The air in most homes becomes extremely dry as furnaces force warm air through the rooms. It’s not unusual for relative humidity (RH) inside the home to drop to 15 percent during the winter heating season. Most houseplants do best at about 35 percent to 45 percent RH.
Keep in mind that most houseplants are outdoor plants in their native climates. Tropical and subtropical species can be damaged by temperatures below 50 F, but being too warm in winter can also be a problem.
Warm indoor temperatures coupled with low humidity can cause plants to lose water faster than they can take it up. So, even though the soil may hold plenty of moisture, the leaves may begin to droop or turn brown along the edges. (Hot, dry, stale air also creates a favorable environment for spider mites to become troublesome.)
The most effective way to increase RH for the comfort of both plants and people is to run a humidifier. Grouping plants together on pebble trays filled with water can also help. However, misting plants occasionally with a spray bottle adds such temporary moisture that it does not effectively change the relative humidity. Keep all plants away from hot air drafts near heat registers. Ferns are especially sensitive to dry air, so take care to place them in a protected area.
Although some plants may grow more slowly during the short days of winter, dry air can cause them to need to be watered even more frequently than when they were actively growing. Monitor the soil moisture to be sure that plants are getting watered as needed.
B. Rosie Lerner is the Purdue Extension Consumer Horticulturist at Purdue University, West Lafayette.