The concept of a greenhouse is fairly simple. It allows in the radiant heat from the sun, but then traps most of it in the greenhouse so that the interior stays warmer, many times much warmer, than the outside air. However, for those who are looking to install a greenhouse or who are just beginning to work in one, there are many things to consider. The materials used and the way plants behave in the greenhouse are among those things that beginning gardeners will need to consider.
The main function of a greenhouse is to provide heat and serve as a place to raise plants in an environment where they often would not be able to survive. Often, this is the way flowers and vegetables are raised when they would normally be out of season. It is also a way to have plants for personal use and enjoyment.
In the past, there was only one way to build greenhouses and that involved using glass. While glass is still an option for greenhouse construction, it is often so prohibitively expensive to the casual gardener that other materials must be used. Polyethylene sheeting, fiberglass and other types of plastics are often common choices for greenhouses. Plastic sheeting is the cheapest, but is also the least durable of materials.
Often, especially in the winter, the amount of light a greenhouse receives from the sun is just as important as the heat. Maximize your greenhouse's value by making sure it is placed in a corner of the yard that receives the most amount of direct sunlight. Greenhouses may be near buildings, but those buildings should not block the sun from the greenhouse for a significant portion of the day. Trees are worth avoiding not only because of the shade factor, but also falling branches could damage your greenhouse, especially if your area has violent storms.
There is a general rule about using greenhouses: the bigger, the better. While this may be an oversimplification, it is certainly a consideration. Greenhouses tend to perform better and generate more heat when they are larger, because they capture more of the sun's rays. Buying the biggest you can afford and are capable of taking care of is a recommendation among many gardeners.
If investing in a greenhouse, make sure the supporting understructure is of good quality and the best you can afford. Some greenhouses will require bracing in the winter, especially if you live in an area that receives a lot of snow. Snow can be very heavy and stressful on a structure, especially if it has a high-moisture content. Greenhouses, however, are often not built to withstand a lot of weight on top of them.
Remember, greenhouses are only responsible for providing heat to an area. They cannot, by themselves, provide light. Those who do not have artificial light sources for their plants may find their greenhouses do not tend to grow much in the winter. This is normal. Flower and vegetable growth may be encouraged with the use of artificial lighting, but this adds to the operating expense of the greenhouse (see Resource section).