Relative humidities of
around 70% are ideal for many plant species. Humidity at this level allows
for satisfactory growth rates, and is low enough to prevent many fungal
Transpiration is the process responsible for
distributing water and nutrients throughout the plant. To maintain active
growth a plant must be able to transpire easily. High humidity restricts
the transpiration rate because the air is less
able to absorb the water vapour that is trying to escape (i.e. transpire)
from the leaves. Once the air's capacity to absorb extra water is reached,
it is 'saturated' (i.e. 100% RH). At this point the plant essentially
stops transpiring which causes the supply of water and nutrients to cease.
In cucumbers and tomatoes this results in symptoms such as 'blossom-end
rot'. This is due to a calcium deficiency at the plant's extremities.
transpiration has ceased (i.e. where RH = 100%) it is common for water
droplets to form on the edges of the leaf. This phenomenon is termed
'guttation' (Fig 18.4). This is able to occur because the plant is able to
pump water by root pressure - but only if the nutrient concentration is
low enough and the temperature is sufficiently high. If guttation occurs
regularly it will become evident by the formation of small white salt
deposits on the edges of leaves. Subsequent osmosis will cause localised
burning (i.e. necrotic spots). These points are vulnerable to botrytis and
many other fungal diseases. When the humidity falls and transpiration
continues the water droplets are reabsorbed into the plant, drawing with
them any fungal spores that may be present.
How to minimise humidity in a grow room or greenhouse?
There are several collective points of action
that must be adhered to:
1. Air exchange via ventilation: The
very act of transpiration causes humidity to rise. The growing area must
therefore be well ventilated to keep the
humidity relatively constant and allow the transpiration rate to continue
achieve this, the humid air must be regularly discarded and replaced with
drier air. Although colder outside air may have an RH of 100%, once it is
drawn inside and its temperature is increased via artificial heating, its
RH will drop significantly. Good air exchange will also help ensure oxygen
and carbon dioxide levels do not
2. Minimise plant density: The more
plants there are in a given area, the quicker the humidity will rise.
Ventilation rates must be increased with increases in plant density.
3. Optimise air temperature: As the
temperature of air increases then its ability to hold more water also
increases. For example, you can lower the RH of a 15OC (59OF)
body of air from 100% to 55% by heating it to 25OC (77OF).
Also, for a given level of RH, plants can transpire more freely when the
temperature is high. For example, at 25OC (77OF) and
80% RH, the plant can transpire 5ml of water per cubic meter of air before
100% RH is reached. Alternatively, at 15OC (59OF)
and 80% RH, the plant can only transpire 2ml of water per cubic meter of
air before 100% RH is reached. Obviously to minimise humidity it would be
best to have the temperature set as high as possible, however, in order to
optimise photosynthesis it is necessary to set
the temperature no higher than around 25OC (77OF)
during daylight hours and then 5OC (10OF) cooler at
4. Circulate air within the greenhouse:
To ensure uniformity of humidity, oxygen and carbon dioxide within the
greenhouse itself, the air must be circulated by use of a fan.
What is 'Relative Humidity' (RH)?
Humidity is best described in terms of
'relative humidity' (RH). RH is simply the ratio, expressed as a
percentage, of the amount of water in the air to the amount that it can
hold when saturated. For example, 1 cubic meter of air at 30OC
can hold a maximum of 30ml of water. Therefore when it contains 15ml (at
the same temperature) it has a RH of 50% and when it contains 30ml of
water, its RH is 100%. Note, at 10OC, one cubic meter of air
holds 10ml of water. With each 10OC rise in temperature air
holds a further 10ml.