Planning for Your Greenhouse

greenhouse inteior with hanging pots

Step #1 – Decide What You Will Grow

The first step in planning for a greenhouse is to ask yourself some questions:

1. Do you want a greenhouse that will be used predominantly for growing plants or do you just want to start seedlings?

2. What kinds of plants do you want to grow in your greenhouse: orchids, cacti, citrus, foliage, vegetables, seedlings, etc.?

Consult a greenhouse gardening book for the environmental requirements of your chosen plants. Do you have a site that will provide these plants with adequate sun or light?

Step #2 – Choosing a Site

Do you have unlimited space or are you space-challenged?

Ideally, one long side of the greenhouse should face south, southeast, or southwest, and one end should catch the morning sun to the east. Many times the perfect site is not available; however, most are workable with the addition of artificial light or increased shading. With a northern exposure, shade-loving plants will flourish.

The south side of the greenhouse should not be shaded by trees or buildings. While deciduous trees with fine foliage may provide dappled summer shade, a bare tree can reduce the sunlight shining on the greenhouse by 50% in the winter. Be especially cautious with a lean-to style greenhouse. The structure to which it is attached will cast too much of a shadow on your lean-to if it attached to the north side of the building.

Consult your local Building Inspector and Zoning bylaws before you make any final decision on the site. You must determine whether there are any laws that will affect your choice. Also inquire about the need for a building permit, and ask if there are any special requirements for greenhouse construction in your area.

You will probably want some utilities in your greenhouse. Can you extend electricity, water (hot and cold), and gas to this site? Plan now for the conveniences you want. All electrical connections in the greenhouse must be protected by a Ground Fault Interrupter Circuit (GFI) because of the dampness.

A final consideration in choosing a site is ease of access to your greenhouse. Can you enter your attached greenhouse without having to go outside? Can you reach your free-standing greenhouse easily in winter?

Step #3 – Choosing a Greenhouse Type

Greenhouses are either attached to a building or free-standing from all other structures.

Lean-to greenhouses are attached to an existing building along one long side. They have the advantage of easy access from the house or other building, and the ready availability of water, electricity, gas, and heat. The lean-to greenhouse is usually simpler to construct than other attached greenhouses.

Attached greenhouses are cheaper to build and heat because they share a wall with an existing structure. In addition it is easy to tap the solar heat generated in an attached greenhouse to help warm the other building.

Free-standing, even-span greenhouses provide the most light for plants. Their configuration allows for flexibility and efficiency in the arrangement of benches and other furnishings. Because of their full exposure to the outside environment, they are more expensive to heat. Some free-standing greenhouses have been modified to collect more light through the south-facing roof and wall, while

providing insulation on the north wall.

Step #4 – Choosing a Size

No matter what size greenhouse you choose the chances are you may one day feel that it is too small. Therefore, buy the largest greenhouse that your budget will allow. While many greenhouses are modular and more units can be added later, it is more efficient and cost-effective to buy the larger structure in the beginning. An added advantage of the larger greenhouse is easier control of heating and cooling. Temperature and humidity fluctuate rapidly in the small greenhouse.

Step #5 – Choosing Your Materials


Most greenhouses have extruded aluminum or steel frames. Some are made of wood or PVC. Aluminum is lightweight and strong. Because it conducts heat and cold, some aluminum framing has a thermal break of a less conductive material between the outside and inside surfaces of the frame.

Western Red Cedar, redwood, and cypress lumber are used to frame some greenhouses. Wooden frames have the advantage of not conducting cold temperatures. They also provide surfaces to which accessories may be easily tacked or fastened with nails and bolts.

Steel and PVC tubing are usually used for hoop-type greenhouses that are covered with greenhouse film. These structures tend to be less expensive than metal- or wood-framed greenhouses.


The original glazing for greenhouses was glass, and many people still consider it the preferred material. Aesthetically, it probably is the best, but practically, it has some drawbacks. Glass allows excellent light transmission and provides a clear view from inside and outside. It is scratch resistant and easily cleaned. It can be shaded with shading compounds or shade cloth. It may be cheaper than rigid plastics and is readily available. It also breaks easier. Double-strength glass should be used for the roof although some greenhouses with glass side panels use polycarbonate for the roof. Tempered glass is also used in greenhouses to reduce breakage.

