Humidity and Houseplants

Why Do Houseplants Need Extra Humidity in Winter?

Winter air and our home heating systems do not mix well with an indoor plant's need for humidity. Cold outdoor air holds less humidity to begin with, and the heating process removes more moisture. The American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) recommends that indoor relative humidity (RH) be maintained between 30% and 60%. During the winter heating season, indoor relative humidity levels can easily fall below 30%. Indoor plants come from a variety of natural environments, so there is not one best humidity level that fits all. However, most indoor plants benefit from a humidity level higher than what is typical for a home heated in winter (some varieties of cacti and succulents are exceptions).

Do Longstanding Remedies for Low Humidity Work?

Like many gardeners, I was convinced that several customary methods to raise humidity for houseplants worked. Though I could not find a definitive study, here is a summary of current thinking about these remedies.

Misting

After a plant is misted, a rise in humidity only lasts until the water evaporates. This can happen in a matter of minutes, so even daily misting does nothing to raise the humidity levels around a plant. (However, misting is an accepted method for watering air plants, aka Tillandsia spp.)

Trays of Wet Pebbles

Another strategy to raise humidity for plants involves standing them on top of a tray of wet pebbles. The theory is that the evaporation of the water below the plants causes a beneficial rise in humidity. This might be the case if the hydrated air from the tray rose straight up and remained, but in fact the evaporated air diffuses into the entire room, making almost no overall impact on the relative humidity.

Placing Houseplants in the Bathroom

Online sources frequently suggest that a bathroom will supply the humidity that houseplants require. This theory is difficult to substantiate or challenge. This probably depends on how your bathroom is used, especially how many times the shower is used regularly. You might gauge the humidity in your bathroom with a hygrometer before settling on this solution. One drawback is that bathrooms often lack the appropriate light source for a houseplant.

What Steps Can You Take to Raise Humidity for Houseplants?

Grouping Plants Together

This is a simple step that most sources still deem beneficial. Its effectiveness most likely depends on the number of plants and the size of the room they occupy.

Growing Under Glass

Placing plants in a terrarium-like environment or even under a glass (or clear plastic) cloche recycles humidity in the plant's immediate surroundings.

Adding a Humidifier

Adding a humidifier to a hot air heating system is beneficial both for human comfort and for houseplants. Although a whole-house humidifier will not raise the humidity to ideal levels for houseplants, it will help. On a smaller scale, you can purchase a portable humidifier. In 2024, Better Homes & Gardens tested a number of humidifiers for use with indoor plants, many with monitors and timers. See BHG Recommends The 6 Best Humidifiers for Plants, According to Testing.

Finally, keep in mind that the optimal humidity for each of your houseplants is not an exact science, and it is not the same for each type of plant. If the humidity in your home is comfortable enough for you, your plants should tolerate it throughout the winter. It is best to concentrate your efforts on key elements that are easier to control, such as light exposure and watering.