Humidity & watering
Requirements of Plants
• A very humid atmosphere reduces the rate of
transpiration and evaporation to a level that may
be harmful to some plants.
• They may suffer damage from overheating
unless cooler, drier air is brought in by
ventilation.
• Many tropical plants from humid climates,
however, require high levels of humidity for
healthy growth, and will not survive in a dry
atmosphere.
Measuring Humidity
• Warm air is capable of holding more moisture
than cold air before it becomes saturated.
• Relative humidity is a measure of the amount of
water vapour in the air expressed as a
percentage of saturation point at the same
temperature.
• A "humid" atmosphere is defined as having a
relative humidity of about 75 per cent;
• A "dry" atmosphere has a relative humidity of
about 35 per cent.
Measuring Humidity
• Wet and dry bulb
thermometers, used in
conjunction with
hygrometric tables, may be
used to measure the
relative humidity of the
atmosphere.
• Hygrometers, which have a
dial that gives readings for
both the temperature and
the humidity, are also
available.
Measuring Humidity
• As a general guide, a relative humidity below 75
per cent, but above 40 per cent, is beneficial for
most greenhouse plants during the growing
season.
• At levels above 80 per cent, diseases such as
grey mould (Botrytis) and mildew may become a
problem.
• In winter, humidity should be maintained at a
lower level, but the exact level required will
depend on the types of plant grown and the
temperature of the greenhouse.
Humidifiers
• Greenhouses may be "damped down" during the
summer by splashing water -predominantly on the floor
and on any staging -from a watering can or hose.
• This has the effect of increasing the level of atmospheric
humidity.
• An automatic spray system simplifies humidity control,
especially for plants that require very high humidity. In a
small greenhouse, mist-spraying by hand or providing a
tray filled with water that slowly evaporates into the air is
usually adequate.
Watering
• A traditional watering can is still the best method of
watering a mixed collection of plants in a small
greenhouse.
• You can readily monitor the flow, so it ensures that all the
plants are watered according to their individual
requirements.
• An automatic watering system is a useful addition to a
greenhouse in summer, if the greenhouse is left
unattended on a regular basis.
• An automatic system becomes essential, since some pot
plants may need to be watered several times a day in
very hot weather.
Capillary systems
• Watering systems that rely on capillary
action to draw up water are often used in a
greenhouse.
• Plant pots may be placed on a 2-5cm layer
of clean sand (which retains moisture well)
that is placed on the greenhouse staging
and kept permanently wet.
Capillary systems
• Capillary matting, which is widely available
in rolls and is simply cut to the required
size, is far lighter, easier to keep clean,
and just as effective as sand.
• To keep the capillary matting continuously
moist, trail the edge into a water trough or
other reservoir of water. The water may be
topped up by hand or supplied
automatically from the mains.
Trickle irrigation systems
• This type of irrigation system consists of a series of
small-bore tubes, each with an adjustable nozzle. The
tubes are placed in the individual pots or growing bags,
or near plants growing in the greenhouse border.
– Most trickle irrigation systems are fed with water from a reservoir
that is filled in turn from a hose connected to the mains water
supply. It is possible, however, to use water supplied direct from
the mains.
• The rate of water delivery must be monitored very
carefuIly and adjusted according to the needs of the
plants; these needs vary depending on time of year and
the vagaries of the weather.