People who love gardens also love greenhouses. The best backyard greenhouses feed the need to dirty our fingers while the rest of the garden is dormant. Even a small greenhouse can provide effective season extension by jump-starting seedlings in spring. When managed properly, a greenhouse is a backyard oasis that yields fresh food year-round.
If a lack of construction skills is keeping you from building a greenhouse, consider a kit. The best greenhouse kit is one that fits your needs and your pocketbook. Kits are easier than building from scratch and don’t require as much building experience (although it certainly helps to have a DIY attitude when you set out to build your own greenhouse, even from a kit).
Shopping for Your Best Greenhouse Kit
Local Regulations. Start by researching required permits, zoning, setbacks, underground utilities and other requirements for your area. Some localities demand a certain type of foundation. A greenhouse attached to your home will likely face more stringent requirements than a free-standing greenhouse. In some places, greenhouses are regulated under a “storage shed” designation. Consider size carefully, as you may be able to avoid many regulations by simply settling for a small greenhouse.
Site. Your greenhouse site must have adequate light — six hours of uninterrupted sun on a clear day. You may have to trim or remove a tree to create more light for your chosen location. Also, consider access to water. Is there a nearby hose bib to provide water, even in winter? Some gardeners add gutters and an interior rain barrel to their backyard greenhouses for a winter water source. Electricity can power heating, lighting and ventilation, so keep an accessible power source in mind, too.
Vendors. Check companies carefully — even a small greenhouse is a big investment, and you should feel comfortable with the supplier. Don’t be afraid to ask questions, such as:
• How long has the company been in business?
• How many kits has it sold?
• Does it manufacture the kits or simply resell them?
• How extensive is the warranty?
• What technical help can the company provide?
• How is the greenhouse shipped and packaged?
• What is the cost of shipping?
You might add other questions to this list. If you have minimal building experience, read a copy of the kit’s manual beforehand to make sure it’s understandable to you. If you’ll be building the kit on weekends, ask whether someone from the company will be available to answer questions on Saturdays and Sundays. You may want to see demonstration photos or videos of the kit’s construction before committing to buy. Tech support may be limited if greenhouse kits are not the company’s specialty but just one of many products it sells.
Most home greenhouse kits have frames made of either wood or aluminum. Wood frames tend to be more aesthetically pleasing but are a bit more difficult to construct. Wood can twist and warp, and wooden greenhouses usually include more parts. Aluminum greenhouses can be fragile or sturdy, depending on the model and the quality. Aluminum lasts virtually forever and is basically maintenance-free. Although generally easier to construct, aluminum frames conduct heat and cold and are a little less energy-efficient than wood.
Using a hoop house frame is another option to explore. A hoop house is a type of greenhouse made from a series of pipe hoops covered with heavy plastic that’s stretched tight and fastened to baseboards. Hoop houses are simple structures to build, and they’re an inexpensive way to control the climate over large areas, but they aren’t as durable as most kit greenhouses.
Glazing Types for Your Greenhouse Kit
“Glazing” is the term for a greenhouse’s exterior covering, such as glass or plastic. There are many variables to consider when selecting the right glazing for your backyard greenhouse kit.
Polycarbonate glazing is the most common type and is sold as either a single corrugated sheet or a multilayered honeycomb panel. Among the multilayered options, twin-wall is frequently used, but triple-wall and five-layered honeycomb panels are also available. Multilayered honeycomb polycarbonate is available in thicknesses from 4 to 16 millimeters. The more layers, the more expensive the glazing — but multiple layers conserve energy better. Multilayer glazings also reduce daytime heat from the summer sun, buffering the interior temperature so you’ll have a greenhouse that’s cooler in summer and warmer in winter. This glazing may be problematic for gardeners in climates with cloudy winters, because multiple layers filter out more light.
Polycarbonate glazing is treated with an exterior coating to filter out ultraviolet light. Because the coating is only applied to one side of the panel, make sure the treated side is facing out during assembly.
