In the Garden:
Northern & Central Midwest
January, 2002
Regional Report

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I set my tropical hibiscus on a bed of moist shells. Pebbles work just as well to raise the humidity.

Houseplants and Humidity

Now that the heat's been on in most of our houses for a few months, is your skin feeling prickly, crackly dry yet? You can bet that if you feel that way, your plants do also. We can run for the body emollients, but plants have to rely on our providing extra humidity for them. If we don't give them the additional moisture, they let us know by browning tips and margins, spider mite outbreaks and generally looking poorly.

It may not seem like a critical issue to deal with, since it's one that will correct itself come spring when the air has more natural moisture and the dry heat from the furnace is more sporadic. But a plant under any kind of stress is just waiting for trouble. Stress can start a cycle of problems that will take all kinds of drastic measures to correct.

What is Humidity?

Humidity is simply the percentage of water in the air. Tropical and subtropical plants (that is, most houseplants) are native to environments with about 80% humidity. Most homes fall within the range of 35-65% humidity, sometimes falling below 20% in the winter, so you can see the problem.

Luckily, many plants are adaptable regarding the amount of humidity they'll tolerate. A house would be uncomfortable at 80% humidity, and furnishings and people would start to mildew. If you can keep your humidity from dropping below the 40% range, however, most plants will be okay. Avoid buying plants that require extreme humidity or you will end up being frustrated.

Create a Microclimate

It's possible to create a humid microclimate immediately around plants. To keep humidity high, you must not only put more water into the air, but also reduce the air circulation that evaporates water from around the plant. Low humidity problems are intensified if the soil is allowed to dry out, if the plant is in a draft, or if it is flooded with unfiltered sunlight.

Some of the most successful ways to raise humidity are to group plants in order to reduce the air circulation inside the grouping, to raise the pot on a bed of wet pebbles, or to double pot a plant, filling the void between the pots with dampened sphagnum or Spanish moss.

Humidifying the House

Managing the humidity in an entire house can be done with a humidifier installed directly on your furnace. This theoretically lets you raise or lower the humidity in the entire house by use of a humidistat. However, even with a system such as this, the humidity may not remain high in all areas.

Hand misting or wetting foliage is a common recommendation for humidity-loving plants, but in reality this puts more moisture into the air only temporarily. If the rest of the room is dry, that moisture will evaporate quickly. You would have to mist every few hours for it to be effective. Besides, wetting the leaves of some plants may invite disease problems.

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