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(COOPI.KA'lVI.. 1EXI ENSIO()N \ )kK IN
A(; I(R I.TCLIRE AND) 110\MI E-.( )N)\\ICS
(Acts of May 8 and June 30, 191,i)
Agricultural extension Service. University of Florida
Florila State University and
United States Department of Agriculture, Cooperating
M. 0. Watkins, Director
by Fred P. Lawrence
AGRICULTURAL EXTENSION SERVICE
UNIVERSITY OF FLORIDA
.irc uiar 1 /74
Probably the most important factor which influences
the susceptibility of citrus to freezing temperatures is
the degree of dormancy of the trees at the time extreme
cold arrives. Cold weather in itself induces a degree
of dormancy in citrus; if it comes gradually it is very
effective in increasing the trees' tolerance to freezing
temperatures. Trees in active growth are much more
severely injured by cold than are those somewhat dor-
mant. Citrus trees, being evergreens, never become
fully dormant and can never withstand temperatures as
low as those tolerated by deciduous trees.
To avoid cold injury, growers should follow cultural
practices that would induce the highest degree of dor-
mancy during winter months.
SYMPTOMS OF COLD INJURY
During periods of low temperatures the leaves of
citrus trees commonly assume a drooping or wilted
appearance but remain firm and brittle if frozen. As
they thaw the leaves first become slightly flabby. If
the injury is not too severe, they gradually regain
normal turgidity and recover. Seriously frozen leaves,
however, gradually collapse and dry out; this is ac-
celerated by warm sunny days immediately following a
freeze. The youngest leaves are the most easily in-
jured, but all leaves may be killed in a severe freeze.
If the twigs are seriously frozen the leaves dry out and
remain attached until they rot and fall off. However,
if the twigs and larger wood are not seriously injured,
an abscissionlayer is formed at the base of the petiole
and the injured leaves are shed rather promptly.
On trunks and larger branches, cold damage may
appear as splitting or loosening of the bark. Certain
areas, especially those at or near crotches, are par-
ticularly sensitive. Localized damage to patches of
bark on the trunk and larger limbs (cold cankers) are
often mistaken for gummosis, since these may exude
gum at a later date.
Rules for Care.-Rules for the care of citrus trees
that have been injured by cold must be flexible and
varied. Time of year at which the cold occurs, con-
ditions of the trees at time of injury, and weather con
editions immediately following injury will influence tc
a marked degree the type of treatment to use.
After a freeze the natural reaction is to do some-
thing right away. Actually there is very little that car
be done at that time, as it is impossible to determine
the full extent of injury. No poison or sour sap forms
passed. If, on the other hand, trees do put forth growth
it should not be allowed to wilt for the lack of water.
Remember that irrigation during warm periods in
the winter following cold injury is hazardous and
should be avoided if at all possible.
Pruning.-Cutting of dead wood from bearing trees
which have suffered heavy damage should not be done
until late spring or summer following the injury. This
delay is desirable for three reasons: first, pruning in
itself is a shock to the tree and tends to stimulate
growth and should be avoided until all possibility of
additional freezing has passed; second, pruning should
be delayed because of the difficulty of determining the
extent of the injury; and third, pruning is a costly
operation and should be delayed until an adequate
supply of trained labor is available.
When the pruning operation is undertaken, it is
well to remember that all cuts should be made into
living wood, and where possible, at crotches, leaving
no stubs or uneven surfaces. All pruning wounds over
1/2 inch in diameter should be coated with a heavy
water repellent tree paint.
It is advisable to remove heavy brush from the
grove immediately following the pruning operation.
CARE OF YOUNG TREES
With Slight Damage.-If the damage is slight and
there are some leaves and green twigs above the bank,
no special steps need to be taken. Trees in such in-
stances have sufficient living wood to develop new
tops. Even so, the banks should be removed from a
few trees and the trunks inspected to see that the bark
is firm. This process should be repeated at weekly
intervals, as damaged trees often "sweat" in the bark
and are attacked by a fungus that can kill the young
With Heavy Damage.-If the tree has been killed
to or into the bank, it may be a good practice to re-
move the bank, or as much as necessary to expose
live tissue, to allow the tree to respond, by growth,
to sunlight and air. Otherwise, the bark beneath the
bank may soften and slough off-resulting in complete
loss of the tree (the extent of this operation will de-
pend upon the acreage involved and the manpower
available.) When trees are unbanked it will be neces-
sary to rebank them when there is a forecast of freez-
ing temperatures. In rebanking, the trees should be
completely covered if the growth is tender. If growth
has progressed to a point of hardening, the tree need
Fig. 2.-Bark cracking, as shown here, sometimes
occurs. Don't try to protect your trees by sprinkling
them in freezing weather; ice is likely to form on the
trees and cause the bark to crack as a result.
not be completely covered. If no growth has occurred
or if it is still tender, the banks should be removed as
soon as the cold has passed. This process may need
to be repeated several times before winter is over.