Most polycarbonate glazing comes with a 10-year warranty. It’s a tough material that can resist rocks and hailstones — until it gets old. The sun will eventually win and the polycarbonate will have to be replaced.
Polycarbonate is lightweight, meaning the greenhouse frame doesn’t have to be as heavy-duty as with other glazings. Unlike many plastics, polycarbonate is fire-resistant. It is also easy to handle and cut. It does expand and contract with heat and cold, though, and small greenhouses made with this type of glazing need a special mounting bar that helps the structure adjust with temperature fluctuations.
Polyethylene glazing is available as a sheet film, woven sheet plastic or honeycombed material. Woven polyethylene is stronger than film. Polyethylene sheets, whether woven or film, are available with varying warranties for up to five years — the more years, the higher the expense. Polyethylene is still cheaper than polycarbonate, but a single sheet (either film or woven) doesn’t have the insulating value of a multilayer product.
Honeycombed polyethylene, sold as “Solexx,” is a double-layered product that looks similar to polycarbonate but is cheaper and doesn’t transmit as much light. It has a limited 10-year warranty. It’s noticeably softer and less rigid than polycarbonate, and it’s harder to weatherize because the softer edges are more difficult to seal.
Glass has the longest life of all glazings, assuming it doesn’t meet a rock or large hailstone. Difficult to cut and heavier to handle than plastic glazings, glass requires the support of a beefy frame structure because of its weight. Glass is available as a single sheet, as well as double and triple layers with better insulating characteristics. Tempered or laminated glass provides increased strength — essential for overhead applications. The positive aspects of glass include that it’s not combustible, doesn’t scratch easily, and has a low expansion/contraction rate in extreme temperatures.
Window glass is commonly sold as “low-e” (low emissivity), which means it has a surface treatment for energy savings. Plants grow more slowly under low-e glass, so I prefer untreated glass for greenhouse kits. Clear glass creates sharp shadows and doesn’t diffuse much light, which is less optimal for growing plants. Plastics diffuse light better, but the clarity of glass is great if you want to admire the view from inside your backyard greenhouse.
Fiberglass is great for diffusing light, which increases photosynthesis in a greenhouse. Commonly found in many brands and grades, it’s available both corrugated and flat, but only in a single-layer thickness. For that reason, fiberglass is often used in combination with a low-cost, insulating inner layer of polyethylene. Some brands turn yellow more quickly than others do, so that the life span of fiberglass products can vary from three to 20 years. Be wary of fiberglass sold in lumberyards — trust only a written warranty. Fiberglass is relatively strong but can be bent around large-diameter curves, such as hoop house frames. It’s easy to cut and work with, but do wear gloves, because the fibers can irritate your skin. It is highly combustible and has a low expansion/contraction rate.
Greenhouse Kits That Can Weather Storms
Consider the wind when building your own greenhouse from a kit. If you live in a windy area, this not only affects your choice of glazing, but also means you’ll need to take extra care in anchoring your greenhouse to a foundation.
Film glazings, such as polyethylenes, are susceptible to wind damage. Woven polyethylene is sturdier than film, but the best material to use in windy regions is a sheet-like rigid plastic, such as multilayered polycarbonate. Fiberglass is also ideal.
Hoop houses are the greenhouse type most susceptible to wind damage. To combat this, hoop house owners sometimes use a double-inflated polyethylene film. The polyethylene layers are inflated by a small squirrel-cage fan. The inflation produces wind resistance by keeping the glazing rigid. Unfortunately, the power can go out when winds are high. Having a backup power source for the little fan is a good idea, otherwise your hoop house could suffer massive damage in a windstorm.
Be sure to check a greenhouse kit’s snow-load rating if you live in a snowy climate. You may risk greenhouse collapse unless you operate a heater, which will melt snow buildup on the roof.
Fresh Air and Sound Footing
Doors and Vents. Long-lasting, solid doors are important. Hinged doors are of higher quality than sliding ones. Some doors have removable or sliding panels that function as vents.
