BY MELISSA SERENDA

This can be a rough time of year for tropical houseplants. After enjoying Iowa’s hot and humid summer—which may have felt a little like “home” for these plants that originated in rainforests and other warm, humid places—they are suddenly plunged into a central-heating desert where the relative humidity (RH) can be 30% or less.

Plants that evolved to live in places where the RH can reach 70-80% lack the adaptations to thrive in drier areas that more hardy temperate plants have to conserve water. As a plant transpires through openings in its leaves called stomata, the evaporation of water through the stomata causes the plant to pull water and nutrients up from the roots to the leaves. Some plants that are particularly sensitive to dry air include ferns, prayer plants (Marantaceae family), nerve plant (Fittonia), and Anthurium spp.

When the plant is in an appropriately humid environment, the water lost through transpiration doesn’t exceed the water it can take up from its roots and from the surrounding air. But if the air is too dry, the plant will lose more water than it can move up from the soil and cause wilting or browning of leaf tips. There are a few things that can be done to help increase the RH for your plants during these dry winter months.

Methods to Increase RH for Houseplants:

1. Misting: often recommended, though this is not generally effective for long and can lead to disease if water is allowed to sit on leaves.
2. Pebble trays: setting the plant’s pot on a tray filled with pebbles and water, which will evaporate from the tray and raise the RH. Make sure the bottom of the pot is not submerged to prevent overwatering the soil.
3. Location: keep plants away from vents and draft windows where hot or cold air can blow on them. If you have a bathroom or kitchen with adequate light, these rooms may also provide a little more humidity.
4. Grouping: position plants together to take advantage of the their transpiration as they release moisture into the air around them, forming a slightly-more-humid microclimate with their neighbors
5. Humidifier: a portable humidifier can be used to increase the RH near plants. This can be especially effective in smaller rooms or other more enclosed areas.
6. Terrariums/greenhouses/cabinets: keeping the plants in an enclosed space prevents the moisture in the air around them from dissipating. Some people even put up small walk-in greenhouses in their homes to house their more humidity-sensitive plants.

I’ve found that grouping plants together and closing vents in the immediate vicinity to be helpful for a group of Calatheas; smaller specimens enjoy spending the winter in a large terrarium where the RH can be kept at 70% or higher. 

You can invest in an inexpensive hygrometer to gauge humidity around your plants to see how well these methods work. You may not be able to replicate the understory of a rainforest in your living room, but you can make it a little more comfortable for your photosynthesizing friends!

Sources:

1. https://gardens.si.edu/learn/educational-resources/plant-care-sheets/houseplant-care/
2. https://arapahoe.extension.colostate.edu/2020/11/08/humidity-for-health/
3. https://pss.uvm.edu/ppp/articles/humidity.html
4. Capon, Brian. Botany for Gardeners, 3rd ed. Portland, Or.: Timber Press 2010.