Cold Protection

Julian W. Sauls, Ph.D.
Professor & Extension Horticulturist
Texas AgriLife Extension

January, 2008

The major risk to Texas citrus is the occurrence of freezes severe enough to damage fruit and trees. The Lower Rio Grande Valley experienced major tree-killing freezes in 1949, 1951, 1962, 1983 and 1989, plus damaging freezes in 1973, 1979 and 1985. Because the industry is concentrated in a relatively small geographic area, tree damage and losses during a major freeze are fairly uniform across the industry. Indeed, citrus acreage declined from nearly 70,000 to approximately 24,000 acres as a consequence of the 1983 freeze, and from 36,000 to perhaps 14,000 acres after the 1989 freeze.

The acreage reductions occurred as a consequence of both relative tree losses and economics. While all orchards suffered significant tree losses, many of those which were bulldozed were situated in rapidly developing urban areas; many others were no longer economically viable because of age, variety or other factors. Many owners, especially absentees, just gave up, especially after the second freeze of the 1980's.

Types of Freezes

There are basically two types of freezes which can damage citrus--advective and radiational. An advective freeze is characterized by a massive outbreak of very cold arctic air which blows rapidly across the plains and into South Texas. Strong, northwesterly winds and cloudy conditions predominate, usually over several days, with occasional precipitation. If precipitation occurs, strong winds cause evaporative cooling, which reduces the temperature of plant tissues below air temperature, thereby causing even more damage.

Towards the end of an advective freeze front, the skies often clear and the winds subside, which results in a radiational freeze. Without cloud cover to trap radiant heat or wind to mix the air, radiant heat energy from the earth is lost to space, resulting in very cold conditions at the surface.

Factors Affecting Freeze Damage

The extent of freeze damage is influenced by a number of factors, very few of which can be attenuated by the grower. Obviously, minimum temperature and the duration of lethal temperatures is of major importance. The 1983 freeze recorded a minimum of 16 degrees F during 72 continuous hours below freezing, while the 1989 freeze recorded a minimum of 15 degrees F in 48 continuous hours below freezing.

Had either of those freezes occurred a month later (both occurred at Christmas), damage would have been less because of increased hardiness of the orchards, as citrus trees acquire cold hardiness as a consequence of exposure to cold temperatures. In addition, the trees are usually more dormant in late January, because of shorter days and cooler temperatures overall. In the absence of winter rains, soil moisture stress usually increases through January, which induces even greater dormancy.
Freeze devastation in 1983.

Rootstock is an important consideration in other areas, but not so in Texas. While Swingle citrumelo and citrange stocks are more cold hardy than sour orange, they are not as readily adaptable to Valley soil conditions as the latter, which accounts for probably 99 percent of citrus trees in Texas. Research with other rootstocks is on-going, but nothing has proven sufficiently comparable to or better than sour orange to replace it as the rootstock for Texas citrus.

There is little difference in cold hardiness among varieties, although there are some differences between types. Navel oranges are considered to be slightly more cold hardy than round oranges, which are only slightly more cold hardy than grapefruit. Lemons and limes are considered among the least cold hardy citrus. The varietal composition of the Texas industry, which is roughly 70 percent grapefruit, 20 percent round oranges and 10 percent navel oranges, is in inverse order to relative hardiness.

Tree age (size) has considerable bearing on the extent of damage. Obviously, the older (and larger) the tree, the better it can survive major freezes. With more undamaged wood present, a tree can recover faster in terms of canopy regrowth and subsequent fruit production. In the two freezes of the 1980's, all unprotected wood smaller than about 4 inches in diameter was killed. Thus, the younger the tree, the closer to the trunk was the wood killed.

Tree density may or may not be of significant importance in attenuating freeze damage. On the one hand, higher densities may be better able to trap and retain radiant energy from the soil, thereby remaining warmer. On the other hand, higher densities are usually characterized by smaller individual trees, especially in terms of smaller trunks and scaffold and sub-scaffold limbs.

Elevation effect in a moderate freeze.

Differences in elevation can make a significant difference in freeze damage, especially in parts of California and central Florida. However, topography of the Lower Rio Grande Valley is primarily flat. In a moderate freeze, Texas orchards that are situated even a few feet above surrounding terrain may be spared damage, but not so in a major freeze event. Nor are Texas growers blessed with an abundance of large lakes, which can attenuate freeze damage to orchards to the south-southeast side.

