Along with receiving the correct amount of water and light, moisture in the air in the form of water vapor greatly affects plant health. Water vapor is the gaseous, invisible state of water in the air known as humidity. Like soil moisture, some plants have evolved and acclimated to very dry, arid air with little or no humidity. Many of these plants are ‘low moisture’ plants with thick, waxy leaves and other adaptations for water retention.
Other plant types, particularly ‘high moisture’ plants prefer higher humidity due to their environmental adaptations. Ferns, particularly Nephrolepis (Boston Fern) are notorious for requiring higher levels of humidity. In lower humidity, susceptible ferns will excessively drop leaves, creating a mess. This is one of the main reasons Nephrolepis ferns are not used extensively in interior landscaping. Most palm types, Ficus, Bamboo, and Schefflera, and others thrive in higher humidity. Most other non-succulent plants benefit from higher humidity because it reduces transpiration, leaf tipping, and can discourage mite pests.
Unlike soil moisture, it’s often difficult or unpractical to raise the humidity in a building. Plant selection is often the most practical method of ensuring adequate humidity. For example, if plants are to be placed in a space with low humidity, selecting plants tolerant of these conditions is advised.
Site selection involves seeking ideal locations for a plant based on its needs. Humidity levels may vary throughout a building. Locations such as pools and water features such as ponds, water fountains, water falls, etc. tend to have higher humidity levels. Plant and site selection can be summarized as Right Plant, Right Place.
If humidifiers are available, they should be used where plants are located, particularly in winter months when humidity is lowest in most climates.
Another method of raising humidity is to group plants close together in a cluster. When plants transpire, they become miniature humidifiers—when grouped together the relative humidity is increased, benefiting all plants around. Green walls often have an advantage of higher humidity due to many plants together in close proximity. Grouping plants together can be beneficial; however, if pests or disease are prevalent, plants should be spaced properly to allow air flow.
Misting plants is sometimes practiced to raise humidity. Although the humidity can be raised through misting, the effect is very short-lived and may only last 10-15 minutes. However, misting typically has a more significant impact on controlling mites.
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