Transpiration
The process by which water moves through a plant from roots to leaves and evaporates through pores called stomata. Drives nutrient uptake, cools leaf tissue, and accounts for the majority of water plants absorb.
Transpiration is the movement of water through a plant: absorbed by roots, transported up through the xylem, and released as water vapor through microscopic pores (stomata) on leaf surfaces. It’s not waste - transpiration is how plants cool themselves, how water pressure drives the uptake of dissolved nutrients, and how they manage the gas exchange required for photosynthesis.
Of all the water a plant absorbs from the soil, 95-99% is transpired. Less than 5% is incorporated into plant tissue.
The Mechanism
Water molecules in the mesophyll cells of leaves evaporate into the intercellular air spaces and exit through open stomata. This creates a slight negative pressure (tension) in the leaf cells, which pulls water molecules upward from cells below in a continuous chain through the xylem. This tension propagates down through the stem to the roots, where it drives the uptake of soil water.
The driving force is solar energy: the sun heats leaves, evaporating water, which maintains the tension gradient. Without sunlight, stomata partially close and transpiration slows. At night, transpiration nearly stops.
Stomata Regulation
Plants open stomata to allow CO2 in for photosynthesis, but every moment they’re open, water exits. In low-water conditions, guard cells flanking each stoma lose turgor and close the pore to conserve moisture. This is why plants wilt in drought - stomata close, photosynthesis slows, and eventually cell turgor throughout the plant drops.
Stomata also respond to humidity. Low-humidity conditions accelerate transpiration even at adequate soil moisture. Hot, dry, windy days substantially increase crop water needs. A tomato plant in still, humid air uses far less water than the same plant in a hot, windy, low-humidity environment.
Transpiration and Irrigation
Understanding transpiration helps calibrate irrigation. Crops need more water on hot, sunny, windy days and less on cool, overcast, humid days - regardless of what the calendar or a simple schedule says. Plants in full fruit load transpire more than young seedlings.
Mulching reduces soil moisture loss through evaporation, but transpiration happens through leaves and isn’t affected by what’s covering the soil. However, mulch keeps roots cooler and soil moisture available longer, indirectly supporting the plant’s ability to maintain transpiration rates without stress.
Transpiration in Greenhouses and Hoop Houses
In enclosed growing structures, transpiration raises humidity substantially. Without ventilation, relative humidity can exceed 90%, creating conditions favorable for fungal diseases like botrytis and powdery mildew. Ventilation in a hoop house serves two purposes: temperature management and humidity reduction from accumulated transpiration.
Guttation vs. Transpiration
Guttation - water droplets appearing on leaf margins and tips, often in the morning - is sometimes confused with transpiration. Guttation occurs when root pressure is high (well-watered plants overnight) and stomata are closed; water exits through hydathodes at leaf tips rather than stomata. It’s harmless and indicates healthy root function. Transpiration is the main, active, continuous process; guttation is a minor, passive pressure-relief mechanism.