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.