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California’s Prune Orchard OF THE Future BY LUKE MILLIRON | UCCE Farm Advisor for Butte, Glenn and Tehama Counties AND FRANZ NIEDERHOLZER | UCCE Farm Advisor for Colusa, Sutter and Yuba Counties AND DANI LIGHTLE | UCCE Orchards Systems Advisor for Glenn, Butte and Tehama Counties AND KATHERINE JARVIS-SHEAN | UCCE Orchards Systems Advisor for Sacramento, Solano and Yolo Counties Figure 1. An unheaded 2nd leaf prune tree in March 2017 in Glenn County with traditionally headed trees in the background. This Glenn County grower is experimenting on their own with various tree training regimes. California will likely have a large prune crop in 2019 following favorable bloom conditions and lower yields in 2018. Unfortunately, in prune production with larger crops typically comes smaller fruit, of which there is currently an over-supply in the world market. High production of small fruit world-wide has come at a time when demand for small fruit from consuming nations like China, Brazil, and Russia has been in decline. California handlers have been strongly urging their growers to use shaker thinning to reduce the fruit number during spring and help deliver large, high-quality fruit at harvest. To be successful, the prune orchard of the future is going to have to thread the needle of achieving earlier production in its life cycle and maintaining high and consistent yields in maturity, all while attaining large average fruit size each year. Prunes come into production later than many other orchard crops. One way to increase early production is to reduce or eliminate severe heading 26 Progressive Crop Consultant September /October 2019 cuts during tree training. Subsequently, yield potential at orchard maturity may be increased over historical production levels in some situations by striking a new balance between spacing and rootstock vigor for increased canopy volume and higher light interception. Finally, the key mechanism for achieving more consistent yields and larger fruit size in prune production has been to thin the crop with mechanical shaking following good bloom conditions like we had this year. Shaker thinning will likely continue to be the foundation of consistent yields for future orchards. Pruning Prunes: Greater Early Production by Avoiding Heading Cuts During Establishment University of California (UC) research has shown that earlier and greater production can be achieved in both almonds and walnuts by reducing the severity of pruning during tree establishment. This has been done by reducing or eliminating the use of severe heading cuts (Figure 1) during tree establishment. In pruning, “heading cuts” reduce the length of a limb, while “thinning cuts” completely remove the limb. Although historical orchard tree training often calls for severe heading cuts (e.g. cutting back ½ or more of the 1st year growth during the first dormant period), leaving limbs unheaded can lead to earlier fruit production. Bill Krueger, now a UC Cooperative farm advisor emeritus, tested several pruning regimes for newly planted prune trees from 1996 through 2000. The most successful of these pruning regimes all minimized the severity of pruning. Yield and profitability were greatest in the treatment that selected three to five scaffolds at the first dormant and bent back competing limbs (nearly to, or to the point of breaking to reduce competitiveness). Competing limbs were again bent at second and third dormant, and finally limbs were left unheaded at fourth dormant. In all four years of this treatment, pruning cuts were made for selective thinning (complete limb removal) to help shape the tree into a vase-shape and elimi-
SOURCES: Figure 2. Measuring both almond yield (kernel lbs/ac) and midday photosynthetically active radiation (PAR) percentage have shown a direct relationship of 40 kernel lb/ac per 1 percent PAR. Photo courtesy of Lampinen Lab, UC Davis. nate competing or crossing branches. Other treatments that both yielded well and had the highest total income selected either 3 or 3-5 scaffolds and either left those scaffolds unheaded or lightly tipped in the first dormant. In the 2nd, 3rd and 4th dormant these other successful treatments had various sequences of either being unheaded (but still making thinning cuts) or leaving the trees completely unpruned. By contrast, the severe pruning treatment that had the lowest cumulative dry yield and income selected three scaffolds at the first dormant and then headed new limb growth back to 30 inches in the 1st through 4th dormant. You can read the full report from 2000, which is the second listed report at: ucanr.edu/sites/driedplum/show_categories/General_Pruning/ Joe Turkovich, Winters area grower and Chairman of the California Prune Board developed his own minimal pruning training method, which he describes as a modified version of the "long pruning" method described by Krueger and others in the UC Prune Production Manual. First used in the early 1990’s, the goal of his approach is to create an upright canopy framework with a strong interior architecture capable of bearing large crops without the need for wiring, rope, or propping. It allows for early bearing and isolates breakage to flatter, bearing side limbs. Continued on Page 28 September /October 2019 www.progressivecrop.com 27
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