Trees That Were Not Banked.-Trees that were not
banked at the time the cold came should not be banked
until the next forecast of freezing temperatures. It is
useless and often detrimental to bank trees immediately
following a freeze; as they are likely to "sweat" in
the bank and be more severely damaged than if un-
banked. Wait until the forecast of additional freezing
temperatures to bank.
Cultural Practices.-If damage is slight, exert
every effort to keep the young tree dormant or as nearly
so as possible. If heavy damage has occurred, in addi-
tion to removing the banks, it may be necessary (de-
pending on conditions) to water the trees. Cultivation,
fertilization and spraying should be delayed until
winter has passed.
in the tree that will back up into the living tissue and
do further damage. Following a severe freeze, twigs
and branches may continue to die for a period of sev-
eral months to as long as two years. This is caused
by injury to the bark and wood at the time of the freeze
and not by a condition of the sap. Citrus trees on
which twigs and branches have been killed by cold
should receive extra care during the following season.
Survival of the Fittest.-Ability to recover from
cold damage depends largely on the vigor of the tree.
Weak trees, showing disease, insect damage or defi-
ciency symptoms, usually are the ones most severely
One of the best ways to lessen cold injury is to
maintain healthy trees and follow cultural practices
that have a tendency to induce dormancy in the early
winter and strive to maintain this dormancy until cold
weather has passed.
Fertilization and Cultivation.-Fertilize at regular
time, but reduce amounts according to amount of dam-
age. Eliminate cultivation until after cold weather has
On severely injured trees it is a good practice to
divide the spring fertilization into two separate appli-
ations, giving only a small amount of soluble nitrogen
on the first application and following this in four to
six weeks with a complete fertilizer. In so doing, the
new growth that puts forth early will be strengthened-
>rovided it is not killed by subsequent cold-and by
he time the second application is due, sufficient time
ill have elapsed to determine the possibilities of a
rop for the next year.
If it appears that there will be little or no crop
following the freeze, this should be taken into con-
sideration and the total amount of fertilizer reduced
Where possible, groves should be cultivated and
rrigated following the late spring application of fer-
Melanose Control.-Cold injury to leaves, twigs
nd smaller branches contributes to the development
f such diseases as melanose and stem-end rot, wither-
:ip, and diplodia dieback, since the organismsrespon-
sible for these diseases forms spores in dead tissue.
Ihis is a serious problem when the cold occurs early
n the winter. When such damage occurs, it is usually
necessary to protect the new foliage, as well as any
ruit that may develop, with copper sprays. As soon
Fig. 1.-Cold can defoliate and severely damage
citrus trees of all ages. Proper aftercare will help the
trees to overcome the damage.
as new leaves are 1/4 to 1/2 inch in width it will be
necessary to spray with a neutral copper spray.
Physiological or Nutritional Sprays.-Deficiency
symptoms in deficient trees will be intensified where
they have been badly damaged by cold, due to the
drain entailed by the large amount of growth necessary
to replace the foliage lost. Because of this, a post-
bloom nutritional spray of copper, zinc, and manganese
usually will be very beneficial to both new growth and
condition of the tree. If a nutritional spray is not used,
a second copper or melanose spray may be necessary.
Spray Program.-Freezing weather does not elimi-
nate mite and insect populations on citrus, but only
reduces them indirectly by dropping the leaves on
which they feed.
Control measures should be delayed on severely
damaged trees. It should be continued where needed
on trees only moderately damaged, for uncontrolled
populations of rust mite, purple mite, and Texas citrus
mite can only increase the dropping of leaves and fur-
ther weaken the trees.
Irrigation.-If freeze damage occurs early in the
winter and soil moisture is reasonably adequate, it
will be well to delay irrigation in hopes of delaying
tree growth until the danger of additional freezes has