Condensation on Leaf and Flower Surfaces | Nursery and Flower Grower

November 1, 2018
By Steven A Tjosvold

Good air circulation in a crop canopy has many important positive impacts. As described in previous blogs, carbon dioxide utilization and consequently photosynthesis is improved. Moreover, temperatures within the leaf canopy and greenhouse are more uniform, and leaf temperature will be closer to the immediate surrounding air temperature. This has a profound impact on when and where condensation occurs as the greenhouse air temperature falls.

Article on RH Figures and Tables — Line chart titled “Temperature, Humidity, and Leaf Wetness” showing changes over time from 12:00 PM to 1:00 PM. Red line (relative humidity) starts around 70%, drops to about 40% at 5:00 PM, then rises to near 100% overnight before decreasing again to about 60% by 1:00 PM. Green line (temperature in °C) stays relatively steady between about 20–32°C, dipping overnight and rising again by midday. Yellow line (leaf wetness) is low in the afternoon and evening, increases sharply overnight to peak in the morning, then declines by early afternoon.

Above are actual temperatures, relative humidity and leaf wetness data I measured in a greenhouse in a 24 hour period. As temperature (green line) climbs in the day the relative humidity (red line) falls. As temperatures fall at night the relative humidity rises. The relative humidity is the amount of moisture in the air compared to what the air can "hold" at that temperature. When the air can't "hold" all the moisture, then it condenses as dew. Leaves get wet when water-laden air cools at night and falls to dew point temperatures. So you can see as relative humidity gets high, approaching 100%, then leaf wetness also increases as dew is formed.

Water condenses first on the coolest surfaces such as the greenhouse structure, and usually crop leaves and flower buds too. Greenhouse and plant surfaces that face the night sky are radiating and losing energy, and can become cooler than the ambient air temperature. Thus they are prone to become surfaces that support condensation as illustrated below.

radiate from leaves — Illustration of a greenhouse at night showing heat loss and condensation. Yellow arrows indicate heat radiating upward through the roof toward the night sky (moon and stars shown). A highlighted section of the roof shows cooling of the glazing. Inside, plants sit beneath the roof. Text explains that greenhouse glazing radiates energy to the night sky, can become cooler than the air, and that condensation often occurs.

radiate from greenhouse surface — Diagram showing plants losing heat at night. A tall plant and a top-down view of a leafy canopy are highlighted with red circles. Yellow arrows point upward toward a moon and stars, indicating heat radiating from the plants to the night sky. Text explains that plants can radiate energy at night and become cooler than the surrounding air.

condenseonrose1 — Close-up of green plant leaves with a waxy surface and visible veins, attached to reddish stems. Tiny water droplets are scattered across the leaf surfaces, giving them a speckled appearance.

Condensation occurs first on the coolest surfaces as air cools and approaches the dew point. Above, dew formed first on the top of rose leaves near the top of the rose plant (which faces the night sky).

Moving air within the leaf canopy at night helps move the relatively warmer greenhouse air around the cooler leaf and flower surfaces. Even very small temperature effects matter. The result is that condensation on those important crop surfaces may be prevented or reduced.

This has strong implications for disease control.....which will be described in next week's blog.