A greenhouse is something many of us would like to have, but it's not something you just toss up, place the plants in and hope for the best. It takes a lot of thought, planning and in some cases, a lot of money.

Let's start with the thought. Is the greenhouse going to be used to propagate plants in the spring for a garden? Is it for overwintering houseplants or container plants? Or rather will it be used for year around plant maintenance? It makes a difference you know. Plants overwintered will need heat--orchids, for example, need a nighttime temperature of 50 to 60 degrees depending on the species. Many foliage plants need a nighttime temperature of 65 to 70 degrees. Propagating plants or growing plants from seed, such as tomatoes for example, requires temperatures above 60 degrees at night and 70 to 80 degrees on sunny days. Maintaining a 70-degree nighttime temperature in an 8' x 12' greenhouse when it's 0 degrees outside, requires a 12,000 to 14,000 Btu per hour heat system. In the spring, when the in-house temperature gets too high, ventilation will be needed. This might be accomplished by opening doors on each end of the greenhouse or by using a fan in one end and an open door on the other. Plants left in the greenhouse during the summer months will definitely need ventilation and plenty of water, so a watering system is something to think about.

You need to consider making a greenhouse as large as you can so that after you've used it a while you won't say "I just wish while I was at it, I had made it a little bigger"--a common response for most construction projects.

Next comes the planning. Orientation of the structure is important. The most ideal direction is facing the south for winter operation and probably the east for summer with the ability to shade it in the summer. Watch out for those trees overhead. A winter storm of heavy snow or freezing rain can bring a tree down branch by branch, destroying a greenhouse in the understory.

What kinds of materials will you use? Double wall plastic sheeting with air blown between the two layers is probably the least expensive material to use. A corrugated fiberglass sheet is easy to assemble, but limits sunlight penetration because of its opaqueness. In some cases a double or triple wall acrylic panel as well as a polycarbonate panel is the best because of its insulating value as well as for light penetration, but the cost may be prohibitive.

When it comes to framing, certain wood species will last longer than others will--cedar, redwood and cypress probably being some of those. However, even the best wood succumbs to moisture and fungi invasion eventually. Metal framing is something to consider. Galvanized steel is heavy and may ultimately yield to rust, although it may take many years to occur. Aluminum and some alloys are lighter and may withstand high moisture conditions better. Plastics are something to consider, but framing materials for greenhouses may not be available. Additionally, some plastic materials are deteriorated by sunlight. Something else to think about is the relationship between the framing material and the greenhouse covering. For example, a plastic covering such as polyethylene lying over a metal hoop may deteriorate quicker because of its susceptibility to heat as sunlight heats up the metal.

For additional planning, you might want to consider the shape of the structure. This might be more a decision about the aesthetics or what you think looks best in your back yard or in the garden area. Options include a hoop structure, gable roof, A-frame, gothic, slant-leg and probably others. Some of these offer a better volume to storage capacity ratio than others, but on a hobby scale this is probably insignificant. The choice then becomes personal preference.