We often hear of the importance of humidity in the culture of orchids, but that has often raised the questions as to which is more important, relative- or absolute humidity, how do we determine what humidity level we have, and how do we control it
Absolute and Relative Humidity
Absolute humidity is defined as the mass of water held in a certain mass of air.
Relative humidity is defined as the ratio (usually expressed as a percentage) of the mass of water in a given mass of air divided by the maximum mass of water that can be held by that same mass of air at a given temperature. That maximum increases with increasing temperature, which is why for example, we see the RH drop as lights heat a growing area.
One of the functions of the plant’s cells is to regulate the flow of moisture into and out of the plant, and they do so in response to the gradient between the inside and outside of the plant. If we take that at face value, one might be inclined to think that the absolute humidity is what’s important, as the same number of molecules of water are present no matter what the temperature.
However, as gases such as moist air expand upon heating, warmer air will have fewer water molecules per volume than cooler air will, and as our plants do not similarly change volume with temperature, the gradient is shifted to one favoring the loss of moisture from the plant, rather than an uptake of moisture. It is for that reason the the relative humidity is the factor of interest in orchid growing. (Before folks take issue with that, let’s consider that we tend to grow our orchids in a fairly narrow temperature range, so controlling the relative humidity also constrains the absolute.)
As an aside, it’s that physical expansion upon heating that makes warmer air capable of holding more moisture, as there is physically more room to fit more molecules in the mix. Upon cooling, the molecules get closer together and have a greater chance of bumping into each other, and when they do, they ultimately condense into droplets, which we see as fog, dew, or rain.
Determination of Humidity
There are several ways to determine the humidity level in your growing area. The simplest is a reactive method, in which we simply observe the aerial roots of our plants; if the roots are plump and their tips are green, the humidity is just fine.
Going to the other extreme is the use of a sling psychrometer, in which two thermometers – one dry, the other maintained wet by a wick and reservoir of water – are whirled through the air for a period of time so that the cooling effect of evaporation from the “wet bulb” can be compared to the absolute temperature of the “dry bulb” thermometer. By finding the intersection of those two temperatures on a psychrometric chart, one may determine the absolute and relative humidity levels of the air.
For the hobby grower, resorting to such means to measure the humidity seems a bit extreme, and there are a number of devices available that do an adequate job. Just remember that the accuracy may be questionable in less-expensive devices, and if you select a stationary wet-,.dry bulb thermometer, you’re going to need a breeze from your circulation fans to get a reasonably accurate reading.
Controlling Humidity
For the orchid grower, we can usually equate “control” with “supplement,” as many of our plants come from regions more tropical than our individual geographies. If you are an outdoor grower, you’re pretty much at the whim of nature, but if you do need to add moisture to the air, frequent misting is a good method.
In the greenhouse, there are a number of techniques available, such as misting, fogging, or evaporative cooling, and there are a multitude of devices available for that purpose. I use a combination of overhead misting, fogging with an atomizer device, and evaporation from a wet gravel floor after watering to maintain humidity levels around my collection.
For those who grow their orchids indoors, in a controlled environment under lights, many of those same options are available. For the in-home grower whose plants are grown in the main living area of the house, the challenge is a bit greater.
Certainly, hand misting several times a day is good – if transient – but just don’t leave them wet as night draws closer. In fact, it’s more effective to mist the air in the growing environment rather than the plants themselves. Some growers move their plants closer together, so that the localized humidity of the moisture evaporating from the plants and growing medium benefits the entire cluster.
Another good alternative is the use of an evaporator of the type used to ease folks with sinus congestion. A few comments about those: if you select a “cool mist” evaporator – the kind that atomizes the water using a rotating disc – or an ultrasonic type, you may experience the formation of a white film on your plants, furniture, and floor from the minerals in your water. Wipe your plants down periodically with a good leaf cleaner (our Rise & Shine works well, and has other benefits), and they’ll be fine. The console-type humidifiers that utilize a sponge or moving wick belt to enhance evaporation of the water achieve the same effect without the fear of mineral buildup on your plants and furniture, but it will, instead, build up in the evaporator, requiring periodic cleaning or replacement of the sponge or wick. The same is true of “warm mist” room humidifiers, where the mineral buildup will occur on the heating element.
The use of “humidity trays” is another common recommendation, but one with which I do not concur. With those, waterproof trays containing pebbles are half-filled with water and placed under the plants. The theory being that as the water evaporates, it raises the localized humidity. The reason I do not recommend them is explained below.
A general caveat about all of the humidity-enhancing methods described for in-home growing: remember that unless your growing area is sealed off from the rest of your home, any effort to raise the humidity level around your plants is actually trying to raise the humidity of your entire house or apartment! Because of that, especially if there is air circulation due to fans or forced air heating, the passive methods like the humidity trays appear to be of very limited value, if any, and room-type humidifiers might offer only a minimal improvement. If you really want to do this right, consider closing off the growing room, or find a means to humidify your entire home, and remember that for many plants it is not necessary to have tropical humidity, but that we are shooting for a sufficient humidity to keep the plants from desiccating. If you’re interested, we have a calculator that can tell you the maximum amount of water – in milliliters or gallons – that the air in a closed room can physically hold. As a reference point, at 70°F, it takes about a gallon of water “dissolved” in the air to maintain 75% RH in 1000 square feet of a typical home. These is also a very simplistic explanation of the science of evaporation and humidification.
Maintaining a reasonable humidity in your home has several non-orchid benefits, as well: it keeps your wood furniture from drying and splitting, reduces “nail pops” in drywall, and keeps your skin from drying out. One more thing – and this may be the biggest benefit of all – if you maintain the relative humidity in your home around 50% to 60% during the winter, you will be able to lower your thermostat a bit, as the evaporation of moisture from your skin is reduced, and you feel warmer at a lower temperature.