Indoor air, often dry (especially during winter heating), challenges houseplant enthusiasts. Many popular indoor plants originate from tropical environments, requiring humidity levels far higher than the typical 30% found indoors. This dryness causes symptoms like brown, crispy leaf edges and stunted growth in moisture-loving species such as ferns and calatheas. The humidity tray is a frequently suggested, low-cost solution intended to create a small, localized area of increased moisture around the plant.
The Intended Function of Humidity Trays
A humidity tray is a simple, low-tech device constructed from a shallow, waterproof tray filled with a layer of pebbles, gravel, or decorative stones. Water is added until it reaches a level just below the top surface of the pebbles. This setup elevates the plant pot above the standing water, preventing the drainage holes from making contact with the moisture. The principle behind the tray is that as the water slowly evaporates from the exposed surface, it releases moisture into the air immediately surrounding the plant’s foliage. This process aims to create a humid microclimate that mimics the plant’s native habitat.
Scientific Limitations and Actual Effectiveness
The fundamental physics of evaporation supports the concept of a humidity tray, but the practical effect in a typical living space is often minimal. Although the tray releases water vapor, the small surface area limits evaporation. The surrounding room air, which is often dry and constantly moving due to heating or airflow, rapidly disperses the added moisture.
Studies confirm that the humidity increase is highly localized and short-lived. A humidity tray typically raises the relative humidity by only 5 to 10% within a few inches of the water. For plants with significant height, the majority of the foliage remains in the ambient, dry air, receiving little benefit. While humidity trays technically create a microclimate, they are often insufficient for plants requiring consistently high humidity levels, such as 60% or more.
Maximizing Results and Avoiding Common Mistakes
To maximize results, precise setup is required to prevent serious issues like root rot. The most common mistake is allowing the base of the plant pot to sit directly in the standing water. This wicking action keeps the potting mix saturated, suffocating the roots and inviting fungal infections. To avoid this, the pebbles must elevate the pot’s drainage holes at least a quarter-inch above the water line.
The effectiveness of the tray increases with the surface area of the water exposed to the air. Choosing a tray that extends significantly beyond the diameter of the plant’s foliage will maximize the evaporation rate. Grouping several plants with similar moisture needs together on a single, large tray also enhances function. Clustering plants reduces air movement, allowing the combined moisture from the tray and the plants’ natural transpiration to build a more stable, localized humid zone.
Alternative Methods for Increasing Houseplant Humidity
When a humidity tray proves inadequate for tropical species, several other methods offer a more significant and consistent increase in atmospheric moisture. The most effective solution for raising the humidity of a larger area is an electric humidifier. These devices release water vapor, allowing users to precisely control and maintain relative humidity across an entire room for extended periods. Placing plants in naturally humid areas, such as a bright bathroom or kitchen, can also leverage existing household moisture.
Grouping plants closely together is a simple technique that utilizes collective transpiration to create a more effective microclimate. The moisture released by each plant becomes trapped within the cluster, leading to a measurable rise in local humidity levels. For species requiring extremely high and stable moisture, such as certain orchids, using an enclosed environment like a terrarium or a clear cloche provides the best results. These sealed systems effectively trap water vapor, consistently maintaining humidity levels near 90%.