Humidity refers to the amount of moisture in the air. Some plants are sensitive to low humidity. Most common house plants, given an otherwise favorable environment, will not be injured by humidity levels found in most homes.
Humidity levels will fluctuate during the year. When the furnace is on in winter, the air will be drier. This will be the time when sensitive plants show low humidity injury. Low humidity means plants lose more water and may need more water in winter. If the air is so dry, plant roots can't replace lost water fast enough, the leaves will drop or die back part way.
Indoor gardeners may combat low humidity problems in several ways. Perhaps the most obvious solution is to run a humidifier. Humidity levels of about 40 percent will be sufficient for all but the most sensitive plants.
Another solution involves trays, or shallow pans, with stones or pebbles in them. Set plants on the stones and put water in the tray. The water should not reach the bottom of the pots. The water in the tray evaporates, raising the humidity around the plants. Check the water level in the tray frequently.
If the plant is both very sensitive and small it can be grown in a terrarium. This provides high humidity and works while the plant is small enough to fit in the container.
There are two other methods of raising humidity but they are not as effective. One is misting. Misting is only effective when done at very short intervals. This is not practical in most home situations.
The second method is called double potting. This method also reduces the loss of soil moisture through the side of the pot. Select a pot, or other container, which is one or two sizes larger than the pot the plant is in. Damp sphagnum moss is put in the bottom of the larger pot. When the pot containing the plant is set on the moss, the rim of the larger pot should be slightly above the rim of the smaller pot. Fill the space between the two pots with more moist sphagnum moss. The moss is kept damp at all times.