Most new owners of home greenhouses don’t give enough thought to the importance of ventilation. Overheating can be a major problem, so be sure your greenhouse comes with both low and high vents. The best greenhouse kits include automatic vent openers that are powered solely by the interior temperature. In areas with warmer summers, you should also consider the addition of an electric exhaust fan controlled by a cooling thermostat. It should be placed on the side opposite the door.
Foundations. Most greenhouse kits do not include a foundation. Usually you don’t need much of one, because most manufacturers recommend using rot-resistant, treated timbers in lieu of a concrete or concrete-block foundation. I prefer not to set greenhouse kits on concrete slabs. I like an exposed dirt floor because this setup gives me the option of planting directly into the ground, and it allows excess water to drain into the soil when I’m watering flats of seedlings. If you prefer something more substantial, check your local zoning laws for specific requirements.
If you opt for a simple foundation on timbers, it may take a day or two to build. Producing a level and square foundation requires a lot of fine-tuning, but it’s essential for proper assembly of your greenhouse kit. If your foundation is skewed, nothing will fit right and you’ll be guaranteed headaches.
Gardeners who live in places with severely cold winters should consider adding 2-inch-thick foam board placed vertically against the exterior of their foundations or timbers, 1 to 2 feet down into the ground. The foam board set into the soil prevents the conduction of cold into the interior soil through the ground by tapping into temperatures deeper in the soil (45 to 50 degrees Fahrenheit in winter). Even when it’s minus 20 degrees outdoors, the foam will insulate the soil inside your backyard greenhouse.
Ready to Commit
You’ve made a multitude of decisions about your greenhouse kit — site, frame, glazing and more — but you’re not quite finished. Here are a few more tips to consider before you buy and build your own greenhouse.
Make an Offer. Many greenhouse kit retailers have regular sales and may be willing to negotiate the final price, especially if you’re knowledgeable about the competition. Don’t just compare kit prices — also factor shipping costs into your negotiation.
Warranties. The longer the warranty, the better the kit. Check the fine print, because the frame and glazing may have separate warranties.
Custom-Sized Options. Manufacturers may allow customers to order custom-sized greenhouse kits. Most gardeners eventually wish for more space, so purchase the largest greenhouse you can afford for your yard and budget, staying within local regulations.
Delivery. If you live in an area with severe winters, be sure to have the kit delivered during a warm season. Having the company store it for you is better than stashing it outside next to your garage. Wood-frame kits may warp if stored outdoors too long.
Before You Build. Check the kit thoroughly for shipping damage before you begin construction. Set aside some time to study the directions. Many problems people encounter with greenhouse kits are a result of not following the written directions.
Greenhouse Kit Suppliers
Buffalo Junction, Va.; 800-487-8502; kits, accessories and supplies
Surrey, British Columbia; 888-391-4433; glass and polycarbonate kits
Mount Vernon, Wash.; 800-322-4707; large selection of kits and supplies
Dyersville, Iowa; 800-327-6835; kits, irrigation supplies, heaters and lighting
Austin, Texas; 877-760-8500; wide range of designs and sizes from German and Belgian manufacturers
Pagosa Springs, Colo.; 800-753-9333; solar-heated dome greenhouses
Tacoma, Wash.; 800-370-3459; one-piece fiberglass greenhouses
Salem, Ore.; 877-476-5399; home greenhouses and cold frames with polyethylene glazing
Glocester, R.I.; 401-965-0753; geodesic dome greenhouses
Building From Scratch
Check out these suppliers and resources if you’re building from the ground up.
Build It Solar
free online design guides and plans for small greenhouses you can build yourself
Building a Passive Solar Greenhouse
drawing and materials list for a DIY solar-heated greenhouse
Manchester, N.H.; 603-668-8186; glazing, heaters and thermal storage tubes
glazing systems and extensive how-to information
Shane Smith is the author of Greenhouse Gardener’s Companion, which covers all aspects of growing food, flowers and herbs in home greenhouses. Smith is the founder and director of the Cheyenne Botanic Gardens in Cheyenne, Wyo. Browse his website Greenhouse Garden.