How to Build a Greenhouse

Among the things to consider when building a greenhouse include:

• Location of Greenhouse
• Types of Greenhouses
• Permits
• Electricity and Water
• Preparation and Floor Surface
• Building
• What to Plant

Probably every gardener has dreamed at some point about having a greenhouse. Some of us dream of being able to grow vegetables year-round. Others want to get an early start in the spring and be able to extend the season into the fall. Some gardeners specialize in something like orchids and would love a greenhouse devoted to their favorite plant. How you intend to use your greenhouse does make a difference in the type of greenhouse you will need to build.

Location of Your Greenhouse

The other most important decision is the location of your greenhouse. Greenhouses need to be located in a sunny location. This means a minimum of six hours a day of full sun–more is better. The best orientation is facing south or southeast to capture the morning sun and the heat after the colder night. If you have a location you would like to build your greenhouse, consider the sun year-round. It may have full sun in the winter but once the trees leaf out, the sun might be blocked for a major part of the day. Some greenhouses are attached to the house. Is that possible or desirable for your home? Are you able to manage not just the sun but the temperature and humidity year-round?

Lastly is the cost factor. Greenhouses traditionally were made of glass. They were miniature houses that were decorative as well as functional. They are still available to purchase, but are also quite expensive. Glass is more fragile and more expensive, so today, most glass is replaced by polycarbonate, acrylic, fiberglass or polyethylene sheeting. Today, with options other than glass, many greenhouse structures are designed with function and cost being the primary considerations. Keep in mind that the fiberglass can discolor over time, which is undesirable for two reasons. The discoloration will diminish the ability of the sun to reach the plants and the aesthetic look of your greenhouse is lessened. The polyethylene sheeting is the cheapest, but is easily punctured or cut and has a shorter life. Depending on the type you buy, it may have to be replaced as early as every two to four years. The sheeting is relatively inexpensive, so this is still the most cost-effective option.

Types of Greenhouses

There are four different options when building a greenhouse. Each has advantages and disadvantages.

• Hire a company that will have designs available for you to choose from and they will build the greenhouse for you at the location you have chosen. This is the best option if you want the greenhouse attached to your home or if you want a more traditional freestanding greenhouse.
• Purchase a kit. There are companies that will sell you a greenhouse structure that includes everything you need to build your greenhouse. This will be delivered to your home and then you can build it yourself according to the instructions, or you can hire a builder. This is usually less expensive than the first option.
• DIY with repurposed materials. There are a number of gardeners who have built their greenhouse from repurposed glass windows and doors. This requires some pretty significant design and building skills. The size of the greenhouse will be determined in part by what materials you have to use. The results can be very charming one-of-a-kind structures that are very functional. The advantage is of course the cost. If you have access to windows and doors at little or no cost, your greenhouse will be very inexpensive.

• DIY High Tunnel Greenhouse. This has become a very popular style of greenhouse. Made with PVC pipe and plastic sheeting instead of glass, the builder with basic skills can successfully build this type of greenhouse. These are especially popular for extending the growing season, and in the growing zones of more Southern areas, it can be possible to grow year-round in this type structure. It is also very inexpensive to build.

Getting a Building Permit for Your Greenhouse

Once you have decided on the type of greenhouse you want to build and the location, the next step is to find out if you are even allowed to build a greenhouse. There may be an ordinance that forbids building freestanding structures (outbuildings) in your yard. If you live in an area with a Homeowners Association (HOA), you will need their approval before you build. Once you have the approval, you may need a building permit.

Prepare the Site of Your Greenhouse

Once you have decided on where your greenhouse will be located, the size and type of greenhouse you are building and you have any necessary permissions and permits required, you are ready to prepare the site. Remove any sod or plant material from the site. You want the floor of your greenhouse to be flat or a very slight pitch to allow drainage. Remember: You will be watering in the greenhouse, and if there is a lot of rainfall or snow melt, you don’t want to be standing in mud. Options for the floor or pathways in your greenhouse include gravel, mulch, pavers or even a solid cement floor. Don’t forget to cover the area with a fabric weed barrier before you add the mulch, gravel or pavers. You won’t need the barrier if you are going with a cement floor.

