Dew, the moisture observed nearly every morning on grass blades, results from specific atmospheric conditions that cool the ground and vegetation overnight. While the presence of these tiny water droplets is a familiar sight, it raises a practical question for homeowners: does this natural moisture fundamentally help the grass grow, or is it merely superficial wetting that evaporates with the morning sun? Understanding the source of dew and the structure of a grass plant reveals the true impact of this moisture on a lawn’s health and hydration needs.
The Physics of Dew Formation
Dew forms when the temperature of a surface drops below the dew point of the surrounding air. This cooling process is driven by radiative cooling, where the grass and the ground rapidly lose heat to the clear night sky. When the grass blades cool below the temperature at which the air holds moisture, water vapor condenses directly onto the surface as liquid droplets.
This moisture is sourced from the atmosphere, not pulled up from the soil by the plant itself. The formation is a passive physical process relying on a temperature gradient and high humidity near the ground. Calm winds also aid the process by preventing warmer, drier air from mixing with the cooled layer above the turf.
Foliar Water Uptake in Grass
Whether dew helps grass grow depends on the plant’s ability to absorb water through its leaves, a process called foliar water uptake (FWU). Grass blades are covered by a waxy, protective layer known as the cuticle, which is designed to minimize water loss through transpiration. However, some water absorption can occur through the leaf surface, especially when the plant is experiencing drought stress.
Studies indicate that perennial grasses are capable of FWU, providing temporary relief during dry conditions. This absorption pathway is a secondary mechanism, not the primary way a grass plant sustains itself. The limited water absorbed primarily reduces water stress by preventing the plant from losing internal moisture, rather than supplying new water for growth.
Dew Versus Deep Root Hydration
The water provided by dew is minimal compared to the requirements for healthy, sustained grass growth. Daily dew deposition is often measured in fractions of a millimeter, averaging around 0.2 mm in some temperate grasslands. While this moisture contributes a small percentage to the overall water budget, it is temporary surface moisture that quickly evaporates after sunrise.
For a turfgrass plant to thrive, it requires deep root hydration. Dew does not soak into the soil sufficiently to meet the needs of the deeper roots, particularly during periods of high heat or drought. An established lawn needs the equivalent of about 25 millimeters of water per week, and dew cannot reliably deliver more than a small fraction of this amount. Therefore, dew is not a substitute for proper irrigation, which is necessary to encourage deep root growth.
The Connection Between Dew and Lawn Disease
While dew offers little hydration, its prolonged presence creates a favorable environment for turfgrass pathogens. Fungal diseases, such as brown patch and dollar spot, thrive when grass blades remain wet for extended periods, typically for 10 to 12 continuous hours. Since dew often forms late at night and lingers into the morning, it provides the necessary moisture duration for fungal spores to germinate and infect the turf.
This surface moisture is a primary trigger for many common foliar diseases. Homeowners can mitigate this risk by watering lawns only in the early morning. This washes existing dew off the blades and allows the sun and air movement to dry the turf quickly. Removing this film of moisture is a cultural control practice to reduce the severity of fungal infections.