Annual Walnut Pruning Unnecessary During Development Phase
March 15, 2016 Most walnut orchards are pruned annually during the establishment phase – a costly, laborious endeavor.
But, there has been some promising research showing that annual pruning might not be necessary.
In the early 2000s, Bruce Lampinen was traveling in Chile and France as a new Cooperative Extension specialist. During his trip, he worked with a number of individuals who had conducted successful trials in which walnut trees weren’t pruned. Lampinen, now Agriculture and Natural Resources Cooperative Extension Integrated Orchard Management Specialist at University of California, Davis, decided to conduct his own trials.
Over the course of eight years, he trialed both pruned an unpruned ‘Howard’ hedgerow trees, documenting canopy growth, tree height, yield, and nut quality, as well as the effects of fruit removal. The research found that pruning did not actually impact canopy development, nor did it have an effect on nut quality or yield. Nut removal in both pruned and unpruned trees did, however, stimulate more vegetative growth, but not enough to significantly affect tree height, yield, or nut quality.
“It had been ingrained in most farm advisors and growers that you have to prune walnuts in order to get them to grow,” Lampinen says. “We found this was not true.”
Lampinen suspects this thinking came from the fact that if walnuts are not pruned, they put on elongation growth in mid-summer in alternate years, while in the other years they put on short spur growth, producing walnuts on this wood.
“Pruning short-circuits the natural controls of the tree and causes expansion growth to take place at the expense of production,” he says. “When you stop pruning, you actually get more production with less canopy area in years two to six or so. This results in higher water use efficiency and increased returns to growers.”
Considering the drought California has experienced in recent years, this is a welcome benefit.
Although this particular trial focused on ‘Howard,’ Lampinen has also conducted similar trials on ‘Forde,’ ‘Gillet,’ ‘Tulare,’ and ‘Chandler’ varieties, which yielded similar results. In fact, a larger ‘Chandler’ trial, now eight years old, has shown even better results than the ‘Howard’ experiment. The full results of this trial are expected to be published soon.
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