As if protecting your citrus trees and tender perennials from an impending November freeze isn't enough to fill your day. Walnut trees, too, are at risk when the temperatures plummet to the 20's in November.
True, the walnut harvest is mostly done by now. But the walnut tree itself could be injured or killed by a November freeze if the tree hasn't entered dormancy.
Check your walnut trees. If you see emerging leaves at the shoot tips this time of year, your walnut tree is not in dormancy.
A prolonged, late autumn freeze following a period of relatively mild weather (remember the 70's last week, including a high in the 80's on Nov. 16) sets the table for walnut tree damage to vigorously growing, non-dormant young trees.
Trees that have been hardened off are less susceptible to damage in an autumn frost. But in the winter, even dormant, mature walnut trees can be injured by extreme cold.
According to UC Farm Advisor Carolyn DeBuse, both types of cold damage (autumn and winter) show similar symptoms of darkening bark and streaks of gray 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 a 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.
The good news: this usually happens only to walnut trees that are drought-stressed or are in dry, sandy soil. The copious rainfall we have had this autumn may lessen the chances of frost damage.
DeBuse points out that a good precautionary measure to take after a frost or freeze is to paint the trunks of the walnut trees. 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% 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% of unpainted trees sustained cold damage while only 8% of the trees painted 8 days after the event showed damage.
And just as we've recommended before with other plants that get frozen back, Dubose says not to prune out damaged walnut limbs in the spring. The buds may be slow to open. Don't prune out any suspected dead wood until the late summer. And she suggests to reduce or delay any applications of fertilizer to your walnut trees in the spring where cold damage has been spotted.
Read about more steps DuBose suggests for reducing freeze damage to walnut trees here.