost twice as much as the kit that I eventually chose (which yielded attractive results).

Greenhouse-kit manufacturers buy materials in bulk, so they are able to keep their manufacturing costs — and therefore prices — low. (DIYers, on the other hand, often must purchase polycarbonate panels in minimum quantities; you might have to buy more than you need, driving up your costs.) In addition, kit manufacturers have already done the hard work of designing a structure that’s both strong and watertight — not an easy accomplishment when most of the building materials are glass or plastic.

Building a kit greenhouse
If you want to save time in designing a greenhouse or you simply appreciate having everything you need in one convenient package, you may want to purchase a ready-to-assemble greenhouse kit. These kits come with everything necessary except for the foundation, and most can be assembled in about a day.

We purchased a Rion EcoGrow greenhouse kit for our project (shown in the main photo), but no matter which brand you choose, you’ll follow the same basic assembly steps (despite differences in hardware and overall appearance). Just remember to work methodically and follow all of the manufacturer’s instructions.

Start by assembling the framework for the greenhouse base. Check it for square by measuring the diagonals; then place it on the prepared foundation.

To secure the framework, drive 1/4-in.-dia. x 3-in.-long stainless steel screws through the greenhouse base into the pressure-treated foundation.

Each section of the greenhouse either snaps or slides together, securing the polycarbonate panels between the frame pieces.

Pins lock each section of the framework together, and the pins can be removed if any repairs are needed in the future.

Snap in place the glazing channels that will fasten the polycarbonate panels to the roof’s framework. Work on a warm day, as the heat will make it easier to snap the parts together.

Set the individual polycarbonate roof panels into the glazing channels; then snap the retaining strips onto the tops of the channels.

After placing the uprights into the base, work your way around the greenhouse and slide the side polycarbonate panels into position.

Have a friend help you lift the roof assembly onto the walls and slowly lock each of the uprights into its corresponding roof socket.

Your last step is to attach the door and install any accessories, such as a potting bench, a mister, an auxiliary heater, etc.

Location, location, location
Although a greenhouse may be a pretty simple structure, there’s more to successfully building one than randomly picking an empty spot in your yard and erecting a plastic- or glass-encased framework. Before you purchase or design anything, analyze the path of the sun in relation to where you want your greenhouse to stand. According to the University of Arizona’s College of Agriculture and Life Sciences, a greenhouse with a north-to-south ridge will get more uniform light distribution than one that’s aligned east-to-west. However, if you live in a northern climate, an east-to-west orientation will allow for the most total light during the winter, when light intensity is at its lowest.

Another consideration is snow. Check with your local building-code authority to determine the roof snow load (if any) in your region; then verify with the manufacturer that the materials or the kit you plan to use can handle that load.

Also look at nearby trees. How much shade do they create? Coniferous trees such as pines can be useful for reducing summertime temperatures in the greenhouse, but because they provide shade year-round, during winter they may block sunlight when it’s most needed for warmth. Deciduous trees such as oaks, maples and other leafy varieties can also offer heat-regulating shade in the summer, and because they shed their leaves in the fall, they won’t block much-needed wintertime sunlight.

Many unique designs are available in kit form. This version from Gothic Arch Greenhouses is ideal for cold climates, as the arched roof design readily sheds snow.

Foundation is everything
Whether you’re building your own custom-designed greenhouse or you’ve opted for a ready-to-assemble kit, you have to start with a strong, level and square foundation. Without a properly constructed base, your greenhouse will settle over time, and as it shifts, it can stress and warp the glazing, causing it to crack or shatter.

To build a foundation that will last, start by digging a hole slightly larger than the perimeter of the greenhouse to a depth of 12 in. Line the hole with landscape fabric; then backfill with 7 in. of compacted Class V crushed gravel.

Use pressure-treated 6x6 posts for the foundation structure. Cut corner half-lap joints at the ends of each of the 6x6s (see illustration, above). Lay the beams in place on top of the Class V and check to make sure the framework is square by measuring the diagonals — the two measurements should be equal. Screw the corners together using either galvanized or stainless steel 3/8-in.- dia. x 4-1/2-in.-long lag screws.

To allow for good drainage, fill the interior space of the foundation frame with gravel; then use additional Class V around the outside of the framework to fill in any space between the foundation and the surrounding soil.

Creating a happy environment
To get the most out of your greenhouse, consider add-ons that extend the growing season and help to keep plants healthy. For example, you may want to purchase a misting system to prevent plants from getting too hot and to maintain proper humidity. (Small misting kits can be found for as little as $50.) Inexpensive additions such as fan kits, louvered windows and temperature-controlled roof vents not only aid cross-ventilation and help to keep the air temperature from getting too hot but also help to prevent the growth of plant-damaging molds and bacteria.

Temperature-controlled automatic roof vents help to regulate the greenhouse’s daytime temperatures by opening when it gets too hot inside the building.

Louvered windows allow for proper cross-ventilation. Notice the temperature-actuated piston — when the temperature gets too hot, the window automatically opens.

Though greenhouses can get very hot during the day, they can also get quite cold overnight or during winter. To battle temperature fluctuations, consider investing in a heating system. Auxiliary heaters include portable radiant units powered by natural or liquified-petroleum (LP) gas, small electric space heaters, ceramic and infrared heaters and forced-air blowers. Prices for small electric or ceramic heaters start at about $50; prices for larger gas heaters begin at about $300. The size, type and location of your greenhouse will ultimately determine what you should purchase.

As with any project, talk to local experts to learn what works best for your area; then determine your budget and buy accordingly. With proper research, planning and preparation, you can create a greenhouse that is a delight to look at and to work in — and save yourself time and money at the grocery store.