Gardening Under Cover-Hobby Greenhouses

When early European explorers went West to find a shorter route to the spice-laden Far East, they brought back with them some of the exotic plants they encountered along the way. One plant of great interest that especially was suited for transport for months at sea in the dank hold of a ship was the pineapple. European nobility were so entranced by this strange new taste-treat, they simply had to find a way to produce it locally. The Greek philosopher Plato once wrote "necessity is the mother of invention" and, indeed, the need to have fruits like pineapple and orange throughout the year was one factor that led to the development of the greenhouse as we know it today.

Early greenhouses were a status symbol among the aristocracy of Europe. Because of the heavy taxation on glass at that time, only the wealthy could afford greenhouses. Although the tax on glass subsided, the considerable expense in constructing a glass greenhouse remained, and for many years greenhouses were beyond the financial means of most people. Modern engineering and alternative covering materials have helped to change that, and today greenhouses are much more affordable than in the past. It is estimated that there are over three million hobby greenhouses in the United States today used for a variety of purposes from providing food for the table to a relaxing place to pursue one’s penchant for gardening. For the avid plant lover, a hobby greenhouse is the "ultimate toy." Building a hobby greenhouse is much like purchasing an automobile-it can be small, simple and (relatively) inexpensive or it can be fully-equipped, automated and a bit more pricey. As with purchasing a car, the choice of greenhouse type depends a lot on one’s budget and what you want the greenhouse to do for you.

If a hobby greenhouse is in your future plans, then several decisions will have to be made. The first is the location of the greenhouse. Since light is life to plants, the primary objective in locating any greenhouse is to do so in a manner that will maximize available sunlight. Many hobby greenhouses are attached to the home, and a southern or southeastern exposure is ideal for such units. Freestanding units should be located adequate distances from buildings and trees so that shadows will not shade the greenhouse. At our latitude an object casts a shadow two and one-half times its height in the dead of winter. Drainage, proximity to utilities and personal convenience also should be considered when selecting a location for the hobby greenhouse. Additional considerations relative to the site surround the need for building permits and the existence of local design ordinances. More than one hobby greenhouse has had to be torn down only after its despondent owner found out about legal issues after construction.

After site selection is accomplished, a decision will have to be made relative to the type of greenhouse to build: attached or freestanding. Attached greenhouses have the advantage of sharing an existing wall (usually) with the home. This reduces expense as it reduces the need for building materials and cuts heating costs. However, it does limit the size of the greenhouse. A lean-to is typical of an attached greenhouse. Since the roof slope of most greenhouses are fixed at 32 degrees from the horizontal, the maximum width of a lean-to is dictated by the height of the wall to which it will be attached. Most lean-to greenhouses are between six and twelve feet in width. Freestanding, even-span (Dutch style) units are separate, "stand alone" structures. Their maximum size is determined more by finances and/or space needs than by physical constraints. Typical widths of freestanding hobby units range between 10 and 20 feet. Even-span units can also be attached, with the north wall of the greenhouse attached to the home.

All hobby greenhouses have two things in common: a semi-transparent covering material and a frame to support the covering. Indeed, the covering material chosen (somewhat) will dictate the type of frame chosen because of factors such as weight, glazing and flexibility. Glass is still the standard-of-excellence against which all other glazing materials are judged, especially for hobby units. Glass possesses good clarity (light transmission) and longevity, two critical factors to consider when selecting a covering material. Additionally, it is considered the most aesthetically pleasing covering material on the market which is an important consideration for a structure associated with the home. A sturdy, rigid frame is necessary for glass since it is relatively heavy and not flexible. Extruded aluminum is a popular frame material in modern glass hobby greenhouses, and kits of various styles and sizes are readily available. Wood frame kits also are available, but their selection is more limited.

Alternatives to glass as a covering material include fiberglass, double-walled rigid plastic and polyethylene film plastic. Fiberglass can be bought in various grades, some actually transmitting more light than glass initially. All grades of fiberglass lose clarity over time because of the action of ultra-violet light. Those types of fiberglass that are clad with a thin layer of Tedlar® (polyvinyl fluoride) discolor at a much slower rate (20 year life expectancy) because of the ability of Tedlar® to resist the action of ultra-violet light. Additionally, fiberglass is light in weight, requiring a less costly supporting frame than glass. Unfortunately fiberglass is very flammable and should not be used on a unit attached to the home.

Double-walled, rigid plastics include glazing made from acrylic or polycarbonate. Both are available in a "honey comb" configuration where two layers of glazing material are separated by a very insulating layer of dead air. Thus, the major advantage to the double-walled, rigid plastics is the considerable savings in heating costs they make possible. With adequate clarity for good plant growth, these materials have a life expectancy of between 12 and 15 years. They are fairly light in weight but do require a sturdy frame to support them because of possible snow loads that might build up due to their insulating nature.

Finally, polyethylene film is a very inexpensive, easy to install alternative to glass that can be installed over rigid or curved frames such as the quonset. A home-made, wooden frame unit covered with polyethylene film represents the least expensive way to build a hobby greenhouse. Unfortunately, polyethylene lacks the clarity of glass to begin with and ages very rapidly. One should expect to replace polyethylene film every third year. Additionally, polyethylene film tends to be less aesthetically pleasing and probably deserves consideration only on freestanding greenhouses out-of-sight from the general public.

Climate control is key to growing plants in greenhouses. Most hobby greenhouse kits include heating and cooling equipment adequate for the particular model purchased. Those that do not include the equipment frequently give the specifications of the equipment that should be purchased (i.e. Btu heater rating and c.f.m. fan capacity). Do-it-yourself models will need to have these values calculated; information for doing so is readily available on the web or from many extension offices.

When all is said-and-done, a hobby greenhouse represents an investment. It allows one to extend the gardening season by growing plants indoors when temperatures and other conditions outdoors make plant growing impossible. Properly designed, it should provide its owner with countless hours of relaxation and enjoyment while they "exercise their green thumbs" while growing plants.

Next Month: Part II--Greenhouse Gardening

David Trinklein, Associate Professor, Horticulturalist, UMC (573)882-9631