Plants increase indoor humidity through evapotranspiration.
The Mechanism of Transpiration
Plants release moisture into the air through a combined process known as evapotranspiration, which is the primary way they affect indoor humidity. Transpiration is the plant’s internal mechanism, where water moves from the roots up through the stem and evaporates from the aerial parts, mainly the leaves. This water vapor escapes primarily through tiny pores on the leaf surface called stomata, which open to allow carbon dioxide uptake for photosynthesis.
This evaporative loss draws water upward from the roots to the leaves. A secondary process contributing moisture is the evaporation of water directly from the potting soil itself. The water vapor released by the leaves and the evaporation from the soil surface increase the overall water content of the surrounding indoor air. Environmental factors like low ambient humidity or higher temperatures increase the rate of this water loss, as dry air draws moisture away from the leaves more quickly.
Quantifying the Effect on Indoor Air
While plants certainly add moisture, their effect on the overall relative humidity (RH) is modest compared to mechanical devices. Relative humidity is the amount of water vapor present in the air expressed as a percentage of the maximum amount the air can hold at a given temperature. The influence of plants is heavily dependent on the volume of the space, the total leaf surface area, and the ambient temperature.
To achieve a noticeable change in a standard-sized room, a significant number of plants are required, often estimated to be at least two good-sized plants per 100 square feet. The moisture from a single plant diffuses throughout the entire room, making the overall impact small. Because air at warmer temperatures can hold much more absolute humidity, a room kept at a high temperature will need more plants to reach a comfortable RH level.
For significant and consistent humidity control, especially in dry winter months when heating systems decrease RH, a mechanical humidifier is far more effective. Plants can help maintain a slightly higher level, but they are not a substitute if the goal is to raise the RH from a very low level to the ideal range of 40% to 60%.
Selecting Plants for Maximum Humidity
The most effective houseplants for maximizing indoor humidity have a high transpiration rate, meaning they possess large, thin, and numerous leaves. Plants native to tropical or rainforest environments are naturally adapted to release more moisture. Selecting plants with these characteristics yields the greatest impact on the surrounding air.
Specific examples of high-transpiring plants include:
- Peace Lily (Spathiphyllum), which has broad, glossy leaves that readily lose moisture.
- Various types of Palms, such as the Areca Palm, known for their large volume of foliage.
- Ferns, like the Boston Fern, are highly effective because their fine, feathery fronds offer a large surface area for water loss.
To encourage maximum moisture release, the soil should be kept consistently moist, but not saturated, ensuring a steady supply of water is available for the roots to absorb and transport.