Cold Protection
Julian W. Sauls
Professor and Extension Horticulturist
Texas Cooperative Extension
July, 2005
The major risk to the Texas citrus industry is the occurrence of freezes severe enough to damage fruit and trees. The Lower Rio Grande Valley experienced
major freezes in 1949, 1951, 1962, 1983, and 1989, plus moderate freezes in 1973, 1979 and 1985.
The extent of freeze injury is influenced by several factors: the minimum temperature; the duration of minimum temperature or of temperatures below the
critical level; the time of freeze occurrence; tree size (age); dormancy; the variety of scion and rootstock; and overall tree condition. For example, 3 to 4 hours
at or just below a threshold temperature may be more damaging than a brief drop to a few degrees below threshold. Moreover, a freeze in December almost
invariably causes more damage than a similar freeze would cause in late January. This is because of acquired hardiness of citrus, i.e., citrus trees can acquire
cold hardiness by exposure to cool temperatures, so trees in January usually are hardier than in December. In addition, crop losses in January are less because
fewer fruit remain on the trees as the season progresses.
The larger the tree, the less likely it is to be killed and the more rapidly it can recover. For example, the 1983 freeze killed almost all wood less than 4 inches in
diameter. Large trees with 4-inch sub-scaffolds recovered more rapidly than younger trees which had to be cut back to the trunk. Similar damage occurred
during the 1989 freeze, which was quite similar in terms of minimum temperatures and duration of sub-freezing temperatures.
Although there are no documented differences in cold hardiness among varieties, there are differences between kinds of citrus. For example, navel oranges are
considered to be slightly more cold hardy than other sweet oranges. Oranges are more cold hardy than grapefruit and grapefruit are more cold hardy than
lemons and limes. Swingle citrumelo rootstock provides more cold hardiness than sour orange, which in turn is more cold hardy than Carrizo or Troyer
citrange.
Differences in the extent of freeze damage within a relatively small area normally occur as a consequence of variable topography which affects air drainage.
Cold air settles into lower areas, resulting in more damage than at slightly higher elevations. However, general tree condition can make a major difference in the
extent of damage and the potential and speed of recovery. Orchards that are healthy as a result of having received good nutrition, irrigation, pest control and
weed control throughout the season can better withstand cold damage than orchards receiving less than optimal care.
Within the Lower Rio Grande Valley of Texas, which is essentially a delta, topography is not a major factor during severe freezes such as those of the 1980's.
Indeed, during such severe freezes, there seemed to have been little difference in damage from one end of the Valley to the other. However, the existence of
minor variations in topography in the western end of the Valley can be a factor during freezes which are at or just below critical temperatures and durations.
Orchard Freeze Protection
Once the orchard is planted, all factors of site, topography, rootstock and scion combination, tree spacing and density and all other permanent considerations
affecting cold resistance have been established. Moreover, as winter approaches, the ability of an orchard to withstand cold has been established by the cultural
practices followed during the season. All such factors may be considered as passive cold protection. In terms of active cold protection, there is very little that
the grower can do.
Soil Banks and Tree Wraps
Trees younger than three or four years old can be partially protected by soil banks or tree wraps. Soil banks and wraps are used to protect young tree trunks to
provide a basis for growing a new top following removal of the existing freeze-killed top. Soil banks are most effective, but are difficult to put up and take
down; the cost of labor is such that soil banks are not practical today. If used, soil banks should be in place from the first of December through the end of
February. Prior to banking, the tree trunks should be treated with a suitable fungicide (to protect against Phytophthora) and a suitable insecticide (to protect
against ants and other insects).
Tree wraps are commonly used in Texas orchards, usually being installed soon after the trees are planted. Tree wraps protect the young tree trunk from damage
by rodents (especially jackrabbits), sand blasting and herbicide damage, while also preventing adventitious sprouting from the rootstock and lower trunk. A
wide variety of wrap materials is available, including polyurethane, fiberglass, polyethylene and other synthetics. In terms of cold protection, however, most
wraps offer only a few degrees of protection and will not save a young tree during severe freezes. The exception is the Reese Insulator, developed in Florida,
which incorporates a reservoir of liquid inside a durable styrofoam shell; it will provide freeze protection comparable to that achieved with soil banks.
