Kat Jarvis-Shean, Orchard Systems Advisor UCCE Sacramento-Solano-Yolo

As discussed in previous newsletter articles, walnuts are one of the highest chill requirement tree crops in California. Though it’s easy to forget given the luxuriously high amounts of chill last year, multiple recent winters have fallen short of the chill accumulation needed for a tight, economical walnut bloom (e.g. 2014, 2015, 2020). Inadequate winter chill accumulation can result in delayed budbreak, scattered or prolonged budbreak and buds on southern sides of branches never opening. Prolonged bloom can result in a wider variety in nut sizes, more small nuts, and multiple shakes, while also complicating timing of control measures for blight or husk split pests. In the next 20-40 years, Central Valley walnut orchards will get 14-20% less winter chill than in the 1950s when many of our grandparents were farming. Anecdotal experience suggests the chilling requirement for ‘Chandler’ is around 60-65 chill portions as quantified by the Dynamic Model. Given decreased chill projections it is likely that currently planted ‘Chandler’ orchards will not meet their chilling requirement in at least one out of ten years in most of the Central Valley in the coming decades, if left to their own devices. While we wait for high quality lower chill varieties to develop, how can walnut growers manage the chilling requirements of the orchards in the ground now? This has been the topic of many recent years of UC research with funding from the California Walnut Board and the California Department of Food and Agriculture.

Looking at a sampling of four CIMIS weather stations using the UC Fruit & Nut Center chill calculator tool, on average the Sacramento Valley has accumulated 41 chill portions to date (written January 14^th). This is about 20% below last year, and more in keeping with the winter of 2019-2020. While 2019-2020 was a low chill accumulation winter, that was due in large part to a fairly warm February. Thus, it’s too early to say if this is a year in which dormancy breaking treatments would be beneficial. Keep an eye on the UC Fruit & Nut Center’s chill calculator. Nonetheless, it’s good to be aware for future winters that there are options in the toolbox.

For three winters, we have been studying the impact of a number of dormancy breaking treatments to give growers tools to deal with low chill winters (see here for more detail on previous years). Rather than wait for low chill years to come along, we’ve created warm winter conditions in large, open-top chambers that we’ve built around mature Chandler trees at the UC Davis campus. These trees were coupled with unheated trees that got sufficient winter chill. Approximately 30-40 days before (what we hoped would be) budbreak, dormancy breaking treatments were applied to different scaffolds in each tree. We then monitored budbreak over many weeks to quantify timing of 50% budbreak, the duration of budbreak and the percent of buds that opened on a scaffold.

Over the course of three years, we’ve tested hydrogen cyanamide, often marketed as Dormex®, a blend of nitrogen compounds marketed as Erger®, an analogue of the plant hormone cytokinin, marketed as Mocksi®, and calcium ammonium nitrate (CAN-17), all of which were compared with a water control. Dormex is the only one of these products currently labeled for use as a dormancy breaker in walnuts (see label for more use details). Erger and CAN-17 are labeled as fertilizers. Over the last two years (2021 and 2022), we found that at least in terms of budbreak timing, it appears Dormex at 2% and 4% and CAN-17 at 20% could prompt heated scaffolds to behave like they had received enough chill, whereas Erger at 6% only partially compensates for lack of winter chill. Dormex at 4% moved timing even earlier than the sufficiently chilled control, whereas Erger moved the timing but only about halfway between the timing of the heated control and the sufficiently chilled control. No effect was seen using Mocksi over two years.

This previous winter-spring, we put Dormex at 2% and 4% head-to-head. Given that last year was a very high chill winter, it was hard to force insufficient chill accumulation, even with our heated tents. Ambient trees accumulated 78-82 chill portions, while heated trees accumulated 10-12% less, 69-74 chill portions. Both Dormex at 2% and 4% moved terminal and lateral budbreak timing on the heated trees to similar values as seen in the unheated control, in essence compensating for the chill difference. Dormex at 2% also increased the percent of buds that opened, to values as high (lateral buds) or higher (terminal buds) than the unheated control scaffolds. Budbreak percent was not different between Dormex at 4% and the water control within heated or unheated trees. In other words, under these conditions of 69-74 chill portions, it would not have been beneficial to use Dormex at a rate higher than 2%. However, even given this range of chill accumulation, which is considered sufficient, 2% Dormex increased lateral budbreak from 29% to 42% and terminal budbreak from 89% to 98%. That said, in unheated trees, which accumulated 78-82 chill portions, there was no significant increase in budbreak from Dormex use at either rate.

With generous collaboration from two grower hosts, we also compared Dormex at both 2% and 4% and CAN-17 last year at a field scale, to be able to collect yield data, in addition to budbreak data. At one healthy orchard just a few years into its prime yielding years (10^th leaf) near Glenn County, where 73 chill portions accumulated (similar to heated trees on campus), we saw the same change in budbreak timing (3 days earlier) across each treatment when compared to no treatment. This did not lead to significant differences in yield, contrary to what the increased percentage of budbreak in the campus heated tented trees might have led us to expect. Yields ranged on average from 6,240-6,690 lb per acre across treatments. Across size quality measurements (percent jumbos, large, average nut weight, edible yield), nuts from the Dormex treatments were not significantly different from the control, though relative to the control nuts, the CAN-17 treatment had a lower average percentage of large and jumbo nuts (52% v. 64%) and lower average weight (9.15 g v. 10.09 g). There were no differences in color quality as measured by reflected light index (RLI) among any of the treatments.

At the Chandler orchard at the Nickels Soils Lab, where 82 chill portions were accumulated (similar to unheated trees on campus), there was a small but significant change in timing of budbreak in each treatment relative to the control (3 days). Surprisingly, however, there was also an increase in yield in the 4% Dormex treatments relative to the control, yielding on average more than 1,400 lb more per acre (5,216 lb vs. 6,857 lb). The average yields in the Dormex at 2% treatment and the CAN-17 treatment were also numerically higher than the control (1,019 lb higher and 719 lb higher, respectively), but there was a great deal of variability in different replicates, making it difficult to statistics to decipher if yield differences can be attributed to the treatments. There were no differences between any of the treatments and the control treatment across size quality measurements (percent jumbos, large, average nut weight, edible yield) or color quality (RLI).

It is perilous to draw conclusions about dormancy treatment efficacy at a field scale based on one high chill year’s data. The difference in yield effects at the Glenn County orchard and Nickels Soils Lab is intriguing. The Nickels site is an older orchard at a tighter spacing, suffering from significant limb dieback from shading coupled with Botryosphaeria infections. One possible explanation for the yield difference would be that the Nickels site benefitted from treatments that encouraged additional budbreak, whereas at the healthy, high yielding Glenn County orchard there was already sufficient budbreak without intervention. Knowing that June drop generally reduces cropload, it’d make sense that increased budbreak, as we saw in the campus trees with 2% Dormex, would not necessarily lead to increased yield. Given high chill accumulation in healthy orchards, dormancy breaking treatments are unlikely to pay for themselves at current walnut prices. If chill accumulation is lower than ~60 chill portions, our heated tent data indicates they may pay for themselves, but we’ll need yield data at production scale to know for sure. The Nickels results point to the possibility that dormancy breaking treatments may be of use in orchards with limb dieback. At the same time, we need to be cautious to watch for swings in yield this coming year that may result from over-taxing the already struggling trees. Particularly given how tight walnut budgets are these days, I’d wait for more data before trying out this orchard-renewal strategy at a large scale if I were a grower.

We’ll continue this project with funding from the California Department of Food and Agriculture to improve understanding of ideal rates and timings, and the physiological response to these treatments inside the trees. Stay tuned!