Overall orchard condition and cultural practices are also important factors in freeze damage. Healthy, well-managed trees are simply better able to withstand cold temperatures and can recover from damage more rapidly. In addition, orchards under complete trunk-to-trunk herbicidal weed control are warmer than orchards which have weedy middles or those in which weeds are controlled by tillage.


Orchard Freeze Protection

Once the orchard is planted and winter approaches, all of the preceding factors that can affect the extent of freeze damage have been established. Consequently, the citrus grower in Texas has very few viable options to help himself and his orchards to survive.

Insurance

First among these is insurance--both tree insurance and crop insurance. These are not the same, as tree insurance covers damage to the trees only, while crop insurance covers the crop itself, both the existing crop and possible future crops. Obviously, neither will prevent freeze damage, but they do provide the necessary funds to aid in the recovery efforts. There are differing levels of coverage (and different premium costs) from which the grower must choose--but coverage must be selected and obtained well before the onset of a potential casualty. In other words, you can't just go out and buy the insurance when a freeze (or hurricane) is already being forecast.

A number of local entities provide both tree and crop insurance, so growers are well-advised to consult with agents about available coverages, costs and payments. You should determine exactly what constitutes an insurable loss, especially in terms of tree damage. For example, the loss of all fruit, leaves and small twigs is generally not considered to be a loss under terms of the insurance. Too, if your orchard has microsprayer irrigation, ask specifically about the relationship between an insurable loss and any freeze protection provided by the microsprayer system, as microsprayer operation may prevent loss to the insured part of the tree.

Soil Banks

Soil bank for freeze protection.

The trunks of trees younger than 2 or 3 years old can be partially protected by soil banks, thus providing a basis for regrowing a new top following a killing freeze. Soil banks are almost failsafe, but they are difficult to install and remove after a couple of years because of the size of the tree canopy. Too, the labor cost involved in banking and unbanking has practically eliminating this means of cold protection in Texas citrus.

If used, soil banks should be in place from about Thanksgiving through the end of February. Prior to installation, the tree trunks should be treated with an appropriate insecticide for ant and other insect control and a fungicide to preclude Phytophthora infection. Banks should be checked prior to a forecasted freeze, as strong winds blowing against a young tree sometimes result in a hole being wallowed out in the top of the bank next to the trunk. To be most effective, the soil at the top of the bank must be in complete contact with the tree trunk.

Tree Wraps

Damage under foam wrap.

Many types of tree wraps are used in the Texas citrus industry. Materials include fiberglass and various synthetic foams. Growers typically install tree wraps soon after planting, leaving them in place until the trees are about 4 years old. Prior to installation, the tree trunks are treated with an insecticide/fungicide mixture to prevent ants, other insects, and Phytophthora. Still, wraps must be inspected periodically for ant infestations, as ants sometimes construct their earthen nests at the top of the wraps, which can result in Phytophthora infection within the scaffolding of a young tree.

While tree wraps are quite effective in preventing sprouting along the trunk and in protecting the trunk from sandblast, herbicide injury and rodent damage (mostly jackrabbits), most are not very effective in cold protection. Because most tree wraps are made from insulating materials, they can only provide a few degrees of protection--and that for only a few hours at most.

Reese Insulators in young orchard.

Consequently, such tree wraps may protect young trees in a moderate freeze of relatively short duration, but cannot be expected to provide the protection needed during a severe freeze.

There is a tree wrap (Reese Insulator®) made of polystyrene which incorporates a reservoir of heat-retaining liquid. It costs considerably more than other wraps, but it provides freeze protection comparable to that of soil banks. A similar wrap was developed by scientists at the Texas A&M University-Kingsville Citrus Center some years ago, but its production was not commercialized.


Wind Machines

Wind machines can be effective during radiational freezes in which strong temperature inversions occur, i.e. temperatures of the air above an orchard are significantly higher than temperatures within the trees. While radiational freezes occur in Texas, including at the end of major advective freezes, strong temperature inversions do not always occur.

Wind machines can also be effective in localized freeze protection in what is known as cold pockets, i.e., areas where colder air settles. The relatively flat topography of the Lower Rio Grande Valley, however, does not favor such cold pockets.

Thus, the limited benefit provided by wind machines and the overall economic costs of ownership and maintenance precludes their use in Texas citrus.

Orchard Heaters

Orchard heaters and fuel blocks were once common in Texas citrus orchards and their use could provide significant cold protection. Indeed, orchard heaters were commonly used successfully to protect even the fruit from freeze damage.

