Tight Budget? Consider Not Pruning Bearing Almond Orchards
Roger Duncan, Pomology Farm Advisor, UCCE Stanislaus County
Multiple, long-term field trials conducted by University of California farm advisors have determined that pruning almond orchards does not improve yields in the short term or preserve yields in the long term. Across multiple studies, some lasting 20 years or more, yields were frequently a little better in unpruned trees, but differences were often not significantly different statistically. These included studies with hand pruning and mechanical hedging and topping.
One of the first California trials was headed by John Edstrom at the Nickels Estate Soils Lab in Arbuckle from 1979 – 1999. Trees that were trained and pruned only for the first two years and then left essentially unpruned for the next 19 years produced an additional 906 pounds over the 21-year study compared to trees that were pruned every year. Statistically insignificant. In the last year of the study, Nonpareil trees that were pruned every year yielded 2,136 pounds per acre in the 21st leaf while “unpruned” trees yielded 2,307 lbs. per acre. This trial demonstrated that unpruned trees stayed just as productive as trees that were pruned every year for 21 years.
In a follow-up field trial at Nickels from 1997-2022, currently monitored by Franz Niederholzer, total yields in “unpruned” trees were statistically similar to the annually pruned trees. They were also similar to trees that were mechanically topped in the second and fourth dormant seasons and then left unpruned. In that trial, “unpruned” trees out-yielded standard pruned trees in the early years, and then production between all pruning treatments was similar after the 6th leaf. The researchers also noted no increase in disease or mummies in unpruned trees, and tree height appeared shorter than annually pruned trees.
Simultaneously, a pruning trial was initiated in Kern County in 1996, planted in a deep, Wasco sandy loam soil. That trial, initiated by former UC farm advisor Mario Viveros, tested several different pruning strategies. They included dormant pruning by hand every year, dormant pruning by hand every other year, and various strategies using mechanical topping and hedging every year or every other year, with and without follow-up hand pruning. All of these strategies were compared against trees with no scaffold selection and no annual pruning. The trial was followed for 11 years. At the end of the trial, cumulative yields were statistically similar for all pruning strategies for all three varieties, although unpruned trees trended towards higher yields. Mario also noted that unpruned trees did not have more stick tights and were shorter than annually hand-pruned trees.
Finally, I conducted a pruning trial here in Stanislaus County from 1999-2019. In this 37-acre trial, we compared trees initially trained to three scaffolds and moderately pruned every year (standard pruning) against trees that were trained only for the first two years and then left unpruned for the next 18 years. We also compared trees that were left completely untrained and unpruned for the life of the experiment. We compared these different pruning strategies in high-density and low-density plantings and on a high-vigor rootstock (Hansen) and a moderate-vigor rootstock (Nemaguard). In the final year of the study, yields were almost identical among pruning strategies. Cumulatively, trees trained for two years and then left unpruned for 18 years yielded 911 pounds per acre more than annually pruned Nonpareil trees, not statistically different. In the Carmel variety, trees that were never trained or pruned accumulated 4,423 pounds more than standard pruned trees over the life of the trial. At the end of this trial, unpruned trees were the same height as annually pruned trees.
At the average almond prices and labor costs during this Stanislaus trial, conventional training and annual pruning would have reduced cumulative net income by up to $14,000 per acre, including pruning, stacking and shredding costs, plus slightly lower cumulative yield. Although untrained and unpruned trees tended to have slightly higher yields, they were more prone to scaffold failure, especially in widely spaced trees. This also led to more eventual tree loss from fungal canker diseases. Counter to expectations, the no-training and no-pruning strategy worked better in the most tightly spaced trees (10’ x 22’) than trees planted 18 or 22 feet apart. That is because trees planted far apart got larger, and branches were more likely to break or get in the way of equipment.
Based on these and other studies, plus observations of many almond orchards in California, almond growers should consider training their young trees for the first 1 - 3 years, depending on variety, vigor, and tree spacing to develop a structurally sound tree. Closely spaced trees (10-12 feet apart) may not need much scaffold selection or initial pruning at all. After the initial training phase, almond trees still need occasional pruning to remove broken or diseased limbs and to remove branches that are in the way of equipment or are a safety hazard for equipment operators. There are reasons to prune almond trees. Yield is not one of them.
*Yields are an average across all tree spacings and two rootstocks.