It gets hot enough in Georgia to give a tree a heat
stroke.

Really.

The north Georgia mountains average more than 20 days per
year above 90 degrees, and
the coastal heartland averages more than 100 days above 90. Such
heat loads can influence
the way trees grow. And extreme heat can injure trees.

As a rule, trees grow best from 70 to 85 degrees. Their
temperature normally runs just
above that of the air. Trees cool themselves by reradiating
heat, transferring heat to the
air and evaporating water from leaves.

The latter is a major mechanism of tree cooling. Without this
evaporative cooling, heat
radiated to the surroundings and wind cooling are the only means
of keeping trees near air
temperatures. And sometimes radiated heat from pavement and hot
breezes keep trees from
dispelling the heat.

Heat makes the air much drier and causes leaves to lose water
fast. As the leaf closes
down, a tree can’t cool itself as well, and its leaf tissues can
heat up above the heat
death threshold — around 115 degrees.

A compounding problem of rapid water loss and temperature
increase is a delay in the
roots’ water intake. Leaves can lose water much faster than the
roots can absorb it. A
water deficit develops in the tree and can start many
problems.

Heat injury is most prevalent during sunny middays and
afternoons. Water shortages in
the leaves partly cause a midday slowdown in leaf evaporation.
The tree corrects the day’s
shortages overnight if water is available. The roots’ night
uptake can amount to 40
percent of the tree’s water needs.

Heat injury in trees includes scorching of leaves and twigs,
and sunburn on branches
and stems. In leaves, wilting is the first major sign of excess
water loss and heat
loading. Then they’ll turn fall colors, die and fall off.

Under heavy heat loading, leaves begin to take themselves
apart, if they have time, and
then brown out and finally fall. Leaves quickly killed by heat
are usually held onto a
tree by tough xylem tissue and the lack of a prepared splitting
zone. Early leaf fall,
even of green leaves, is a symptom of water and heat stress.

The bare soil surface, or other hard surfaces, reflects and
absorbs heat. In full
sunlight, the soil surface can reach 150 degrees. It radiates
and reflects this heat onto
trees, causing tremendous heat loading. New plantings are
especially vulnerable.

Heat loading causes large amounts of water to be evaporated,
starts major metabolic
problems and can generate heat lesions just above where the
ground and tree meet. Heat
lesions usually first appear on the south and southwest side of
stems.

Trees in containers in full sunlight can be under large heat
loads that quickly injure
roots and shoots. Depending on their color, exposure and
composition, containers can
quickly absorb large amounts of heat. For instance, black
plastic containers can absorb
radiation at 9 degrees per hour until they reach 125 degrees or
more.

Treatments for tree heat stress syndrome include:

* Watering, sprinkling and misting to improve the water
supply and cool tree tissues.

* Partial shading to reduce incoming radiation and control
hot, dry winds.

* Reflecting and dissipating heat using colorants and surface
treatments around the
landscape and on trees.

* Using low-density, organic surface covers such as mulches,
ground covers and hard
surface blankets.

* Using well-designed active shade structures in the
landscape, such as arbors and
trellises.

* And establishing better tree-literate design and
maintenance practices that deal with
heat problems.

Watch your valuable trees for signs of heat damage. And give
them a cool drink of
water.