Thursday, June 18, 2015

A relative humidity deficit of 20% impacts plants at 35 Celsius much more than at 20 Celsius

Hotter air holds more humidity.

Plants that acclimatize need around them a space of which 80 % is filled with water vapour ( humidity ). If the hygrometer indicated only 60 % there would be then a shortage of 20% ( 80% - 60% = 20 % ). How many grams of water vapour is this shortage of 20 %, hence how fast and by which means it can be filled depends on the temperature. Plant growers should always consider the % of humidity with the room temperature, not the % of humidity only.
At 20 C  about 60% of humidity translates 11 gm of water vapor per cubic meter of air.
In order to target our 80 % at this temperature ( a necessary % during the rooting or the acclimatization phase of tropical plants stressed by international travel ) the water vapor must increase up to 14 gm per cubic meter, which is the amount of 80 % water vapour when temperature is 20 C.

Photo credit: University of Missouri Cooperative Media Group
At 20 C this humidity gap of 20 % ( 80% - 60 % )is 3 gm ( 14 gm - 11 gm ) per cubic meter, what if the air is hotter ?

At 35 C the situation is very different : 60% of humidity is 24 gm of water vapour per cubic metre of air ( more than twice the value at 20 C ! ) and  80% of humidity with these 35 C means 32 gm per cubic metre of air !  Hence the 20 % humidity gap is much wider : 8 gm at 35 C instead of 3 gm at 20 C,  and this gap of 8/3 = 2.66 times will be 3 times at temperatures just above 35 C !

Hygrometer and thermometer are essential during the rooting or acclimatization period until the plants are released to a dryer environment. The subjective perception of heat , the " heat index " is not reliable for an " intuitive measure " of the heat and humidity levels in the greenhouse. 

Why are Geneva and Stockholm better places than Madrid or Las Vegas considering the effect of the relative humidity - temperature relationship on tropical plants ?

Hoya and other plant collections are for sale on Plant care is available in various posts of this blog. Contact : Find all Aleyagarden posts on

No comments: