Adapted from the article “How to Avoid Freeze Damage in Young Walnut Trees” by Katherine Pope, UCCE Farm Advisor Sacramento, Solano and Yolo Counties in the September 2014 Sacramento Valley Walnut News.
It’s tempting to push young tree growth as long as possible in the fall, but that’s risky. In severe cases of freeze damage, young trees can die back almost to the roots. Freezing temperatures can also severely damage trees in mid-winter if the trees are in dry soil.
Vigorous young trees are more vulnerable to freeze damage than older trees. Fully dormant walnut trees can withstand temperatures well below freezing. The problem comes when temperatures drop to 28o F or below before the trees have experienced a few nights near 32o F to become acclimated. Trees can be managed to encourage cold hardiness by following these key steps:
- Don’t apply nitrogen fertilizer past August. This avoids encouraging new, tender growth.
- Withhold irrigation during September until a terminal vegetative bud sets.
- Consider protective painting with white latex.
- Delay pruning young trees until March after the threat of frosts have passed.
Water management is perhaps the most critical step. To encourage terminal bud set, avoid irrigating in September until growth at the tips of the branches has stopped. Hold off on water until you stop seeing new growth and red leaves but not so long that you get yellowing leaves and defoliation. Once growth has stopped, resume irrigating in October if there is no rainfall. Make sure soil is moist before the possibility of a November freeze event.
In addition to water management, painting young trunks and shoots white can prevent freeze damage if applied before a freeze event, and can minimize damage if applied as soon as possible after a freeze. Research by UC Walnut Specialist Bruce Lampinen has shown painting after leaf fall with white interior latex paint diluted 50% with water minimizes damage to shoots and buds, especially on the south-west side of the tree. The paint moderates large day-to-night temperature fluctuations after sunny winter days.
If you don’t paint early, painting trees after a freeze can still help decrease severe damage. Wilbur Reil, Farm Advisor Emeritus, found that when trees were painted a week after a freeze event 18% showed damage, compared with 46% damaged in the unpainted trees. Painting any parts that may be damaged should improve recovery and protect against winter sunburn of affected tissue. To know if you have freeze damage that warrants painting, look for darkened cambial tissue below the bark. Symptoms resemble sunburn. It’s most likely to be found on the south and west side of the tree, especially on horizontal branches.
For almost a century, it’s been observed in California that after a freeze event walnut trees on dry soil are more likely to suffer damage than trees on moist soil. Hopefully, we’ll have a wet fall, but if not, this risk will be important to keep in mind. Walnuts on lighter soils are thus particularly susceptible. If rainfall is minimal in November and December and temperatures threaten to drop below freezing, irrigate to moisten soil.