The amount of soil moisture a plant requires for optimal health varies from plant to plant. Similar to light and the forest canopy concept, various plant types have evolved and adapted to different environmental conditions based on moisture availability.
Indoor plants that require less soil moisture have developed modified plant parts and structures to help them cope with drier conditions—similar to the way camels have evolved and adapted to dry conditions. For example, many plants native to dry (arid) regions have developed thick waxy leaves with fewer stomata, effective at storing water and reducing water loss. Sansevieria, Zamioculcas Zamiifolia (ZZ plant), Jade plant and Aloe are examples of these. Many of these plants are considered ‘succulents’—a large group of low-moisture, ornamental plants. Plants native to extremely dry areas, including cactus such as Cereus, don’t have any leaves at all. Instead they have a modified stem(s) that hold abundant moisture and carry out photosynthesis.
Some plants native to dry areas also store water in thick, fleshy modified underground roots. ZZ and Sansevieria both have these. Due to these plant modifications, these plants are adapted to drier soil conditions. Excessively wet conditions are detrimental to these ‘low-moisture’ plants as they have no adaptations for this. Although these plants prefer a drier soil, they still require watering as appropriate.
Conversely, other plant types have grown and adapted in environments with more abundant and consistent moisture. Ficus, Black Olive, Spathiphyllum, many Chamaedorea, Areca and Rhapis palms, Dieffenbachia and ferns are examples of plants that prefer conditions that are moist and rarely if ever get dry.
Some ‘high-moisture’ plants react quickly to inadequate soil moisture—Ficus drop leaves and Spathiphyllum wilt. Characteristics of many of these plants include abundant leaves with a thick, fibrous root system and are relatively fast growers. However, even these high-moisture plants that prefer abundant soil moisture can be over-watered.
Saturated versus Moist
‘High moisture’ plants prefer a moist soil—not a ‘wet’ soil. A ‘wet soil’ or ‘fully saturated soil’ is a soil that has all pore spaces filled with water devoid of air. For example, a sponge that is completely full of water and no air would be considered fully saturated. Roots need air—even high moisture plants. When no air is available, root failure occurs and rotting potential is high. For this reason, the goal is to keep the soil of high moisture plants moist (but not fully saturated) and not allow it to get dry.
Many plants fall somewhere in the middle of the soil moisture spectrum. Plants that are not considered high or low moisture plants typically prefer a thorough watering with the soil surface getting slightly dry before the next watering.
Environmental conditions must also be considered when watering plants—factors such as light, temperature, humidity, plant health, soil type, etc. will impact how much and how often a particular plant needs to be watered. In general, plants in higher temperatures and higher light will require more frequent watering than plants in lower light and lower temperatures. All of these factors need to be considered when watering plants—making watering interior plants a science. Hands-on practice is also a key to watering plants successfully.
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