Morning dew is atmospheric moisture that condenses as liquid water onto surfaces exposed to the night sky. This occurs when the surface temperature drops sufficiently, causing water vapor in the air to change phase directly into liquid droplets. This collected water is a common, non-rainfall source of moisture that many plant species utilize. The ability to collect and use this condensed water is a survival mechanism for vegetation in many global ecosystems.
The Physical Process of Dew Formation
The formation of dew is governed by atmospheric and thermodynamic conditions. The primary mechanism is radiative cooling, which occurs on clear nights when the plant surface emits longwave radiation toward the sky faster than it absorbs heat from the surrounding air or ground. This heat loss causes the temperature of the leaf surface to fall below the temperature of the air immediately surrounding it.
As the leaf temperature continues to drop, it eventually reaches the dew point—the specific temperature at which the air becomes saturated with water vapor. At this point, the excess vapor condenses directly onto the cooled surface, forming liquid dew droplets. Clear skies facilitate this process by allowing maximum heat loss through radiation. A calm atmosphere also helps by preventing the mixing of the cool, moist air layer near the ground with warmer, drier air from above.
How Leaf Surfaces Influence Dew Collection
A plant’s leaf surface actively influences how dew forms and is retained. The plant cuticle, a waxy, protective layer, determines the surface energy, leading to a spectrum of wettability from highly hydrophobic to hydrophilic. Hydrophobic surfaces, characterized by a high contact angle, cause water to bead up into distinct, near-spherical droplets. This “lotus effect” promotes self-cleaning by allowing droplets to roll off easily.
For plants that actively harvest dew, the “rose petal effect” is often employed. These surfaces are also hydrophobic, causing droplets to form distinct beads, but they possess a hierarchical micro- and nanostructure that creates a high adhesion force. This adhesion effectively sticks the droplets in place, preventing the water from being lost to wind or gravity and maximizing the time available for absorption.
Many species, particularly those in arid regions, use specialized outgrowths called trichomes, which are hair-like structures on the leaf surface. These trichomes can be hydrophilic, absorbing water directly and channeling it into the leaf tissue. Alternatively, they can create a dense, porous layer that traps and retains water vapor, increasing the surface area for condensation. Combining hydrophobic structures for initial collection with hydrophilic areas for absorption creates a highly efficient system for water capture, seen in species like Lupinus regalis.
Dew vs. Guttation: Clarifying the Difference
Dew and guttation are often confused. Dew is atmospheric in origin, resulting from external condensation of water vapor onto a cool surface, making the liquid essentially pure water. Guttation, conversely, is an internal process driven by root pressure when soil moisture is high and transpiration is suppressed, typically at night.
The water involved in guttation is not pure; it is composed of xylem sap forced out of specialized pores called hydathodes, usually located at the tips or margins of leaves. This fluid contains various dissolved inorganic and organic compounds, including salts, sugars, and amino acids, which often leave a visible white residue when the liquid evaporates. The source of the water (air versus internal circulation) and its chemical content distinguish the two phenomena.
Ecological Importance of Plant Dew
For many plant communities, especially those in semi-arid and desert environments, dew is an important water resource during prolonged dry periods. This non-rainfall water can significantly contribute to the local water budget, sometimes accounting for 7% to over 10% of the total annual water input in certain grasslands. Absorbed dew helps maintain the plant’s internal water balance, providing relief from water stress.
The moisture from dew can be absorbed directly through the leaf surface, a process known as foliar water uptake. This helps rapidly rehydrate the plant tissue and activate photosynthetic activity. Even when not directly absorbed, dew reduces the amount of water a plant loses through transpiration, conserving existing water reserves. Dew can also play a role in the initial hydration of seeds, priming them for germination in harsh desert conditions where rainfall is infrequent.