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Care for
lawns during periods of extreme heat?
When a plant, irrespective of whether it is your
prize pelagonium, or just a single grass plant in a lawn,
suffers heat stress, many of the fine hair roots fall away
from the root system. This is the plant’s normal response to
stress, when biological functions shut down and leaves begin
to wilt.
As humidity around the leaf zone decreases, the
mechanism of plant transpiration causes water to pump
vigorously from roots to leaves to maintain humidity. If
evaporation exceeds transpiration then the plant shuts down.
Leaves wilt as the stomata close. The chloroplasts, the food
factories in a leaf, stop manufacturing sugars and starches
via the sun’s energy, and the plant goes into shock. The
biological system shuts down, and the plant effectively goes
into dormancy. Many of the fine root hairs drop off and the
plant is unable to provide adequate water or nutrient to
sustain life. Compare the myriad of fine root hairs in a couch
grass stolon during the active growing period with the number
of fine root hairs in winter when dormant.
It takes a long time for the plant to recover
from this kind of stress. Growth tends to be weak and spindly
because those fine roots that account for much of the root
system surface area have been lost. We see this in hydrangeas,
in fuchsias, and in turf grasses that receive a daily sprinkle
rather than deep watering. The shallow root zone is unable to
cope with extra demands for soil moisture.
The only way that the plant will recover is to
keep the soil temperature down to the optimum for that plant
where maximum growth will occur and by maintaining humidity
about the leaf zone.
Under extreme temperature conditions it is
acceptable to apply overhead watering to cool the plant. But
overhead watering in more moderate temperatures may cause
infection by fungal pathogens such as black spot and downy
mildew.
Four and seven-day weather forecasts make it
possible to care for our gardens more efficiently. Ornamental
shrubs and turf plants will survive heat better if the soil
has had a deep soaking down to the root zone 24 to 36 hours
before the temperature soars above 32oC.
Otherwise we see thin, weakened turf,
susceptible to invasion by weeds that compete for available
food, water and light much more efficiently than turf
grasses.
While kikuyu grows best in temperatures from
15oC to 30oC, a protracted spell of hot
weather over 35oC will allow soil temperature to
build up and force the plant into dormancy. Under these
circumstances, many of the fine root hairs are shed from the
stolons and the plant loses its ability to take up water and
nutrients from the soil. The kikuyu has suffered from heat
stress and has been shocked into summer dormancy. The best
management practice is not to let the soil temperatures rise
above a critical 32oC by thoroughly irrigating when
the weather forecasters first predict a heat wave, four or
five days before the heat occurs.
Excessive heat can weaken or, if soil
temperatures become hot enough, kill turf. But heat alone may
not be the only reason why turf may begin to decline in
summer. Drought stress, particularly if the turf is not
irrigated is a factor. And even if the temperature doesn’t
soar above 35o, cool nights around 12o,
dewy mornings and overcast days provide conditions ideal for a
range of diseases. Environmental stresses can be a real
challenge in our maintenance of turf.
Neylan (2000, Australian Turfgrass
Management Volume 2.2) recorded surface temperatures
of grass and synthetic bowling greens. At an ambient
temperature of 39.7° C one synthetic grass surface had a
temperature of 62° C while a couchgrass surface was 41.2° C.
On a day of 30° C a second synthetic was 50° C while the
bentgrass surface was 29° C.
It’s probably a good idea to keep a soil
thermometer handy, and to use it when the temperatures start
getting uncomfortable. Soil temperatures measured at 25 to 50
mm depth, for bluegrass and creeping bent are very narrow.
10o to 17oC for optimum root growth and
15o to 23oC for optimum shoot growth.
When the soil temperature exceeds 23oC the root
systems decrease by 50 percent. While the grass may look nice
and green, underneath the plant is losing its root system and
that can cause problems.
Average Soil Temperatures measured at 10 cm
depth are frequently deeper than the root zone in many lawns,
so we can expect soil temperatures to increase closer to the
soil surface (Table 1). January and February are the critical
months, when heat waves can elevate these average soil
temperatures above the 35o threshhold for
summer-active grasses. If the turfgrass goes beyond the
Permanent Wilting Point, then the lawn will die, and it is too
late to water the lawn and expect it to
recover. | 
Thin
and stringy kikuyu growth during the summer is a result of
heat stress on the lawns. This will occur when the soil
temperature exceeds the optimum temperature at which kikuyu
will grow ie 32oC. When the soil temperature goes
above this, the plant literally shuts down and goes into
dormancy, even in summer. If the soil is dry through
inadequate irrigation, then the effects of heat can be even
more severe. Most of the fine root hairs are shed and side
shoots (tillers) that give a lawn that nice, dense texture
wither and die.When soil temperatures
exceed 35o root growth ceases and many of the fine
root hairs fall away from the roots, just as deciduous trees
shed their leaves when the temperature falls below a
threshhold. The scanned image of a kikuyu stolon was taken 1
week after a heat wave with temperatures of
38-39oC.
White grubs are the larvae (the
young) of certain beetles that can cause devastating damage to
your lawn. They also attack the roots of flowers, vegetables
and ornamentals. Grubs are easy to identify but often go
unnoticed because they live underground. To identify grubs in
your lawn, look for these key features: 
Birds, especially magpies and
blackbirds are frequently digging into your lawn