Next to be considered are the technical aspects of greenhouse operation: temperature, heat, ventilation, water, shade, light.

THREE RANGES OF TEMPERATURE
When cultural directions for plants include growing temperatures, these refer to night temperatures, unless there is specific advice for germination of seeds or rooting of cuttings under special conditions. It is usual to run a greenhouse in one of three general ranges:

Cool greenhouse      45-50 F
Moderate greenhouse      55-60 F
Warm greenhouse      65-70 F

Of course, it is not possible to maintain the entire greenhouse at exactly the specified range, and this is an advantage. For within each range, it is possible to provide widely different climatic conditions. When you run a check with several strategically placed thermometers, you will find that there are pockets, sections, corners, and high and low locations where the temperature varies five to ten degrees from the general setting. In these areas you can indulge yourself with a plant or two from the list recommended for the moderate greenhouse even though you run your house cool; or you can have success with plants on the moderate list although most of your plants are grown warm.

You will discover that corners are warmer than the center of the greenhouse, and that rising heat results in more warmth at head height above benches—a good place for hanging baskets or shelves for orchids. If yours is a glass-to-ground greenhouse, you will surely have some cool, humid areas even in a moderate-to-warm greenhouse, as moisture from above and below the benches combines with the rising heat. Furthermore, recommended temperatures for various plants often "straddle" two ranges, and many plants can be grown in a cool or moderate house, or in a moderate or warm house. Obviously, then, you have considerable leeway in your selection of compatible plants for any one of the three rather arbitrary temperature ranges.

In my own greenhouse with a night temperature setting of 52 F, in February I usually have in bloom camellias, cattleya and cymbidium orchids, Christmas cactus, crown-of-thorns, geraniums, and fibrous-rooted begonias. Coming along nicely will be amaryllis, gloxinias, tuberous begonias and maybe a dendrobium orchid or two in bud.

Each temperature range has its advantages but this cool greenhouse —45-50 F at night—is the one I prefer, and of course it costs the least for heating. The low night temperature does not preclude a pleasant warm daytime atmosphere when the sun makes the house a most agreeable place to work. And the cool house marvelously prolongs the life of flowers. The same cyclamen in bud and flower before Christmas will continue in handsome bloom through April. The temperature range best suited to the plants you want to grow will directly affect your decision about the heating equipment you will buy for your greenhouse.