To most people when the term orchids is spoken they think of hot steamy jungles with oppressive humidity and constant rainfall. To a small extent they would be right but most orchids are found in areas that stay in temperatures between 50'-80'. How do you explain this when the majority of orchids are found south of the Tropic of Cancer and north of the Tropic of Capricorn?
To answer that question we must remember that all land is not flat. Mountain ranges have large areas that even in the tropics manage to keep snow and ice year round. As you come down in elevation to areas that are more temperate you encounter more and more orchids. So the answer to the dilemma here are microenvironments that exist because of elevations or the lack thereof.
When you see orchids in books that say that they are cool growers, they are telling you that this plant is found at high elevations, between 2500 and 3800 meters, and may get quite hot while the sun is out on a clear day but can quickly cool off as night or cloud cover approaches, sometimes as much as 25'-30'. So in a greenhouse what you would most like to mimic is that temperature change range and not particuarily the exact temperatures. These plants more often than not will need more constant humidity and watering because of the effects of the paramo, which is a Colombian term that describes the phenomena of year round cloud cover that creates a fog bank at a certain elevation. This occurs because of the hot humid air from the jungles below meets the cool dry air of the snow covered mountain peaks above creating a dense ground hugging fog that never seems to dissapate. Plants that grow from these regions in cultivation must be kept extremly humid but not overly wet, and they need that day to night, or diurnal teperature change.
When you see the term 'intermediate growers', the day to night temperature change or diurnal change is closer to 20'. These plants will be found in elevations of 1800 to 2500 meters. Here the day can be quite hot and the rainfall may also be more seasonal than in the cold climes. For this reason a rest period may be required for some species.
Warm growing plants come from elevations of 1000-1800 meters. In this clime it is possible to have either very constant conditions or seasonal ones. These seasonal changes are why we say that certain species need a rest period. The rest period is nessesary in cultivation because in nature it does not rain all the time, and in many areas this is because they have a rainy or non-rainy season. Here the temperatures can get quite warm, up to 90' in the daytime, but at night the temps fall off about 15-20'.
Lastly we have hot growing orchids which can be found at all elevations above sealevel and up to 1000 meters. Here we find plants that like to be wet year round and enjoy a high heat and humidity level. Usually these plants do not need as much of a rest period as plants from intermediate or warm conditions. Generally these plants require less of a diurnal tempearature change, 10'-15' is average, and as a rule have more constant water and fertilizing requirements.
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