If you are going to run water or electricity to your greenhouse, this should be done by a licensed professional. Electricity is needed for running a fan, lights and supplemental heat. Depending on how you are going to use your greenhouse, electricity may be a convenience or an essential. The same is true of access to water.

Note: If your greenhouse is the High Tunnel type and will be covering raised beds, consider constructing the boxes for your raised beds and filling them before you build the High Tunnel. If you fill them first, you can use some heavy equipment to move the soil into the raised beds. If you have to fill them after the greenhouse is built, all that soil will have to be moved by wheelbarrow. Talk about a very heavy and labor-intensive proposition! Renting a front-end loader for a day would seem well worth the expense.

Building the Greenhouse

If you are building your greenhouse from a kit, you should be successful as long as you follow the directions carefully. Most companies that sell kits also have a hotline you can call if you run into questions or trouble. Don’t be afraid to use it!

If you are free styling your greenhouse with repurposed materials, don’t forget to provide a way to release the heat on really sunny or hot days. Sometimes, we are so busy concentrating on how to keep our plants alive when the cold comes that we forget they can also die from too much heat. You will need an exhaust fan, ceiling vents or operable windows. There are systems that you can install that automatically open the ceiling vent when the temperature reaches a certain level, and it will close them again when the temperature in the greenhouse drops. Try to find windows and doors that have tempered safety glass in them. Tempered glass shatters into tiny pieces while regular glass breaks into shards that could cause serious injury. Lay your windows on the ground to figure out the best layout with the windows you have available. It is easier if most of the windows are the same size. Corrugated roofing is a good choice as the rain and snow will slide off if you angle the roof. 

If you are building a High Tunnel greenhouse, there are a couple of additional factors to consider. If you live in an area that has a lot of high wind, you may need to provide extra stabilization to the structure to counteract the force of the wind. This can be as simple as a couple of ropes running from the door frame to stakes in the ground. The other concern is damage from the weight of snow or heavy rains. If you live where either of these occur, you will need to take extra steps.

What happens is that the weight of the snow or water causes the roof to dip, holding a puddle of water or snow. Once the dip in the roof starts, it keeps growing until you have a total collapse of your greenhouse roof. Gardeners who live in the North where there are more frequent snowstorms have added a support beam down the center of the roof and attached their hoops to the beam. This eliminated the danger of collapse due to the weight of the snow or rain.

The long PVC pipes are attached to rebar or pipes that have been pounded into the ground. The PVC pipe is then attached on one side and then bent into a curve and attached to a corresponding pipe on the opposite side. This is easily done with two people. For extra stability, if you are not going to supplement the structure with a support beam, you can add a PVC pipe down the middle. This won’t be enough to support a heavy snow load, but it will stabilize the hoops. The end caps will have the doors and/or windows to allow ventilation as well as access.

Once the frame is complete, the structure is covered with the polyethylene sheeting. The sheeting can be designed to be raised up during the heat of the summer, making the High Tunnel usable all year round.

What to Grow in Your Greenhouse

Of course, you can grow almost anything in your greenhouse depending on your growing zone. If you live in more Northern areas, start with planting organic vegetable seeds of cool weather crops in the fall or spring. Vegetables like kale and broccoli do very well in cool weather. Spinach also likes the cold. Try growing root crops like carrots and parsnips. Leave them in the ground and you may be able to pull them as needed throughout the winter months. Don’t forget to make room for herbs. Plant your organic herb seeds in the greenhouse and harvest throughout the winter.

Enjoy Your Greenhouse

Having a backyard greenhouse may seem like a luxury, but it will be so worthwhile by extending the planting season and providing so much pleasure and satisfaction for the gardener lucky enough to have one. Use it to start seeds, preserve your favorite potted plants over the winter or to try new plants you aren’t quite sure how to take care of. The greenhouse can protect your new plants—especially if you don’t have a lot of room in your home to do so—while you figure out the best way to tend to that new-to-you plant.