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Growing Requirements
by David Horak
Temperatures
This orchid leaf has suffered frost damage from sitting too near a cold window during freezing weather.
Different authorities may disagree on the ideal temperature ranges for certain orchids, but most people put orchids into one of three categories: warm-growing (90°F maximum temperature on a summer day to 65°F minimum temperature on a winter night), intermediate (85°F to 60°F), or cool-growing (75°F to 55°F). Most tropical orchids are content with daytime highs from 75°F to 80°F and nighttime temperatures that are about 10 to 15 degrees cooler. The diurnal drop in temperature is very important for the plants to produce strong growths and, in many cases, to initiate blooming. Orchids grown in buildings with climate-control systems that maintain stable, nearly constant temperatures may not experience sufficient variation in temperature to bloom. For most plants, temperatures over 90°F are damaging. When the daytime highs exceed the recommendations for a plant, try improving conditions by increasing humidity and air movement and by providing additional shading.
Orchids such as Phalaenopsis and Cattleya types will be harmed if temperatures drop below 50°F. If you are moving your orchids outdoors for the summer, wait until nighttime temperatures are regularly 55°F and above. In fall, move the plants back inside before the nights become too cold. But always be sure to check the needs of individual orchids. Cymbidiums, for example, require nighttime temperatures in the low 40s in the fall to initiate flower spikes.
If you are growing orchids on a windowsill, be particularly careful in winter when the temperature close by the window can easily fall below freezing and potentially damage leaves or even kill plants.
Humidity Levels
Humidity is most often measured as a percentage of the amount of moisture actually in the air relative to the air's capacity to hold it. As air heats up, its ability to hold moisture increases substantially. Most orchids tolerate humidity ranges from about 40 to 80 percent. Within this range, higher humidity levels accommodate more types of orchids and produce better growth and fewer problems with blooming. Humidity is often highest early in the morning when temperatures are cooler, then it steadily declines as the temperatures rise. The amount of moisture in the air has stayed the same, but the capacity of the air to hold moisture has increased, so the relative humidity drops. In order to maintain higher humidity levels, more moisture is needed.
Too little humidity often causes buds or inflorescences to become stuck in their sheaths, and as the flowers try to emerge they get twisted and deformed. In the worst cases they will desiccate and die. Low humidity can also keep new leads (vegetative growths) from emerging properly and result in wrinkled, accordion-pleat leaves. This is a common problem with Oncidium-type plants, especially Miltoniopsis. The leaves may be unsightly but the condition is rarely damaging to a plant unless it is severe enough to cause the loss of a leaf. Raising the humidity will take care of all of these problems.
A plastic pot is a suitable home for Masdevallia strobelii 'Janet', which needs to stay constantly damp. As with all other orchids, it also needs good air movement to assure that its foliage dries quickly after watering.
It is fairly easy to achieve adequate humidity in a greenhouse with the help of humidifying foggers, misters, or by wetting down the floors on a regular basis. In the home or office it can be difficult to maintain adequate levels of humidity. In summer, air-conditioning draws moisture out of the air, and during the winter the ambient relative humidity can easily drop below 10 percent in a centrally heated room. A humidifier placed close to the plants can usually provide enough humidity. Even misting the plants occasionally during the day with a spray bottle can help. When the humidity is adequate, the rate at which a plant gives off moisture through its leaves is balanced. It is important to remember that it's not possible to compensate for low humidity by increasing watering. When the humidity is low, a plant will often transpire faster than the water can move through its system to compensate. It becomes desiccated yet may lose its roots because of excessive moisture in its pot.
Many people set their plants on shallow trays lined with gravel and partially filled with water, so that the pots are just above water level. The premise is that the evaporation of the water will raise the humidity around the plants. In practice, without enclosing plants and trays to confine the evaporated moisture, water will quickly dissipate into the environment without effectively raising the humidity level. The best reason for setting the plants on trays is to make watering easier, as the trays catch the excess runoff when the orchids are watered in place. In this way orchid growers might be more inclined to water whenever the plants need it rather than try to postpone the job of hauling the plants to the sink or shower for as long as possible.
In the evening, as temperatures sink and the ability of the air to hold moisture decreases, the humidity begins to rise. In response, it's best to turn off humidifiers to prevent moisture from condensing on the plants. In general, when the humidity is too high and there is little air circulation, conditions are just right for the spread of fungal and bacterial disease problems. Of course, in nature plants are commonly wet at night, but they are exposed to constant air movement, which prevents the stagnant conditions that can lead to problems.
Fresh Air and Air Movement
Growing orchids in a greenhouse offers many advantages, such as higher humidity levels than easily accomplished in the home, more variation in microclimates, and more light levels.
Fresh air and air circulation are vital for orchids grown in the home or greenhouse. The plants' leaves should move gently in a light breeze created by constantly running fans. Oscillating fans are the best choice; they are better at providing active air movement than fixed straight-line fans. Good air movement delivers oxygen and carbon dioxide to the leaves, helps keep leaf temperatures down when it is hot and sunny, and allows foliage to dry quickly after watering, minimizing disease problems. Air movement is a key component to be considered when trying to balance watering, humidity, light, and temperatures in a healthy environment. While good air circulation is important, the growing area should not be a wind tunnel. Too much air movement, especially if combined with low humidity, can pull moisture from the plants and desiccate them.
David Horak has been growing orchids for more than 25 years. He is the curator of Orchids and the Robert W. Wilson Aquatic House at Brooklyn Botanic Garden. He is the current president of teh Greater New York Orchid Society and the chairman of the New York International Orchid Show.
Photographs: Charles Marden Fitch