Irrigation
Irrigation can provide one to five degrees of cold protection during a radiation freeze (clear skies, light wind), but this protection would be negated by
vaporative cooling during an advective freeze (windy and cloudy). During most freezes, ground water and most surface water are warmer than air temperature
and can release radiant heat to the trees. Additional heat is released when water freezes (the heat of fusion), thereby providing added cold protection. However,
the evaporation of water during an advective freeze requires heat; thus resulting in colder temperatures of the plant tissue than if water were not involved..
Flood irrigation has been used from time to time in an effort to provide some cold protection. However, most such efforts have had little impact during a severe
freeze. To be effective, water must be applied quickly and more or less continuously throughout the freeze to provide protection. Problems of inadequate water
availability, extended application time, and saturation of the root zone limit the practicality of flood irrigation for cold protection in most orchards. Moreover,
concrete pipelines may break during irrigation under freeze conditions. Too, water may not be available from irrigation districts during freezes.
Microsprayer or microsprinkler irrigation application at a minimum rate of 20 gallons per acre per minute can provide a few degrees of cold protection. Slightly
greater protection is provided at higher rates of 30 or 40 gallons per acre per minute. Application of water must begin before the air temperature reaches
freezing and must be continued until the wet bulb temperature rises above freezing (or until ice in the shade outside the irrigated area begins to melt). Any
interruption in the application of water during the freeze, caused by either a temporary loss of power or lack of water, usually results in the freezing of many of
the emitters, even if the outage is no more than half an hour. For those reasons, most such systems use diesel pumps and are assured an adequate supply of
water, either from a permanently charged irrigation canal or from an on-site reservoir. The continuous formation of ice resulting from the continuous
application of water maintains the temperature of the plant tissue under the growing ice layer at just below freezing, thereby precluding damage to that portion
of the tree. If the water application stops, for whatever reason, the temperature of the tissue beneath the ice will drop drastically below air temperature, possibly
resulting in greater damage than if irrigation was never started.
Wind Machines and Heaters
Wind machines can be effective during radiation freezes that have a strong temperature inversion, but such inversions may not occur. Although wind machines
used to be fairly common in Texas, economics and limited effectiveness do not favor their use in citrus orchards in Texas.
Heat provided by orchard heaters and fuel blocks can be quite effective in cold protection under most circumstances. However, increased costs of heaters, labor
and fuel have made such supplemental heating uneconomical.
Preventing Ice Formation
Freeze damage occurs when ice crystals form inside plant tissues and break membranes and cell walls, thus, causing death of cells and plant tissues. Water can
supercool, i.e., remain liquid at temperatures below freezing, in the absence of ice nucleating agents. Ice nucleation active (INA) bacteria have been identified
in citrus. It has been reported that the control or elimination of certain ice nucleating bacteria can result in some control of ice formation within citrus tissues,
thereby reducing freeze damage. However, once-promising research into this approach to cold protection has not provided adequate information on which to
base recommendations; and such research has been discontinued.
Nursery Freeze Protection
Because cold damage in orchards cannot be prevented and most active cold protection measures are not economically feasible, citrus nurseries must be
protected by whatever means are available so as to be able to quickly provide the trees required by the industry following a major freeze. Container nurseries
generally are enclosed during winter and should have supplemental heat available. Field nurseries can be protected by banking, flooding, overhead sprinkler
irrigation, polyethylene row covers, orchard heaters, helicopters to mix warmer air in inversion layers or a combination of these methods.
The Texas Citrus Budwood Foundation is committed to the protection of its mother trees and increase blocks so that certified budwood will be available to
Texas citrus nurseries in the event of a major freeze. Assuming that nurseries can protect seedling rootstocks, there should be little delay in the production of
new trees for replanting.
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This page revised July 25, 2005.