A study evaluating central leader training, minimal pruning of walnuts in comparison to traditionally trained walnuts has been ongoing within Merced County for the past six years. Over this period, the unheaded, minimally pruned ‘Chandler’ trees have cumulatively out-yielded the headed, or traditionally pruned, walnuts by an estimated 1500 lbs/acre over the first four harvests (p<0.05). The greatest yield difference between the two treatments occurred during the first harvest (3rd leaf) in which the unheaded walnuts outproduced the headed walnuts by 750 lbs/acre (p<0.05). Since that time, the trees have had relatively similar crop loads with no observed differences in limb breakage between treatments.
Leaving trees unheaded after the first growing season provides the opportunity for the walnut tree to develop lateral fruiting positions during the 2nd growing season. These positions set with crop the next year. Headed trees, in contrast, respond to the 6 foot heading cut made following the first dormant season after planting with vegetative growth, reducing the energy directed toward developing lateral positions. The differences in response as well as the reduced canopy associated with pruning are believed to explain the differences in yield.
Observations made within the local field trial have provided some considerations if this technique is planned to be adapted. In-season limbs should be removed to develop the central leader in the dormant period after the first year of growth. Trees should not be painted during dormant period between the first and second year as it may suppress bud growth in the lower positions. Selected limb thinning should occur to allow orchard equipment access. Work in other varieties is ongoing, and should be done more experimentally within an operations as there does seem to be some differences among varieties. ‘Chandler,’ however, is performing consistently with this method across the state.
Observations of the six year old orchard can occur during a field day being held May 31st, 9:00AM-11:00AM, at the farm location near Merced, CA. Signs will be posted at the corner of Olive Avenue and Fox Rd. on the east side of Castle Air Force Base. More specific details can be found at this link.