Home' Grower : February 2012 Contents The South Australian Grower -- February 2012
Be prepared for
Adopt commonsense strategies
By JEANETTE CHAMPMAN,
DOMENIC CAVALLARO and ANTHONY FOX
IN SOUTH AUSTRALIA, heatwaves are five or
more consecutive days of temperatures above
35C degrees. They are expected in summer, but
are now being experienced in late spring and early
autumn. They are becoming hotter (more than
40C) and longer (lasting for more than seven
days). Crops may experience water stress and heat
related damage to tissues leading to loss of yield
and quality. Being prepared for heatwaves can
reduce the extent of damage.
STRATEGY 1: Let plants cool themselves
PLANTS have their own systems for changing
temperature and humidity conditions, and they
will actively regulate their tissue temperatures by
evaporating water from the leaves and stems.
The cooling effect is the result of the water using
energy from the surrounding air to evaporate.
Crops able to actively cool themselves can readi-
ly maintain the temperature of plant tissue at least
several degrees below the outside air temperature.
Plants will need plenty of water to cool them-
selves on hot days. Make the roots grow and keep
them healthy: Plants with deep and plentiful roots
will be able to rapidly access more of the stored
water in the soil. Plants that are made to search for
water as they are being established will develop
deeper roots. Ensure that the soil is prepared to
allow easy root penetration, and that there are no
artificial barriers such as concentrated bands of
salt. Healthy roots are essential for healthy plants.
Essential plant nutrients, especially trace elements,
have vital roles in the uptake of water and process-
es that minimise heat and water stress related dam-
age of plant tissue. Plants already affected by dis-
ease, salinity or poor nutrition will be readily dam-
aged during heatwaves.
Know the amount of water that can be stored in the
soil: Readily available water is the part of soil stored
water within the root zone that crops can take up
quickly. Well structured clay and loam soil types high
in organic carbon will contain more readily available
water than sand and poorly structured soil.
Comparing the amount of stored soil water available
for uptake with the amount of water required by the
crop each day during heatwave events will determine
how regularly irrigation will need to be reapplied.
Maintain an adequate supply of stored soil water:
Irrigate before heatwave to refill the soil within
the root zone. Irrigate to replace water lost each
day using the 'banked' plant available water as a
backup supply if your irrigation system cannot
keep up with the demand for water. Most plants
can tolerate varying degrees of wilting before suf-
fering permanent damage or loss of marketable
product. Put water in the bank -- refill the soil
before forecasted heatwaves. Make each drop
count: Place the water where it is needed whether
within the root zone itself or within the drip zone
of the overlying canopy where the most abundant
roots are located.
Watering weeds along the sides and ends of crop-
ping rows and throwing the water high in the air
is a waste of a valuable resource. Get around
quickly enough: Irrigation system turnaround
times must be quick enough to avoid permanent
crop loss or damage to marketable product.
If your irrigation system is the limiting factor in
minimising crop damage during heatwave events,
it may be time to upgrade.
Keep salt out of the root zone and off the leaves:
Salt dehydrates plants.
Minimising build up of excessive salt in the root
system will help free-up water for plant uptake.
If you have access to better quality water, use it
in preference to other more salty sources of water
during heatwave events.
Information sheets on managing salinity can be
found on the HortEx Alliance Inc. web site.
Do not add to a salinity problem -- avoid apply-
ing fertilisers or other amendments that add salt
immediately before and during heatwaves.
STRATEGY 2: Block out the sun
MOST plants do not need to be exposed to full
sunlight to produce enough energy for growth
Shading is an adaptation used to strike a balance
between the need for sufficient light for growth
and the need to control potentially damaging tem-
As temperature increases, plants are forced to
reduce air and moisture exchange from the leaves
to conser ve water and avoid wilting.
This reduces the available carbon dioxide
required by plants to convert sunlight into energy
for growth and development through a process
Bright light can have the same effect.
High temperatures can also physically damage
PLANTS have a number of ways to shade vital
parts: growing a leaf canopy which acts to shade
edible stems, their flowers or developing fr uit;
covering developing fruit with protective layers or
with waxes; developing fine hairs; developing edi-
ble parts underground.
MAINTAINING a protective layer of leaf canopy over
Healthy roots essential for healthy plants
Irrigation systems limiting factor inminimis-
ing crop damage
External retractable shade systems can be
AT A GLANCE
Shading effect of white wash on right side of the
Potato crop lacking sufficient irrigation.
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