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Houseplants and Humidity

Houseplants and Humidity

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Your nice, warm comfy house may be the worst thing that ever happened to your houseplants. All the ingredients for successful growth, primarily water, light, humidity, air circulation, and soil conditions, may be less than ideal in winter. Let's go explore household humidity to better understand the problem and discover how to correct it.


To fully understand how harsh a heated home can be, we must appreciate the workings of relative humidity.

Just as warm water can hold more dissolved salt than cold, so too warm air can hold more "dissolved" water than cold air. If the air is holding half of what it can potentially hold, we would say it has 50 per cent relative humidity. That means, relative to what it can hold at this temperature, the air is now only holding 50 per cent.

If the air were heated, it could hold a lot more water, just as heating water lets a lot more salt be dissolved. The warmer the air, the more moisture it can hold. This is why wet clothing dries very quickly on a hot day as opposed to cold weather.

In addition, when the air moves it carries away the water that has evaporated and brings new dry air in contact with the wet clothing. Blow dryers for hair work on this principle. The air is heated and now can hold a lot more moisture, and the fan moves away the air that has just picked up moisture and brings new dry air in contact with the wet hair.

The reverse is also true - that cold air can hold less moisture than warm air. Air holding the maximum that it can at that temperature it is called saturated. If it cools, then it is can no longer hold that much moisture - the water drops out of the air as dew.

dew drops

When large masses of saturated air hit cold air, or rise to the extent that it cools, the result is rain. If moist air contacts a cold surface, at that very localized place the water will condense - giving fogged windows, mirrors and eyeglasses.

Now, what will happen when cold outside air comes into a house and is heated to room temperature? The amount of water in the air stays exactly the same, but, because the air is warmer, it can hold much more water. Thus the relative humidity drops. Colder outside air and warmer air in the house will cause the humidity to plummet and the air will feel extremely dry. Heating the air does not remove any moisture; it just makes the air feel much drier.

How dry can your household air get in winter? Some sources say that air in a heated home in winter could have less than 10 percent humidity. For example, if the outdoor temperature is a little below freezing (25 degrees Fahrenheit or - 3 degrees Centigrade) and that air were heated to 70°F (21°C), 60 percent relative humidity would plummet to about 15 percent. Considering that deserts have 10 to 20 percent relative humidity, your home atmosphere is really an approximation of Sahara conditions. Maybe worse.

What effect does low humidity have on your houseplants?

Considering that most houseplants are from tropical areas, the dry conditions of a heated home are extremely stressful. Just as the dry air can irritate lungs and cause dry skin, so too it can damage your houseplants. Symptoms may include a browning "burn" along the edge or at the tips of leaves. Even if there is no tangible damage, the stress may add on to other difficult cultural aspects to push the plant into a decline from which it may never recover.

What to do about dry air?

The key to helping your plants is microclimate. Although your house air may be very dry, there are ways to keep a higher humidity in the air around your plants. Of course, a humidifier may be the all around answer. It may be good for everyone's health - yours as well as the plants. But even without a source of additional moisture in the air, the following tips will help you transform houseplant hell into a phyto-friendly environment.

1) Keep plants away from direct blast of heaters and also away from other sources of heat, such as radiators. A bathroom may be an ideal location due to the moisture from showers, etc. Kitchens also may have higher humidity than the rest of the house.

2) Place the pots on trays or saucers with gravel and allow water to stand in the gravel under the plants. Evaporation from the wet gravel will add to the humidity around the plants.

Never allow the pots to actually sit in water. This is an invitation to root rot, among other problems from waterlogged soil. Put coarse gravel in the saucers so the plants can sit above the water.

3) Group your plants together. Besides being a more attractive composition, the plants will benefit from the moisture given off by their neighbors. Being grouped will also reduce air circulation so the more humid air will stay by your plants longer. This is a double-edged sword, though, because plants do need air to move for their health. After all, they don't have lungs, but still need to exchange oxygen and carbon dioxide.

4) Mist your plants with a fine spray of water. Some areas, such as the Western US, have a high salt content in tap water. This gives the white deposit of "stone" in kitchen pots. It also gives the white residue on the surface of soil and on the outside of clay pots. Such water sprayed onto the leaves of your plants will evaporate and leave a harmful deposit.

Best for spraying leaves is to use pure water, like distilled or purified through reverse osmosis. That can be a bit expensive if you are buying it, but it may still be practical for the limited use of misting. Misting may be an overrated activity. It is unclear just how useful it is unless done regularly. Any hairy leaved plants, like African violet, should not be misted.

There are other factors involved in winter care of your houseplants, but humidity - the lower-than-desert humidity of a heated home - is a major problem. We will deal with the other aspects of houseplants in the winter home in another article.

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