The moisture coating a lawn at sunrise, known as dew, is often assumed to be a natural form of morning irrigation that hydrates the grass blades. However, the relationship between this surface moisture and the grass plant’s physiological needs is complex. This raises the question of whether dew actually contributes meaningfully to the plant’s overall hydration.
How Dew Forms and Grass Structure
Dew is not precipitation, but rather atmospheric water vapor condensing directly onto the grass surface. This process begins with radiative cooling, where grass blades lose heat rapidly to the night sky, causing their temperature to drop below the surrounding air’s dew point. The dew point is the temperature at which air becomes saturated and water vapor changes phase into liquid droplets. Since grass blades cool faster than the soil or air, they become the preferred surface for condensation.
The physical structure of a grass blade is designed to resist water uptake from its surface. The outermost layer, the epidermis, secretes a protective, waxy covering called the cuticle. This cuticle is a hydrophobic, water-repellent barrier composed of lipid polymers and waxes, whose primary function is to prevent water loss from the plant tissue. The presence of this waxy layer causes the condensed water to bead up into distinct droplets rather than soaking into the leaf.
The Mechanics of Water Absorption
Grass plants absorb the vast majority of their water from the soil through their root systems. This absorption is governed by water potential, which dictates that water moves from an area of higher potential energy (the soil) to an area of lower potential energy (the roots) via osmosis.
Once inside, water is drawn upward through the xylem tissue by the transpiration stream, a continuous “pull” created by the evaporation of water vapor from the leaves’ internal air spaces. This transpiration results in a significantly lower water potential within the leaf tissue compared to the root system, maintaining the gradient that pulls the water upward against gravity.
The water potential of the dew resting on the leaf surface is relatively high, closer to that of pure water, and the waxy cuticle acts as a physical shield preventing this water from moving into the leaf cells. While plants can absorb some nutrients through foliar absorption, this mechanism is inefficient for water uptake. The integrity of the grass cuticle severely limits the entry of liquid water. Even the small amount of water that may enter through the stomata is too minimal to replace water lost through transpiration or fulfill the plant’s daily hydration needs. Therefore, dew remains external, failing to contribute meaningfully to the grass’s internal water balance.
The Actual Impact of Dew on Grass Health
Since dew does not significantly hydrate the grass, its main ecological consequence is creating a prolonged period of leaf wetness. This surface moisture is a major environmental factor that directly affects the health of the turfgrass ecosystem. The presence of free water on the leaf surface provides an ideal microclimate for fungal pathogens.
Fungal spores, such as those causing diseases like dollar spot, rust, or brown patch, require continuous moisture to germinate and initiate infection. The longer the dew remains on the grass blades, the greater the opportunity for these pathogens to spread. Turf managers often employ cultural practices, like mowing or dragging a hose across the turf in the early morning, to physically displace the dew and reduce the incubation time for these diseases.
Dew does offer a slight, temporary benefit by reducing the rate of transpiration. The layer of moisture on the leaf surface temporarily slows the escape of water vapor, which provides a momentary cooling effect and conserves a small amount of internal water. However, this conservation is a passive physical effect, not an active hydration mechanism, and the long-term risk of disease usually outweighs this minor advantage.