Plant Temperature
The evaporation of water through the leaves not only keeps the
plant hydraulically pumped up but also causes a temperature drop
around the plant. If the evaporation slows, the plant temperature
can get too high and destroy the plant very quickly because the
temperature is already at its upper limit.
Relative Humidity
This is a term used often but not well understood. In the absolute
simplicity, relative humidity is the amount of water in the air
around the plant compared with how much more could be added before
the leaves show droplets.
The morning dew is 100% relative humidity around the leaves;
so is a winter mist. On a warm sunny day, the air around a plant
has only about 10% of the water vapor it could carry - this means
that its r.h. is 10%. Air temperature and available moisture in
the air each changes the relative humidity.
Plants in dry areas do grow well with low relative humidity,
but they have to constantly balance how far to open the stomata
in their leaves to take in CO2 and how far to close them to stop
excess moisture loss.
In a hydroponics garden, one is trying to get the plant to go
full speed ahead. To get the plant to fully open its stomata to
draw in maximum CO2, it must be given a good average. An r.h.
of up to 70% is good. After that, fungi and moulds grow too easily.
Remember also that propane-burning CO2 generators add to the
relative humidity in an indoor space. They make almost a pound
of water for every pound of gas burned.
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