Managing a greenhouse is a lot like milking cows. A greenhouse requires attention seven days a week. Watering plants, winter heating, summer cooling, and constant pest and disease monitoring consume time by the hours. Even with all the options and automated bells and whistles, plant life ceases when the electricity "goes" in the heat of summer or cold of winter. Hurricanes and freak snow storms add to the anxiety. Even with all the obstacles, the allure of a greenhouse is irresistible. I succumbed to a 20 x 100 foot Quonset plastic greenhouse and for three years lived with the monster. Even with all the problems, it was fun. Not that I would do it again but it was good for me at that time in my life and it will really make you appreciate greenhouse grown plants. Begin with a cold frame, and then slowly move up to a greenhouse.
Most vegetable and ornamental plant seed can easily be started in cold frames, hot beds, under artificial lights, window boxes, and greenhouses. The least expensive and easiest structure to build is a cold frame. A simple cold frame can be constructed of wood, bricks, cinder blocks, or a combination of these materials. Cold frames have been used to get a jump on vegetable production for over 500 years. A cold frame should be 6 to 8 inches below the soil surface, have solid sides of wood or masonry with a transparent or translucent covering that can easily be lifted for ventilation. The back of the frame should be 6 to 9 inches higher than the front. The front can vary in height from 6 to 12 inches (taller structures cast more shadows!). Old window sashes, old storm doors or windows, or a plastic film covered wooden frame will make an inexpensive transparent cover for a cold frame. Pay special attention to ventilating your cold frame since young plants can get "cooked" on warm days.
A cold frame can be converted to a hot bed by simply installing a thermostatically controlled heating cable (available at most garden centers) or heating mat in the soil. A single 2.5 x 6 foot cold frame or hot bed can produce hundreds of vegetable and ornamental plants if properly managed. Locate your cold frame or hot bed where you get ample direct sunlight. A southern exposure is optimum.
A more expensive alternative is to build a greenhouse. Glass or rigid-plastic covered greenhouses can be quite expensive to construct and maintain. An inexpensive and economical quonset plastic film greenhouse can produce beautiful plants.
Contrary to popular opinion, a small, unheated greenhouse can be built for under $300 providing you are willing to do the construction yourself. The automatic irrigation, heating, and cooling can easily end up costing three to four times (or more) the cost of the structure. North Carolina State University engineer Dr. M.D. Boyette and nursery specialist, Dr. Ted Bilderback, put together a terrific publication that lists all the materials and complete directions for building a 12 by 14 foot greenhouse. A small greenhouse is useful for starting vegetable or ornamental plants, over wintering houseplants, and growing orchids or other exotic plants. This is a great starter greenhouse. This greenhouse is constructed of 3/4 inch PVC (schedule 80), treated wood, a single sheet of 4 mil plastic, 1/2 inch galvanized EMT tubing, and lots of PVC cement. Complete construction details can be obtained by going online (http://www.bae.ncsu.edu/programs/extension/publicat/postharv/green/small_greenhouse.pdf) and downloading "A Small Backyard Greenhouse for the Home Gardener - AG426".
However maintaining a greenhouse is similar to maintaining a diary farm; certain chores must be done every day or you are inviting a catastrophe. Commercially available hobby greenhouses are available with all the "bells and whistles" for just about any price range. Check with your local garden centers for reputable greenhouse dealers. In general, read as much as possible about the materials and routine maintenance before putting any money down. Check out the following publications about hobby greenhouses: