Everything you need to know about a greenhouse

Everything you need to know about a greenhouse

If you’ve always yearned for your own home greenhouse, or if you’re considering purchasing one in the future, there are a few things you need to know. Pick the right greenhouse, with the right features for your particular needs, and you’ll find your gardening takes on a whole new rhythm, with the garden moving from outside to inside in response to the season.

What is a greenhouse?

A greenhouse is basically an enclosed environment that has been designed with plants in mind. In the marketplace, greenhouses run the gamut from lower-end, lower-cost structures to elegant but expensive permanent buildings with architectural detailing.

In the case of greenhouses, both from a standpoint of useful lifespan and looks, you get what you pay for. Extremely inexpensive, limited-use, entry-level kits start under a couple of hundred dollars. Small, entry-level permanent greenhouses that you create from a kit can be had for under £1,000. Top-of-the-line, year-round greenhouses with electric hookups, water, automatic venting, and so on can run from £5,000 to £10,000 or even more, depending on size.

Greenhouses are a uniquely accessible product for home gardeners. A large number of hobby greenhouse manufacturers provide “kits” that allow you to construct your own greenhouse, using the manufacturer’s modular components. Many kits can be upgraded, as well, if you choose to improve your basic greenhouse at a later date.

How to get growing

One of the keys in selecting a greenhouse that is right for your style of gardening is to identify what you want to accomplish. If you simply want to extend your season a bit in either spring or fall, you may be most content with a “temporary” greenhouse that employs coverings made of “greenhouse film” or “poly film,” which are flexible sheetings much like a shower curtain. These sheets are difficult to rip or tear, and provide some protection from cold and wind. Sometimes this type of greenhouse is constructed with two layers of plastic with an airspace in between that is kept inflated with a fan to provide additional insulation. However, this type of greenhouse will generally not provide year-round protection for plants in areas where winters are cold.

When purchasing this type of greenhouse, pay attention to the warranty. The manufacturer’s guarantee is your key to the product’s useful life. Look for coverings that are UV treated, meaning that they have been specially made to resist the damaging effect of the sun. Frames for these structures can consist of anything from PVC pipe to aluminium to wood and many are semi-portable. Because of this portability, however, these structures may not be able to withstand strong winds or severe weather. Take that into account when deciding whether this will work for you.

For hobbyists who want to care for specialty plants not hardy in their areas, or who want a spot in which to start plants for next season, a higher-end, more flexible structure is needed.

However, these structures do share a few key features in common. Evaluating these features and their importance to you will help you find a greenhouse that meets both your needs and your pocketbook.

The frame

First is the frame construction. Some greenhouses offer a “skeletal” frame, where the glazing materials (the coverings that let in light) extend all the way to the base of the structure. Others consist of a frame attached to a base wall that runs around the structure with glazing above it. The choice is up to you, but remember that any solid material, such as the base wall, will restrict the amount of light that enters. However, the base wall may improve the building’s stability or insulating ability.

Redwood and aluminium are two common framing materials found in greenhouse kits, but frames can also be found which are made of treated wood, galvanised steel, or reinforced PVC. Some manufacturers do not provide actual framing materials, but instead specify a list of materials that you purchase separately from your local lumber store. You then use your frame with the manufacturer’s glazing materials. This can reduce costs, but you’ll need to be fairly handy to construct the frame itself.

Of these framing options, redwood is elegant, naturally rot resistant, and long lasting. However, manufacturers often encourage homeowners to stain redwood so it is higher maintenance. It is also among the more expensive framing options. Aluminium is strong and light, but it does not insulate as well as wood and it can also be expensive. Treated wood needs to be stained to improve its appearance and preserve its life, but it is less expensive. Reinforced PVC can be a reliable option, providing the structure is installed correctly, but some people don’t care for its appearance. The choice is yours.

The glazing

While frames may affect the aesthetics of a greenhouse, by far the most important feature is the glazing. This is the covering that lets light into the greenhouse, and yet retains heat to protect the plants inside. All glazings admit varying amounts of available light. Glass transmits the most, and it looks elegant. However, clear glass is among the more expensive glazing materials, and in areas of the country where snowfall is heavy, or weather is uncertain or quite cold, glass can be easily damaged. Single-pane glass also provides little insulation so the greenhouse will require additional insulation. In hot or sunny weather, plant scorching can be an issue unless the glass is carefully shaded to minimise light transmission.

