XI. RELATIVE HUMIDITYA. RH range outsideB. High RH conditions in greenhouse

• Apr--May & Aug--Sept–bright cool days–cool nights

• June--July–bright warm days–cooler but warm nights

C. Low RH conditions in greenhouse• Oct--Mar

–usually heating cooler air–dry to very dry environment

Psychrometric Chart

75%95%

46%

C. Object of control– High humidity

• Lower transpiration– leaf temp increases

• Disease potential greater

– Low humidity• Greater transpiration

– water stress possible– Stomata close– CO2 uptake less– Death

Tran

spira

tion

rate

In g

H2O

/g le

af

100% 0%

% Relative humidityOr vapor pressure deficit

Water vapor

Relative Humidity

Vapor Pressure Deficit

Object: High enough not to cause stress or limit metabolism

60%

Low enough to prevent disease

less than 85%

D. Reducing humidity

• Ventilation

D. Reducing humidity

• Circulation

D. Reducing humidity

• Increase temperature

• Ventilate and heat

• Turn off pump to cooling system

E. Increasing RH

• Shade roof

• Evaporative cooling

• Lower temperatures

• Spray walks and foliage during day

• Continue mist into night

F. Special problems1. wilting after dark weather

• Rapid loss of water from tender foliage• Roots cannot supply enough water

2. edema - oedema• Cells expand; cell wall cracks

–Warm soil; High humidity• Cause

–rapid uptake of water - low transpiration

3. Guttation

• Small drops of sap on edge of leaf• Water evaporates

– High concentration of salts• Cause

– high R.H.– rapid water uptake; slow

transpiration