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