In an area where there are severe hailstorms or icing conditions, or where vandalism is a problem, plastic glazing may be the answer. Rigid sheets of acrylic and polycarbonate are popular for their light weight and ease of fabrication. Single, double and triple-wall panels are available. Some plastic panels are clear. Others are translucent, providing even, diffused light throughout the greenhouse. Fiberglass is also used extensively, but should be treated so that it will retain good light transmission as long as possible.

Several polyethylene films are available for glazing. Their durability depends upon their ultraviolet resistance and their thickness. Their light transmission varies. Polyethylene film, however, is the cheapest covering available and is very easy to apply. It can also be used inside the greenhouse to provide an inner lining to reduce heat loss.


The foundation must be completely level. Some manufacturers provide a base for their greenhouses, but usually you will have to provide your own. Depending upon the manufacturer’s recommendations and the climate, the foundation may be anything from landscape timbers or cement blocks to fully constructed masonry walls. Some greenhouses are glass to the floor and others have a kneewall around the bottom.


Greenhouse floors must drain well. Many greenhouse floors are first excavated to the desired depth and then filled with rock or gravel to provide both drainage and a heat sink. Sometimes the walls of the excavated area are lined with insulating material to prevent heat loss. Bricks or paving stones can be placed on top of the fill if desired. Poured concrete or other solid floors should slope to a drain. Dirt floors should have a paved walkway.


Adequate ventilation and good air circulation are extremely important in controlling temperature, and to prevent mildew and condensation. Plants need fresh air and carbon dioxide. Greenhouses should have roof and floor-level vents to promote natural air convection. Most greenhouse kits come with various types of vents.

Exhaust fans will help to move air through the greenhouse and can be moved as needed.

Step #6 – Choosing Your Heating and Cooling Elements


Most greenhouses will need some type of heater even if it is only used during unusually cold weather. Your type of greenhouse, its glazing and exposure, your desired interior temperature, your plants, and your climate will dictate your heater requirements. Consult the greenhouse manufacturer, or books on greenhouse maintenance to determine the size heater you will need.

Electric, oil, or gas space heaters are an efficient and frequent choice. You may need to install a separate 220 volt circuit for some electric heaters, but the cost is offset by the savings on lower electric rates.

Provide proper venting to the outside for combustion heaters and check state and local laws for any restrictions on their use.

Solar greenhouses rely on the sun for all or part of their heat. Consult an expert on solar heating.


While fans and vents provide adequate cooling for many months of the year, during hot weather further measures are usually needed. Shading is the most common way to cool the greenhouse, as well as to protect the plants from burning. Greenhouse roofs may be painted with whitewash or special shading compounds that wear off by winter. These materials may be harder to remove from plastic glazing, so it’s best to read the label. The roof may be covered with shade cloth, available in several densities, or roller shades may be utilized.

Watering and misting systems help to cool the greenhouse interior. A swamp cooler can also be installed.

Step #7 – Choosing Accessories

Plants can be grown directly in beds in the ground, in containers, on benches, and on shelves. Frequently all types of growing areas are utilized in the greenhouse at once, including hanging pots. This provides several different interior climates allowing a variety of plants to be grown.

Maximum/minimum thermometers and a humidity gauge will help you to monitor the interior environments of your greenhouse. A temperature alarm is good insurance against plant loss due to overheating, a power failure, cold, or a malfunctioning heater. These instruments are readily available.

If you have a water supply line, there are various manual and automatic watering and misting systems which may be added. A sink such as a common plastic washtub on legs is also a handy piece of equipment. It can double as a potting bench with the addition of a cover.

Small fans can circulate air in hard to reach areas and provide general air movement when vents are closed.

A self-coiling hose with an attached watering wand makes irrigating the plants much easier.

Benches are available, or you can construct your own.

Time and care

Greenhouses require time and care to be successful, but they need not limit your other activities. How much time you devote to your greenhouse will depend upon your goal, your plants, and your lifestyle. Automatic venting, heating, cooling, and watering devices are available that will allow your greenhouse to function well without your constant attention. The hours that you do spend in your greenhouse will bring you all the benefits and pleasures of year-round gardening.


When you have selected your greenhouse, you fully understand the product warranty and who is responsible if there is a problem after purchase or construction.

Enjoy your greenhouse! Experiment! Learn to grow extraordinary plants!

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