However, as costs of the heaters, the labor to service them and the fuel they burned began to increase, growers could no longer afford to use heaters to protect the fruit. Ultimately, costs escalated to the point that it was no longer economical to use orchard heaters even to try to protect the trees. The few orchard heaters that remain in Texas citrus today are mostly in the hands of nurseries which may use them to try to protect field nursery trees.

Preventing Ice Formation

Freeze damage occurs when free water in plant tissues freezes, thus forming ice crystals which rupture cell membranes and cell walls. In the absence of ice nucleating agents, water will supercool, i.e., remain in liquid form at temperatures well below freezing.

At one time, there was promising research into the control of ice-nucleation-active microbes in citrus, which could result in less freeze damage by preventing the formation of ice crystals. Unfortunately, this approach to freeze protection did not achieve sufficient success to warrant further work.

Irrigation

Flood irrigation can provide some measure of freeze protective during a radiational freeze, but such potential protection could be negated in an advective freeze. The problems with flood irrigation, however, essentially preclude its use for freeze protection. Even if sufficient water were available, it would be nearly impossible to provide enough water quickly to the entire orchard and to maintain it for any length of time. One possible exception might be citrus nurseries, where a sufficient volume of flood irrigation water may protect some of the trees.

Microsprayer irrigation, however, is a viable cold protection option--if water, and the power to pump it, are and will be available throughout the freeze event. Growers must make arrangements with the irrigation district to assure an adequate water supply, or an alternative water source, such as a reservoir and/or wells, must be available. Too, because uninterrupted electrical service is not likely during a major freeze event due to "rolling brownouts", growers with electrical pumps must have a backup generator of sufficient capacity to keep the pump running. Most growers who have microsprayer systems, however, have diesel-powered pumpsets.

Microsprayer and PVC stake. Icing from microsprayer use.

To be effective, microsprayers must deliver an absolute minimum of 20 gallons of water per minute per acre, but higher volumes would be desirable. To check the system's capacity, open it up to full volume, divide the gpm pumping rate by the number of acres being watered. If pumping capacity is borderline or below, close one or more submains, if possible. Submains to the south or southeast side of the orchard would be the preferred ones to be closed.

Application of water must begin before the air temperature reaches freezing and it must continue until the wet bulb temperature rises above freezing. If wet bulb temperature cannot be determined, irrigation should continue until ice in the shade outside the orchard begins to melt.

It is important to understand that ice does not protect the tree--protection results from the continuous freezing of the additional water that is being applied. As new water is applied and freezes, the ice layer gets thicker and thicker--but the temperature of the tissue under the ice layer is maintained at or just below the freezing point. If the water supply is interrupted, the temperature of the tissue beneath the ice will quickly drop, usually to levels below air temperature.

For freeze protection use, the microsprayers should be raised to 3 or 3.5 feet above ground and near the center of the tree on its north side. The emitter of the microsprayer should not be higher than about 42 inches above ground--to raise it any higher risks leaving the base of the tree unprotected. Most growers installed 30- to 36-inch lengths of half-inch PVC pipe, into which the microsprayer stake can be optimally placed for freeze protection.

There is also a double-sprayer system in which the normal microsprayer stake is installed in the irrigation position and a second assembly is installed in the cold protection position. A small selector valve on the lower assembly directs the water to one or the other.


Nursery Freeze Protection

Because cold damage to orchards cannot be completely prevented, citrus nurseries must be protected by whatever means are available so as to provide the new trees required for replanting in the wake of a severe freeze. Container nurseries are usually covered and enclosed during the winter, but a far greater number of trees are still produced in field nurseries.

While field nurserymen usually dig and move indoors as many trees as possible before a major freeze event, the undug trees and unbudded rootstocks should also be protected. Unlike the past, the Texas industry now has a Citrus Budwood Foundation which will protect both its foundation trees and its increase blocks during freeze events, so nurseries should be assured of a supply of certified budwood. However, nurseries must have buddable rootstocks in order to take advantage of this opportunity.

Soil banks put up with border machines are especially effective for unbudded stocks. If necessary, the tops can be cut back to facilitate tractor operation. Smaller seedlings and budded trees can be protected with polyethylene--the ends and sides will have to be well-anchored to resist being blown off during strong winds. The plastic can be allowed to rest atop the plants, although tissue which touches the plastic is likely to be killed. Precipitation would complicate matters by collecting atop the plastic, which could break a number of plants.

The key to successful nursery protection, obviously, is preparation. Nurseries should have a plan in place as to how and what they can protect--and have the necessary materials, equipment and personnel on hand to accomplish it quickly, should the need arise.


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This page last updated January 7, 2008.