Greenhouse Buying Guide

Sizing your greenhouse

Before buying a greenhouse think about the types of plants you’ll grow, how many you’d like to grow and, of course, how much space you have in your yard.

Generally, you should choose the largest greenhouse that will fit on your land and that you can afford. Before buying a greenhouse, make sure to measure the land you have for it. When measuring your site, remember to leave some space around the perimeter of the greenhouse base. You want enough room around the greenhouse to maneuver materials and plants and for cleaning.

Width is the most important greenhouse measurement, because it determines the placement and sunlight-collecting ability of the structure.  Most greenhouses are wider than they are long, and widths are scaled in two-foot increments.

For most people, a greenhouse offering 120 square feet of interior space is ideal. Though you may not think you need much space, the greatest complaint of greenhouse owners is that they have outgrown their original purchase. It is easier and cheaper to buy a large greenhouse than it is to try to expand an old one. Generally, the smaller the greenhouse, the fewer features it will have and the cheaper it will be.

You’ll also need to decide whether you want a stand-alone greenhouse or a lean-to. Lean-to greenhouses attach to your home or to a structure like a garage or brick wall. A stand-alone greenhouse will be separate from your home and is larger than a lean-to model.

Temperature control

The number one reason for buying a greenhouse is temperature control; without it, you might as well plant outside. A minimum of 45 degrees Fahrenheit is needed to grow plants in the winter, though some specialty plants may need more temperature. Think about plant needs when deciding on the heating features for your greenhouse.

There are a number of greenhouse heating choices, including electricity, paraffin and gas. Electric fan heaters use more energy than gas heaters, but they circulate air and can be used for ventilation in the summer. Electricity is convenient, but if a storm knocks out your area’s electricity, your plants will suffer.

Paraffin heaters are a good source of heat and are relatively inexpensive. However, they can produce a lot of water vapor and condensation, which can be an issue. These greenhouse heaters can also smoke unless the wick is trim and well-adjusted, and you’ll need to refill a paraffin heater outdoors to avoid contaminating the greenhouse with vapors.

Gas heaters can run from a gas main, if you have one, or replaceable propane tanks. Gas is more cost-effective than electricity, but you’ll need to keep an eye on pilot lights and make sure that propane tanks are fueled.

Look for ventilation options when choosing your greenhouse. A greenhouse should have roof windows to help with air circulation. Some companies offer expanding tubes that open and shut the roof windows depending on the temperature. Though an added expense, these tubes ensure a consistent environment for your plants. If you want to handle ventilation yourself, poles with hooks on the end can be used to manually operate ventilators.

There are also thermostatically controlled extractor fans which are easy to install and cheap to run. These fans are fitted to the gable end of a greenhouse and provide continuous movement of air and reduce condensation. For greenhouses with polyethylene tunnels, opening the end doors is a good mode of ventilation.

Siding and frames

The siding of your greenhouse is important because it affects the cost, heating efficiency and protection of your plants. You can choose from glass siding or one of the plastics: fiberglass, polycarbonate or polyethylene.

Glass siding is more expensive than plastics, but it lasts much longer and makes it easier to control your greenhouse’s environment. Double- or triple-wall polycarbonate greenhouses are more expensive than glass, but this material is nearly unbreakable and its insulation abilities are better.

When choosing your siding, think about the amount of light entering your greenhouse. You want as much light as possible to enter your greenhouse, but you do not want direct sunlight on your seedlings, as it can harm them, much like a magnifying glass directed at an insect.

Greenhouse frames can make all the difference when it comes to the durability of your investment. Wood greenhouse frames require periodic painting or coating with wood preservatives. While wood is aesthetically pleasing, aluminum, galvanized steel and PVC are more durable. Aluminum is also maintenance-free. Heavier glass siding requires heavier frames.

Foundation

You can build your greenhouse directly into the soil, but it will be more secure if it is built into a strong, hard surface. Make sure your greenhouse’s foundation is level or temperature and stability will be negatively affected.

A greenhouse foundation can be made from concrete or paving stones. If you pour a concrete slab, you’ll need a drain in the floor. Texture or rib the concrete as it dries to protect you from slipping when the floor is wet. Pavers don’t need additional drainage because of the spaces between them. However, weeds can grow between these spaces.

Watering systems and connections

Some greenhouses come with automatic watering and misting systems. These systems will ensure that you are not tied to your greenhouse and plants, and that you can leave for the weekend. Self-watering systems also take some of the guesswork out of maintaining proper moisture for plants, but it’s important to remember that plants close to windows will dry out faster than those residing in the middle of the greenhouse.

Even if you just use a hose, you’ll need a source of water as well as a source of electricity in your greenhouse. Be sure to include the cost of an electrician and, perhaps, a plumber as you budget for your greenhouse project.

Need to know more?

• Greenhouse Gardener’s Companion
  http://www.greenhousegarden.com/
• Greenhouse Foundations
  http://www.gardeners.com/Freestanding-Greenhouse-Foundations/default/5280.page
• How to insulate your greenhouse
  http://gardening.about.com/od/toolschool/g/R_Value.htm