The Importance of Humidity and Air Movement in Successful Orchid Culture
The simple secret to good orchid growing is achieving a balance between the six or so factors; light levels, temperature, humidity and air movement, watering, potting and potting media, and nutrient supply, as they relate to your plants. This isn't really difficult, but it does require knowledge, understanding and careful observation. Humidity and air movement are best handled together because they are intimately linked in both positive and negative ways. First let's look at each factor individually and then we'll take a look at their relationship to one another.
Virtually all orchids do best when humidity ranges from 40% to 70%. Even those that are adapted to growing under rather desert like conditions such as Zelenkoa (Oncidium) onusta are exposed to periods when the humidity is in this range. In this particular example, the species flourishes in coastal deserts where breezes from the pacific bring nightly dues and resultant relatively high humidity even though it may not rain for long periods.
Investing in an inexpensive humidity gauge is a necessity for all but the smallest collection if you want to grow orchids well. In virtually all parts of the United States outside of Hawaii and Puerto Rica, at some part of the year, it will be necessary to add supplemental humidity to the growing area. In some parts of the country such as the South and Southeast, humidity during the summer months will be adequate to even excessive while winter humidity levels may be too low, especially following strong cold fronts. In the Pacific Northwest, the situation may well be reversed with humid, cool winters and relatively low humidity during the warmer summer months. If you grow your plants in areas where supplemental heating is a requirement such heat sources dry out the air making humidity measurements critical. Having a simple humidity gauge takes the guesswork out of the growing area. Also keep in mind that humidity is usually highest at night and lowest during the afternoon peak of sunlight and warmth.
There are a number of ways to increase humidity in your growing area. If you only have a couple of plants, addition of some ferns or other houseplants to your growing area may be sufficient. These plants do a reasonable job of adding humidity to the air around them. For small collections grown under lights or windowsills, humidity trays may be sufficient. These trays are nothing more than a water-holding tray filled with small gravel (aquarium gravel works well). The gravel-filled trays are filled with water to a level just below the surface. To prevent plants sitting on constantly wet gravel the plants are placed on small saucers or pieces of plastic or metal grid placed on top of the trays. The next step up in the humidity game would be an ultrasonic humidifier or one of those fog-generating ultrasonic devises you see sold for terrariums. If you are fortunate enough to have a greenhouse or grow your plants outside, you might want to invest in a set of misting nozzles connected to a timer or humidstat but just simply wetting down the growing space during the mid to late afternoon may be sufficient.
Orchids like air movement. Indoors, an overhead paddle fan set on the slowest speed or a small oscillating fan set to face AWAY from your plants may provide adequate air movement. Air movement at night is just as important as air movement during the daytime so fans should run continuously. One of the mistakes that new growers make, especially those with new greenhouses, is to put their fans on a timer and shut them off at night! Don't forget that what you are trying to simulate is a buoyant atmosphere and close, humid nights are anything but buoyant. If you don't believe that, try walking on the beach in August in Miami without a breeze.
What is adequate air movement? The answer depends on humidity to a certain extent (see below), however in general enough to cause GENTLE movement of thin foliage is about right. A hurricane is a hurricane - not air movement. If the leaves of your cattleyas are moving you might have too much air movement depending on humidity levels.
How are the two factors related?
Simply put, the higher your humidity, the higher can and should be your air movement. Air movement is beneficial to your orchids but, under certain circumstances it can be detrimental as well. If you have adequate humidity, air circulation helps to carry stale air away from your plants and replace it with fresh air. This is especially important on hot humid nights. High humidity coupled with minimal air movement is a terrific recipe for the growth of fungus or physiological problems created by the buildup of moisture within the leaf tissues.
Moving air keeps leaf temperature down. Some orchids close the pores that allow transpiration of air and water from their leaves during the day. The leaves of these plants can rapidly become overheated and damaged without adequate air movement to cool them. Air movement avoids the stratification of cool moist air below the growing area and warm dry air above, where the plants are and “dead spots” are minimized and, equally important, damp stagnant areas - breeding place for disease - are eliminated.
However, where natural humidity is low rapid air movement can be destructive by draining away humidity in the growing area, drying out the plants and retarding growth. Under these situations the roots of the plants simply cannot take up enough moisture to balance that lost through the foliage resulting in shriveling of the leaves and growths and, in the worst cases death of the plant.
When balance of humidity and air movement is achieved, coupled with adequate water at the roots, your orchid plants will thrive and their physical appearance will clearly be healthy. That look is hard to explain but it's one of those things that once you've seen it you will recognize it. Many orchids have growth cycles that involve the formation of pseudobulbs that are full and smooth in their early stages followed by the formation of features like angular edges or furrows at maturity but outright wrinkling isn't normal. Even the pseudobulbs of Dendrobium sulcatum that are curious, flattened canes that look like they've been pressed are still smooth and firm when these factors are balanced. Many orchids have thick, fleshy leaves but their surfaces should be smooth and their texture should be firm. Leaves that appear to be wrinkled or crumpled aren't normal. Lastly, the roots of plants grown in an environment with balanced humidity and air movement will be plump and their tips will remain green and active throughout the growing season.