More than a decade of Almond Board-funded research is challenging once-held assumptions about almond pruning and tree spacing and their impacts on efficient almond production.

University of California Cooperative Extension Farm Advisor Roger Duncan, Stanislaus County, says that based on what he has learned through his own trials and additional research, the goal in designing an orchard should be to maximize light interception through minimal pruning and training, proper rootstock selection and optimal spacing to maximize yield while being careful not to cause orchard management problems.

Duncan’s ongoing 37-acre Stanislaus County trial confirms past research in almond growing regions throughout California: There are many reasons to prune an almond orchard, but yield does not appear to be one of them. The trial has also shown that traditional tree spacing of 22 feet by 22 feet or more is probably too far apart to intercept the sunlight necessary to optimize production in most orchards.

Since 1999, the trial has looked at various pruning and training strategies for Nonpariel and Carmel on Nemaguard and Hansen 536 (peach-almond hybrid) rootstock combinations.  The spacing range includes 10’ x 22’, 14’ x 22’, 18’ x 22’ and 22’ x 22’.

Pruning Strategies

Pruning strategies range from untrained trees with no annual pruning (except to remove broken limbs or for equipment access and safety) to the traditional trees trained to three scaffolds with open centers and annual pruning.

Two intermediate treatments include trees trained to three scaffolds with open centers and unpruned (except for breakage and equipment access) after the second dormant season; and minimal training with four to six scaffolds, open centers, followed by minimal pruning of no more than three pruning cuts annually.

The goal of this research is to find an orchard spacing configuration and pruning/training technique that produces the most benefit in yield potential and profitability without creating problems related to crop drying, limb breakage, orchard diseases, and worker safety.

Research by UCCE Almond Specialist Bruce Lampinen on mid-day light interception—also supported through the Almond Board’s Production Research program—has examined various rates of light interception and their impacts on yield, nut quality, diseases and orchard profitability. Lampinen’s research illustrates that every 1 percent increase in sunlight interception equates to a 50-pound-per-acre higher yield potential. Therefore, theoretically, if 100 percent of the mid-day light is intercepted by the tree canopy, it would provide a yield potential of 5,000 kernel pounds per acre, not accounting for other factors in the orchard that would detract from this potential.

80/20 Rule for Light Interceptions

Too little light reaching the orchard floor, however, can lead to problems with crop drying and promote food safety and orchard disease issues. The research shows that high productivity can be maintained and there is enough sunlight for drying when 80 percent of sunlight is intercepted and 20 percent reaches the orchard floor. Practical on-farm methods to measure light interception are being developed. 

At a recent field day, Farm Advisor Duncan reviewed this trial and said that growers can often achieve the above goal without creating additional problems in the orchard through a balanced tree spacing and pruning strategy.

Tree Spacing

With respect to spacing, Carmel yields in this trial have been significantly higher on closely planted trees (10’ and 14’ feet down the row), and the increase in cumulative yield ranges more than 1,500 to 2,400 pounds per acre compared to wider spacing. On the other hand, there has been no obvious cumulative yield advantage to close planting of the Nonpareil variety, which is a larger tree than Carmel. Cumulative yield represents the total of annual yields from fourth leaf through 12th leaf, except for one year in which yield was not recorded, for a total of eight harvests.

In most cases, tree spacing has a greater effect on tree failure and breakage than pruning technique. More-closely spaced trees are less likely to have scaffold breakage, which may ultimately lead to longer orchard life, though we won’t know for sure until we look at this trial orchard as it progresses into the latter portion of its lifetime.

Advantages of Closer Spacing

Closer spacing, which produces shorter trees, may also provide better spray coverage and a better shake at harvest, providing fewer mummies and less shaker injury. Trees that are too closely spaced, however, can create problems related to access and worker safety. Duncan said a general rule, on good ground, is to plant trees in the neighborhood of 21- or 22-foot row spacing with 14 or 16 feet down the row to optimize light interception without creating problems. Weaker ground might benefit from closer spacing down the row.

With respect to pruning, Duncan notes that in most years, Nonpareil yields are statistically similar in conventionally pruned, minimally pruned and non-pruned trees. Cumulatively un-pruned Nonpareil trees have yielded 1,330 pounds/acre more than conventional pruning, but this is not statistically significant.  In contrast, cumulative Carmel yields in minimally trained and pruned trees is significantly higher by 1,500 to 2,400 pounds/acre compared to conventional programs. However, young trees trained to multiple scaffolds are more prone to scaffold failure and tree blow over, particularly in wider spacings. Taking pruning costs and yields into account, Duncan estimates that conventional training and pruning would have reduced gross income in his trial orchard under a commercial setting by $4,800 per acre.

Pruning/Training Strategies

Duncan concluded that a good compromise to develop tree structure and minimize pruning costs is to train trees to three scaffolds the first year, prune trees for open centers the next year and leave trees unpruned afterwards, except where needed for breakage, worker safety other factors.

We will continue to learn more about orchard pruning and tree spacing and their impacts on efficient almond production as this trial orchard progresses into the later part of its productive life. For now, consecutive years of consistent results in this trial, which confirms results from previous trials, are offering promising recommendations for growers for planting, training and pruning young orchards.

To view the 2011 season results and detailed data from this trial,  go to www.AlmondBoard.com/FarmPress28. Roger Duncan can be reached at the UC Cooperative Extension office in Stanislaus County at (209) 525-6800 or raduncan@ucdavis.edu.