Reducing Winter Kill in Walnut

Straying from the usual almond entries, I thought I would address the issue of winter kill on walnut. There have been a lot of new walnut blocks planted through Merced and surrounding counties.

Winter kill damages trees that are growing very vigorously in late fall. Damage can occur on both young and old walnut blocks, but young trees are more frequently damaged. Prevention of winter kill requires two steps - hardening off the tree by stopping the new growth by late September, then re-applying water to ensure adequate soil moisture before the first frost. Specific recommendations are below:Younger Trees: In mid- to late September, stop irrigating the trees until the the reddish growth has stopped and the terminal bud has formed. After this point is reached, dependent upon soil moisture, the normal irrigation program should be re-started to prevent the loss of leaves. Water needs to be re-applied to provide adequate soil moisture.Older, Bearing Trees: At harvest, the reduction of water will stop all terminal growth. After harvest is completed, irrigate to prevent the loss of older leaves. This may require more than one irrigation if the fall is hot and dry.Winter kill is more severe when warm weather extends late into the fall, and a cold snap occurs. Without hardening off, damage has been observed on new tissue at 28F. After a few nights of cold temperatures, the trees will be capable of withstanding colder temperatures without damage.