There are two types of cold damage that can happen to your walnut trees when winter arrives. Autumn frost damage occurs if the temperature drops below freezing rapidly in the late fall or early winter following mild warm autumn weather. This type of autumn frost often damages vigorous non-dormant young trees.
The second type is “winter kill” which happens when extremely low temperatures occur during the winter months damaging mature and young dormant trees. Water stressed or trees planted in sandy soil are most susceptible to this type of injury.
Both types of cold damage show similar symptoms of darkening bark and streaks of grey on the inner wood. In the spring, buds are slow to break or fail to break altogether. In cases where the branch dies, the winter kill acts as severe pruning and vigorous shoots grow from below the damaged area. Sunburn often accompanies the cold damage increasing the amount of injury. In the harshest instances, entire young trees can die.
Primary steps to reduce the risk of cold injury:
• Hardening off the trees: Hardening off is a process where the tree stops growing and begins to enter dormancy. With a warm autumn and plentiful irrigation, walnut trees may not naturally harden off early enough to avoid frost damage. You should help the trees harden off by withholding irrigation in September. Hold off irrigation until the terminal growth has stopped and there are no new emerging leaves at the shoot tips. Do not let them dry out so much that leaves are turning yellow or dropping. Trees that have been hardened off are less susceptible to damage in an autumn frost.
• Irrigating for winter: Dry trees are more susceptible to cold damage so make sure trees are not stressed as they go into winter. After trees are hardened off in September, restart the normal irrigation schedule and continue until the first rains. The soil profile should be rewetted by beginning to apply the normal ETc that is needed for that time of year.
Secondary steps to take if cold injury occurs:
• Just after the freeze event: If a freeze/frost occurs in the late fall or early winter before the trees have hardened off or acclimatized, you can reduce the wood damage by painting the trees with white paint. This is effective in reducing damage to young trees after a November frost. Paint the tree trunks and primary scaffolds above the crotch with a whitewash made of white interior latex paint diluted with 50 percent water. The paint will help prevent sunburn and help heal the damaged wood by reducing evaporation from the injury. In a study by Wilbur Reil, Yolo/Solano farm advisor emeritus, 46 percent of unpainted trees sustained cold damage while only 8 percent of the trees painted 8 days after the event showed damage.
• In the spring: If you suspect cold damage, do not prune out the damaged limbs. The buds may be slow in opening or buds from deep in the bark may grow to rejuvenate the limb. In the late summer, prune out the dead wood that did not revive. New scaffolds that grew can be trained to replace the damaged wood. Reduce or delay spring fertilizer applications where cold damage is evident.