While air movement is not really a concern when growing plants indoors, it most definitely is a concern in high humidity environments such as a greenhouse or vivarium. Air movement is directly correlated to water loss in plants, and regulating humidity, and can even influence plant pests and diseases.
Air Movement, Humidity, and Watering
Air movement directly affects watering. The more air movement you have, the more frequently you will want to water. While this may sound bad, air movement is imperative in environments that have trouble drying out, or don’t need watering as often. In areas with frequent rains, air movement helps dry off plants so that they don’t get infected. When plants or media are too wet for too long, it sets up the perfect conditions for fungi to infect the plants. Additionally, if it’s not particularly wet, but the humidity is too high, then light condensation can form on the plants, also encouraging an infection. Even in high humidity environments, air movement prevents fungal spores from settling on and infecting your plants, as the air keeps moving the spores along.
How Should I Increase Air Movement?
If growing in an enclosed space like a vivarium, any waterproof fan will do. Most folks use computer fans for enclosed spaces. If in a greenhouse, further up towards the roof should be powerful waterproof fans that run fast during the day, and slowly at night.
What If There’s Too Much Air Movement?
Indoors, this is rarely a problem unless you are using too powerful of a fan, or are putting a miniature, easy-to-dry-out species in front of a fan (such as a Pleurothallis in front of a fan which may knock around the leaves and dry it out too much).
In greenhouses, this is of concern by vents or by fans. It’s ideal to have the fans blow in a void direction – i.e. not on your plants, but nearby them. When plants get too fast of air movement on them, they run the risk of drying out too fast. If you water less, you’ll dry the plant to death. If your greenhouse is more humid, this is tolerated for a short while, but the plant should be moved or the fan be redirected.
Outdoors, winds can be a problem from time to time, but especially during storms or cooler weather. Storms can blow debris that can damage your orchids, and constant winds can dry out your orchids faster than you can water them. Additionally, the lower the humidity, the more harmful wind can be. When cooler or drier humidity, try to shelter the orchids from airflow by placing them next to walls or fences. Also consider placing them in the shade to help them from drying out.
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 20 min 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 groundwater, 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 leaves.
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.
