Atmospheric Humidity: Botany for the Tropical Gardener
Jeff Cameron
Staff Writer, Tropical Gardener Magazine
Miami, Florida
Reprinted from Tropical Gardener Magazine, Spring 2001

Understanding the role of atmospheric humidity in the life and well-being of tropical and subtropical plants can make the difference between success and failure in their cultivation. A brief lesson in basic botany with respect to ambient moisture can help the gardener improve the quality of life of a host of ornamental garden and conservatory plants that thrive in humid environments.

Atmospheric humidity is the amount of water vapor present in the air. It is measured as a ratio of the quantity of vapor actually present to the greatest amount possible at the given temperature. The higher the temperature, the more water vapor the air can retain; the lower the temperature, the less it can hold. Theoretically, atmospheric humidity can range from zero to 100 percent. In reality, it ranges from 10 to 15 percent during a drought period in an arid region to 100 percent just before or during a rainfall or heavy fog.

Usually, low atmospheric humidity characterizes areas where the soil moisture supply is likely to be limited. High atmospheric humidity is found in areas of abundant rainfall. Consequently, it is often difficult to segregate or distinguish clearly between the influences of the two moisture factors - soil moisture and ambient moisture - in the life of a plant. While these influences are clearly interrelated, it is important to understand how each affects a plant's response to the environmental pressures likely to be encountered in the garden or greenhouse.

Consider a plant in a hypothetical environment where the relative humidity is 100 percent. The plant is in a state of dynamic equilibrium vis-à-vis the ambient moisture - the plant is absorbing as much water from the air as it is losing by transpiration. Transpiration is the process of water loss from a plant's leaves. It is akin to the purely physical process of evaporation, but is distinguished from evaporation in that the plant exercises a considerable degree of control over it. Both temperature and atmospheric humidity influence the rate of transpiration.

If the temperature in our hypothetical environment is increased, the air can hold more water. If more water is indeed available, then the dynamic equilibrium is maintained. But if the amount of water present in the air is fixed, then the relative humidity drops as the temperature increases. At this point the equilibrium collapses - the plant is losing more water than it is absorbing from the air. If the temperature continues to rise and no attempt is made to increase the moisture content of the air, the plant will reach a point where it is transpiring water so quickly that its roots and vascular system cannot keep up. The plant wilts, even though the soil may be wet.

Nature does not maintain a perpetual state of dynamic equilibrium. But in its native habitat, the plant in our scenario lives in a state of natural equilibrium with the environment in which it evolved. The size of its root system is in balance with the size and number of its stems and leaves. The root system is large enough to supply the water needed to replace the moisture lost through transpiration. In other words, the size of the root system is a function of the regular patterns of ambient moisture and air circulation found in the plant's habitat.

If we take this plant out of its home and place it indoors on a sunny windowsill, the change may be adverse. The relatively dry air of the average home and the intensified heat of sunshine through glass quickly increase the rate of transpiration. If the root system cannot keep up with the rate of water loss, the plant will wilt. Increasing soil moisture will not correct the problem, but will cause the roots to rot or make the plant susceptible to fungal pests. To restore balance, the relative ambient moisture must be increased, either by misting the plant, moving it to a cooler spot, placing a pan of water near it, or a combination of all three.

Over time, the plant will adapt to the reduced humidity by increasing the size of its root system and reducing the number and size of its new leaves (thus decreasing the surface area of transpiration). Horticulturists refer to this process of adaptation as "hardening." The more gradual the hardening process, the less stress a plant will endure. The less stress on the plant means the less susceptible it will be to diseases, insects and microbial pests.

When growing tropical and subtropical plants - many of which have evolved in areas of relatively high ambient moisture - it is critical to understand how changes in atmospheric humidity can affect their performance. Many varieties of orchids, begonias, ferns, fuchsias, vireyas, anthuriums, bromeliads and other ornamental plants require high humidity and resent sudden changes in ambient moisture levels. Anyone who has tried to grow maidenhair ferns ( Adiantum sp. ) in less than optimal conditions knows the sting of defeat. The gardener should be able to recognize the symptoms of inadequate ambient moisture and correct the problem with sound solutions.