One note here: Some do-it-yourselfers who have built their own greenhouses using insulated windows designed for houses have discovered to their dismay that warm, wet greenhouse conditions caused the windows to deteriorate quickly. Good greenhouses are specially built to accommodate extra humidity and heat and the glazing materials available for them reflect that.

Some greenhouse manufacturers offer clear acrylic as an alternative glazing material to glass. This has the benefit of being stronger than glass, while still offering a clear view through the structure.

Flat or corrugated fibreglass is another glazing choice. It is much stronger than glass, but it also is more opaque or cloudy, so it transmits less light. This can be useful in areas with the possibility of hail or where summer sun is relentless. However, fibreglass also usually requires additional insulation to be used in cold areas. Look for fibreglass that carries a good guarantee against sun damage. If you notice structures from years ago that employed fibreglass, you may notice some serious discolouration or hazing. UV treatments available today can minimise this problem.

Yet another choice for glazing is a material commonly known as polycarbonate. This plastic-like material typically consists of two thin sheets of nearly clear wall with honeycombed channels extending throughout the sheets. This construction creates trapped air space inside the chamber which provides excellent insulation. Polycarbonate transmits light well, is usually UV treated to insure long life, stands up to much harsher conditions than glass, is lighter than glass, is impact resistant, and is a good insulating material. However, it is also among the more expensive greenhouse glazing options.

Other manufacturers offer similar double-walled glazing products, made of different base products, that mimic many of the good qualities of polycarbonate, only with less expense.

Heating and cooling

Because greenhouses trap heat efficiently, in summer you’ll find this is too much of a good thing. All greenhouses need some type of venting to exchange the hotter air inside with the cooler air outside. On the least expensive type of greenhouse, these vents may be as low-tech as screened openings on either side of the shelter.

On higher-end models, a more efficient system employs roof vents. Since hot air tends to collect near the roof of any structure, these vents allow the heat to drift out of the top of the building, while vents nearer the floor draw cooler air back in. This passive air exchange is very efficient.

One note: Some vents are manually operated, meaning you’ll need to open and close them as needed. That may mean twice a day, morning and night. Forget once and a suddenly sunny day will fry your plants, or an unexpectedly cool night will freeze them. A much more convenient system employs a self-powered roof vent opener which works on a spring system. As a waxy material within the opener heats up, it expands, gradually opening the vent as the temperature rises. When the material cools, it contracts, thereby closing the vent. Thermostats are also available which will automatically open your vents or turn on a system of exhaust fans when the temperature hits a predetermined level. These vents flush hot air out while drawing cooler air in at floor level.

Although vents will help cool a greenhouse, in summer preventing heat is critical. Shading a greenhouse is one of the best ways to minimize heat build-up. For some greenhouses, you can apply a special removable “paint” to your glazing that is somewhat opaque when dry. This restricts the amount of light that enters. There are also shade cloths, aluminium roll-up walls, fibreglass panels, and so on that will all provide some shade and therefore cooling.

These passive measures will not be enough in greenhouses in very hot areas. Some greenhouses may need evaporative coolers, sometimes called “swamp coolers,” in order to be used year-round. In these systems, fans are used to evaporate water, which draws heat from the air thereby cooling the structure.

While keeping a greenhouse cool is important in summer, keeping it warm is key in winter. Correct siting is important to insure that your greenhouse will take advantage of all available winter light. (Any greenhouse manufacturer will be able to help you find the best siting for your particular area.)

Even with good siting, if you use your greenhouse only to overwinter marginally hardy plants, some supplemental heat will probably be necessary in cold areas. However, if your dream is to raise tropical plants or heat-loving orchids, you’ll need to keep the greenhouse much warmer (and you may need to supply supplemental light, as well.) Again, a manufacturer can offer you more specifics.

The most efficient greenhouse heaters are generally either electric or propane-based. Other forms of heaters are usually discouraged either because they are not safe, or not good for the plants. Since heat rises, you may need a fan near the ceiling to force warm air back down when you are heating the greenhouse. Special recirculating fans are available that keep air moving gently around your plants, insuring a proper distribution of carbon dioxide and moisture.

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