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Rehabilitation of Freeze-Damaged Citrus, Avocado and other Sub-Tropical Fruit Trees

As in the freeze of 1990, your trees must be cared for in the same way during this post freeze period. In
1990, advice was issued to the grower about the rehabilitation of their trees, both citrus and avocado. We
would like to review that information for you at this time. How can we best aid tree recovery so that tree
growth and yield will proceed most rapidly?

Citrus and avocado leaves appear wilted or flaccid during periods of low temperature. This is a natural
protective response to freezing temperatures and does not mean the leaves have been frozen. Leaves will
be firm and brittle and often curled when frozen. Leaves become flaccid after thawing, and if the injury is
not too great, they gradually regain turgor and recover, leaving however, dark flecks on the leaves.
Seriously frozen leaves collapse, dry out, and remain on the tree. Foliage form recent flushes are most
susceptible to this damage. If twigs or wood have been seriously damaged, the frozen leaves may remain
on the tree for several weeks. If the twigs and wood have not been damaged severely, the leaves are
rapidly shed. Trees losing their leaves rapidly is often a good sign and is not, as many growers believe a
sign of extensive damage.

Cold damage to the twigs appears as water soaking or discoloration. In older branches and trunks it
appears as splitting or loosening of bark where the cambium has been killed. Bark may curl and dry with
many small cracks. Dead patches of bark may occur in various locations on limbs and trunk.
Sensitivity to frost is dependent upon many variables. In general, mandarins are the most cold hardy
followed by sweet orange and grapefruit. Lemons are very frost sensitive with Eureka decidedly more
sensitive than Lisbon. For avocados, Hass is about as cold tolerant as lemons, while Bacon is more cold
tolerant. Limes are the least cold hardy. Healthy trees are more tolerant than stressed ones. The rootstock
also imparts sensitivity onto the scion.

Injury to the foliage and to young trees may be immediately recognizable but the true
extent of the damage to larger branches, trunks, and rootstocks may not appear for on to
four months following the freeze. No attempt should be made to prune or even assess
damage from the frost until spring when new growth appears.

The only treatment that should be done rapidly after a freeze is whitewashing. Often the
most sever damage following a freeze results from sunburn of exposed twigs and
branches after defoliation. Avocados and lemons are the most susceptible to sunburn,
oranges not as much; but, if the tree has been defoliated, applying whitewash would be
precautionary. Temperatures do not have to be extremely high to cause sunburn.

Pruning should be carried out to prevent secondary pathogens and wood decay organisms
from slowing tree recovery. Again, however, there should be no rush to prune. Premature
pruning, at the very least, may have to be repeated and, at the worst, it can slow tree
rehabilitation. It should be remembered that when pruning, all cuts should be made into
living wood. Try to cut flush with existing branches at crotches. Do not leave branch
stubs or uneven surfaces. Tools should be disinfected in bleach or other fungicide before
moving on to the next tree.The extent of pruning is dictated by the amount of freeze damage:

Light Damage
Where only the foliage
and small twigs are
injured,pruning is not

Medium Damage
Where a considerable
part of the top has been
killed but the trunk and
main crown limbs show
little damage, branches
should be removed back
to living wood above
vigorous sprouts

Severe Damage
Where the top and crown
limbs are severely
damaged but there are
sprouts above the bud
union, the tree should be
cut back to the
uppermost sprout.

Extreme Damage
Where trees are killed to
the bud union or the
rootstock has been
girdled, the trees should
be removed and replaced
with new trees.

Irrigate carefully! Remember that when leaves are lost, obviously evaporation from
leaves is greatly reduced, and, therefore the amount of water required is also greatly
reduced. A frost-damaged tree will use the same amount of water as a much younger or
smaller tree. Over irrigation will not result in rapid recovery. Instead, it may induce root
damage and encourage growth of root rotting organisms. This is particularly true for
avocados. Irrigation should be less frequent, and smaller amounts of water should be
applied until trees have regained their normal foliage development.

Fertilization of freeze-damaged trees should be carefully considered. There is no
evidence to indicate that frozen trees respond to any special fertilizer that is supposed to
stimulate growth. If trees are severely injured-with large limbs or even parts of the trunk
killed-nitrogen fertilizer applications should be greatly reduced, until the structure and
balance of the tree become re-established.
Trees should be watched for evidence of deficiencies of minor elements. Deficiencies of
zinc, manganese, copper, and iron are most likely to develop. For citrus, these materials
should be applied as sprays, and they should be used as often as symptoms are observed.
Two or more applications may be required the first year.

Reprinted from the Cooperative Extension of Ventura County. For the full transcript please visit the following link


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