Citrus Freeze Injury and Care

Monte L. Nesbitt, Extension Program Specialist - Pecans & Fruit

The 2009-’10 winter season has been tougher on citrus plants in Texas than other winters have been for quite some time. The Rio Grande Valley has not been affected by low temperatures to date this year, but Houston had snow in early December, and then the big arctic blast of early January pushed damaging temperatures through most of the Gulf Coast, Hill Country, and Winter Garden regions. Temperatures in the Hill Country fell below 10°F; Uvalde experienced lows of 13-16°F, and the Houston-College Station-Beaumont areas saw lows of 17-20°F. Additionally, hours below freezing were more protracted than usually seen in south Texas during advective freeze events. Mainstream or popular types of citrus grown in these areas, including sweet oranges, grapefruit, Meyer lemons, most tangerines and mandarins, will have some level of freeze injury from these temperatures if they were not given some type of protection. Satsumas, kumquats, Ichang lemons, Changsha tangerines, Yuzu, and the Poncirus trifoliata hybrids will all fare better, having greater cold hardiness. Of this latter group, Satsumas and Changsha will incur varying amounts of damage below 23°F, depending on age, health, site, duration of the low temperature, but could survive temperatures as low as 13-14°F, depending on acclimation.

Fig 1.

Very minor freeze injury to citrus leaf.

Fig 2.

Moderate freeze injury, citrus leaf.

Fig 3.

Abscission of citrus leaf blades, with petiole stubs attached to stem, caused by cold temperatures.

Fig 4.

Frozen citrus leaves, turning brown and remaining on the tree, indicating leaf and stem injury.

Fig 5.

Magnified view of Asian Citrus Psyllid, nymphs and string-like honeydew.

How do you know whether a citrus plant has been injured by freeze?

Damage to the plant progresses from dead spots on leaves that also remain on the plant (Fig. 1), broad dead patches on leaves (Fig. 2), abscission of the entire leaf blade, leaving behind a petiole stub (Fig. 3), and dead leaves adhering to the stems, with both leaves and stems turning brown (Fig. 4). Minor dead spots to leaves or even loss of foliage indicates that the tree should recover and grow. In cases where even 90+% of the leaves fall off but stems are not damaged, the tree will force new buds and produce abundant flowers. However, fruit set (retention of fruit) will be very low to none, depending on how many leaves were lost. If leaf loss is 30% or less, a normal crop of fruit may be even be produced. Plants that have curling and browning of leaves that adhere to the tree, as in Fig. 4., likely have extensive woody stem damage, and for young trees can even mean complete death (to the ground).

No immediate action is needed when freeze injury is suspected. There is no benefit to pruning the plant until spring growth commences, and the full extent of injury is manifested. Pruning may in fact be counterproductive by stimulating faster bud activity before the danger of additional frost/freeze events has truly passed. On young plants where leaves are adhering and serious damage is suspected, scratching lightly through the bark close to the ground to look for green phloem tissue. If finding such above the graft union, the plant can still be saved from additional freezes this winter by wrapping with insulation or burying with a mound of soil. After late winter and spring freeze risk is over, pruning of dead limbs and branches can be done as needed anytime through the spring and summer. It is often wise to delay pruning until the fruit has set (May) to know exactly how much fruit is set and where on the tree it is. Prune dead limbs and twigs back to live tissue before mid-summer to decrease the inoculum level of Melanose and wood-rotting fungi in the tree.

Not all freeze injury in citrus shows up quickly. In some cases, even where leaf damage was minimal, bark and phloem injury may occur to one side of branches or trunks. Such injury partially girdles the limb or even the canopy, and the tree may grow and flower normally for several weeks only to show sudden stress or even death when exposed to high summer temperature. In such cases where trunk injury can be identified, reducing crop load and pruning trees to lessen transpiration demand can improve survival.

Citrus trees that are frozen to the ground often lose the graft, and the suckers that regenerate from the stump must be re-budded or grafted. It is important to insure the identity of the suckers that arise to identify the rootstock variety. In many cases of freeze damage, the rootstock should be replaced and better results will be obtained by replanting altogether.

Freeze-injured citrus need good water and fertilizer to restore foliage or regrow the canopy. For large trees, rates of fertilizer should be moderate if extensive canopy dieback occurred, because in the absence of fruiting, vegetative growth can be rampant. Use balanced fertilizer and micronutrients to replenish nutrient reserves. Excessive fertilizer, especially late in the spring or early summer, will cause strong flushes of growth that are targeted by Citrus Leafminer and the Asian Citrus Psyllid (Fig 5), a new pest in Texas that vectors Citrus Greening Disease. Psyllids are attracted to vegetative flushes of growth, and citrus growers in Texas with abundant vegetative growth should be inspecting flush growth for psyllids. If psyllids are suspected or discovered, growers should contact their local county extension office for verification and control recommendations.

Fig 1. Very minor freeze injury to citrus leaf. Fig 2. Moderate freeze injury, citrus leaf. Fig 3. Abscission of citrus leaf blades, with petiole stubs attached to stem, caused by cold temperatures. Fig 4. Frozen citrus leaves, turning brown and remaining on the tree, indicating leaf and stem injury. Fig 5. Magnified view of Asian Citrus Psyllid, nymphs and string-like honeydew.