Results of two walnut field trials conducted by University of California researchers over an 11-year period question the conventional wisdom that pruning prevents tree growth from stalling out.

The research was conducted by Bruce Lampinen, Extension specialist, UC Davis; Janine Hasey, farm advisor for Sutter/ Yuba/Colusa Counties and Carolyn DeBuse, former orchard systems advisor, for Yolo/Solano Counties. They describe the studies in the Winter 2014 issue of Walnut News, the California Walnut Board’s industry newsletter.

Traditionally growers train young walnut trees by removing about a third of the previous year’s growth on scaffold limbs. The researchers studied the effect of pruning versus non-pruning on walnut tree growth and productivity.

The first trial, involving Howard walnuts, began in 2004 during the dormant season prior to the third growing season. In the first two years of the orchard’s life before the trial, the trees were pruned using standard practices and scaffolds were selected. Coming into the third season in the pruned plots, the researchers removed about one-third of the prior years’ growth on all branches that had grown the previous year. On the unpruned treatment trees, they removed only branches that were likely to hinder travel through the orchard or impair visibility of the trunk for shaking.


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The pruning treatments were continued during the dormant season from 2004 to 2010. In the first and second year after treatments started, yields in the unpruned treatment trees tended to increase slightly. However, after seven years of treatments, the researchers found no significant differences in tree growth (as measured by trunk circumference or midday canopy light interception), cumulative nut yield, or nut quality between the pruned and unpruned treatments.

To determine the impact of crop load on tree growth, all nuts were removed from both pruned and unpruned trees when the nuts were about one-quarter inch in diameter in the third, fourth and fifth growing seasons. This resulted in a slight but insignificant increase in tree growth. Meanwhile, cumulative yields over the seven years of the study decreased. “This suggests that cropping is not causing trees to stall out in growth,” the researchers note.