Tips for winter care of young walnut trees

Wait until the threat of freezing has passed before making heading or thinning cuts on walnut trees.

Last year, newly-planted trees grew well in many California walnut orchards. By the end of the season, trunk growth on a number of them had reached 10 feet or more.

Proper care this winter can go a long way in making the most of that growth, say Janine Hasey, University of California Cooperative Extension farm advisor for Sutter, Yuba, and Colusa Counties and Bruce Lampinen, UCCE walnut specialist, based at UC Davis.

They explain how in the January issue of Orchard Facts newsletter, published by the Butte County Cooperative Extension Service.

Although pruning can help reduce wind breakage, they advise waiting to make heading or thinning cuts on young walnut trees until March when the threat of winter freezing temperatures has passed. Otherwise, freeze damage could occur lower on the trunk than if they were not pruned.

“If there is some breakage during strong winds, then a few trees get an early heading cut,” they write. “This is far better than subjecting all trees to possibly more severe lower trunk freeze damage.”

Trees tied loosely but securely to the stake so they can move and develop a stronger trunk during the growing season are less likely to break during high winds than if they are secured tightly to the stake, Hasey and Lampinen note.

The manner in which trees are painted can affect how they leaf out. A study in the winter of 2013-14 showed that bud temperatures fluctuated more from day to night on the south and west sides of the trees than on the north and east sides. Most likely this was the result of relatively warm temperatures that winter, which included many sunny days with little fog, they say. (On foggy or cloudy days, bud temperature very closely tracks air temperature.)

Daytime temperatures at the bud positions on the south to southwest side of a tree rose to 12-18 degrees (8-10 degrees C) higher than the air temperature, falling 12-18 degrees (8-10 degrees C) lower than the air temperature at night.

That compares to the north side of a tree, where high daytime bud temperatures were about equal to air temperature. At night, however, temperatures at this positions again fell 12-18 degrees (8-10 degrees C) lower than the air temperature. This fluctuation from low to high temperatures probably cancelled chilling hours on the southwest side of the shoot leading to low chill conditions and causing uneven leafing, the researchers note.

They were able to overcome this by painting limbs with a 50/50 mix of white interior latex paint and water. They used an ATV, equipped with a 15-gallon spray tank and electric pump, to apply the paint quickly.

Hasey and Lampinen recommend painting or whitewashing the southwest side of the trunk — as high up as possible — to minimize uneven bud break. This can be done before defoliation and will also protect tissue from being damaged in a freeze or from subsequent winter sunburn of affected limbs, they note. There’s no need to paint side shoots that will be thinned the following spring, they add.

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