Orchids enjoy the same temperatures that we do 70° to 80°F (21° to 27°C) during the day, with a drop of 10° to 15°F (5° to 9°C) at night. As you see catalogs you will encounter the terms warm-growing, cool-growing, and intermediate. These all refer to a species’ minimum winter nighttime temperatures – that is, the coldest conditions it will tolerate.
Cool-growing orchids thrive in nighttime temperatures of 50° to 55°F (10° to 13°C), rising to 60° to 75°F (16° to 24°C) during the daytime. This group includes cymbidiums, odontoglossums, masdevallias, and many paphiopedilums. Some cool growers tolerate night temperatures into the low 30s (-1° to 2°C); predictably, these plants can be difficult to grow where summer temperatures are consistently high. Still, even in this group some individual species tolerate higher temperatures.
Intermediate temperatures 55° to 60°F (13° to 16°C) at night, 65° to 80°F (18° to 27°C) during the day – are satisfactory for cattleyas, dendrobiums, many oncidiums, paphiopedilums, and a large number of the so-called botanical or species orchids. Warm growers such as vandas, phalaenopsis, and some tropical paphiopedilums appreciate nighttime temperatures of 60° to 65°F (16° to 18°C) and daytime temperatures of 70° to 85 OF (21° to 27°C).
If temperatures are too high for more than a brief period, orchid plants will suffer. If bright sunlight accompanies high heat, leaves will show sunburn – bleached areas that turn brown or black. Depending on the amount of damage, the leaves may fall off or (in extreme cases) the plant may die. High heat and strong light also encourage plants to transpire, or lose moisture through their leaves, to their detriment. Even if they do not look sunburned or otherwise stressed, plants will not thrive in excessive temperatures they will assume a yellowish tint and fail to grow or flower.
Higher summer temperatures will not harm most orchids, however, if they are sheltered from hot sun and given increased humidity as temperatures rise. Daytime temperatures to 90°F (32°C) are actually beneficial, especially for the warm growers. Many hobbyists move their orchids outdoors for the summer, where dappled shade and moving air can reproduce the conditions they enjoy in their native forests.
Atmospheric humidity – the amount of moisture in the air – is as important as temperature to orchid health. Although most orchids grow naturally in humid environments, indoors they can make do with a humidity level of 30 to 40 percent – which fortunately is comfortable for most people as well. Lower humidity levels, like strong light and high heat, will increase their transpiration rate.
If the humidity level is inadequate you can raise it with a portable electric humidifier, by adding a humidifier unit to your central-heating forced-air furnace, or by keeping a kettle gently steaming on the stove. As temperatures rise indoors, the humidity level should be increased in compensation. As temperatures drop in the evening so should the humidity level, in order to thwart the disease organisms that thrive in the combination of low temperatures and high humidity.
Many people maintain adequate humidity for their orchids by placing their pots on trays filled with gravel and water. Evaporation from the wet gravel keeps the air around the plants fresh and moist. Be careful not to let water in the tray rise to pot level, however; stagnant water around orchid roots will cause them to rot. Growing a number of plants close together also helps maintain humidity, because each plant’s transpired moisture is shared with its neighbor.
If the indoor environment is dry, spraying your orchid plants with a fine mist of water is another way to supplement atmospheric humidity. The mist should be fine enough to leave a thin, quickly evaporating film on the leaves; a coarser spray can cause problems by drenching plant, pot, and planting mix. Spray orchids early enough in the day so that the leaf surfaces and plant crowns can dry out before evening. This is especially important when humidifying phalaenopsis (moth orchids).
Free air circulation is another requirement of healthy orchids, in part because stagnant air tends to trap heat around their leaf surfaces. Home ventilation is generally satisfactory during the summer, because some windows are almost always open. In climates that require summer air-conditioning, or that prohibit keeping a window ajar in the winter, you’ll need a small oscillating fan to keep air moving and help in modifying your growing conditions. Do not direct the fan at the orchids, as they will not tolerate drafts; instead, position it so that the flow of air is directed above or below your orchids. If a forced-air heating or air-conditioning duct is located beneath a window where orchids are growing, cover it with a deflector to direct air into the room and away from the plants.
In most climates your orchids will benefit by spending the summer out-of-doors, under light shade. The fresh, moving air and humidity are to their liking, as are the occasional cleansing rains.