Humidity for Plants

Orchids and Humidity

Humidity is one of the least understood factors about plant growing.  You will see many online articles that espouse that you should “mist your plants every day”, but that’s not really the case for most other tropical plants, or even some orchids.  Spritzing your plants is an added bonus – something that keeps the plant from losing water temporarily.  Spritzing your plants does NOT raise humidity; if it does, it’s temporary, maybe for 20min until it dries out.  The volume of air around a plant is too big to have the humidity go up from simply spritzing. 

Most orchids are epiphytes, and as epiphytes, will respond to spritzing – spritzing can be used as a tool for yielding certain results.  Lithophytes will respond to spritzing too, but terrestrial orchids will not respond to spritzing.  This is because of the mechanisms that each habit (growth form) has developed.  Epiphytes and lithophytes do not know when their next drink is coming, and have no media to draw water from.  If the mist hits their exposed roots, they will absorb that water.  Terrestrials have a supply of ground water, so they never really evolved a way to utilize mist.  It’s pointless to mist terrestrial plants in general because they have no mechanism to absorb the water on their leaves.

Interestingly, orchids and other epiphytes, which can absorb water through their leaves, have a mechanism to absorb water vapor through their stomata, but not water on their leaves4. Virtually all orchids do best when humidity ranges from 40% to 70%.

Raising the humidity lowers the rate of transpiration (water loss) from the plant.  For outdoor and greenhouse growers, the higher the humidity is in your greenhouse, the less frequently you will have to water your plants.  The caveat to that is, is that you lose your leverage to cool the greenhouse if the humidity is too high, and that fungi, pests, etc. also like moist conditions.  The catch-22 to recreating the natural environment for your plants is that you also are creating an environment that is favorable to all the other organisms that would live with the orchids too, both good and bad!  That requires diligence that we will discuss later, but for now, just know that if the humidity is high, you water less; if the humidity is low, you water more. 

Indoors, humidity is typically low, and that is especially challenging when growing mounted orchids.  We generally recommend not growing mounted orchids indoors unless they are in a vivarium/terrarium.  For folks who want to raise humidity indoors, the only effective way to raise humidity indoors is through a humidifier – one that can push several quarts per hour into the air.  The smaller humidifiers have a negligible impact on the humidity of a room.  Humidity trays barely raise the humidity at all, and are terribly ineffective.  Remember that most of the water that a plant absorbs is through its roots – the humidity only affects the time that any one plant will dry out.  For many of the more succulent species like Cattleya, growing in low humidity is not a problem – provided they are watered frequently enough.  However, for thin-leafed species, and smaller orchids, humidity is essential, and humidity must be maintained at higher than average levels.

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.

In greenhouses, 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.

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.  In general, 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.