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

Depending on where you grow and how you count, this is shaping up to be a low-to-moderate chill winter. As discussed previously in 2023 and 2024, walnuts are one of the highest chill requirement tree crops in California. Multiple recent winters have fallen short of the chill needed for a tight, economical walnut bloom (e.g. 2014, 2015, 2020), with more low chill winters expected to come. Recent UC research funded by the California Walnut Board and the California Department of Food and Agriculture have been looking into tools to help walnuts cope with low chill winters.

Chill so far this winter
The graphs below show chill accumulation through January 20th over the last six years at five CIMIS stations in the Sacramento Valley counted in chill portions and chill hours, with this year shown by the green bar farthest to the right in each station grouping. Research has found chill portions more closely represent chill accumulation in Mediterranean climates like ours than chill hours, better quantifying the chill-deleting effect of warm days that follow cold nights and, most importantly for this year, giving some credit for cool but not cold temperatures. Chill hours stop giving any chill credit at all when temperatures go above 45° F. Chill portions give some credit up to 54° F. Many folks are still more comfortable counting in chill hours and thus are understandably very concerned about our low chill hours accumulation to date. Given the additional information provided by chill portions and all the fog we had in December, I’m not panicked about the chill accumulation this year. It looks like we’re on track to be less than the high chill of 2022-2023 (yellow), but more than the low chill of 2019-2020 (dark blue) that led to straggled and decreased budbreak in many orchards. That said, it does look like chill will be low enough in many orchards to potentially see an impact of dormancy breaking treatments.

You may be thinking “Another spray? In this economy, Jarvis-Shean?” I know prices are tight, and walnut growers are still clawing their way out from years of not even being able to afford to just irrigate and harvest. The tools I’ll talk about here aren’t for every orchard every year in every situation. What’s more, our understanding of these tools and how to best utilize them is still evolving. But given that these tools are currently on the market, and many growers are wondering if and how they may fit into their management, I’m sharing below what we know so far, acknowledging that our understanding will evolve as we get more experience with these tools.

Early research showed us tools to move budbreak
Rather than wait for low chill years to come along, we created warm winter conditions in large, open-top chambers that we built around mature Chandler trees at the UC Davis campus. These trees were coupled with unheated trees that got sufficient winter chill. Dormancy breaking treatments were applied roughly 30 days before anticipated budbreak and we then then monitored budbreak over many weeks. The most promising treatments to come out of years of these trials were hydrogen cyanamide (Dormex®) and calcium ammonium nitrate (CAN-17). Dormex at 2% and 4% and CAN-17 at 20% could prompt heated scaffolds to behave like they had received enough chill, whereas CAN-17 at lower rates (5% and 10%) only partially compensated for lack of winter chill. Heated trees showed Dormex at 2% often also significantly increased the percentages of buds that opened on heated scaffolds, whereas Dormex at 4% and CAN-17 often increased budbreak numerically, but not to a level that statical analysis could differentiate from the control. Dormex is the only one of these products currently labeled for use as a dormancy breaker in walnuts (see label for use details). CAN-17 is labeled as fertilizer. Taken all together, our tented tree trials indicated that hydrogen cyanamide and CAN-17 were worth testing at a field scale.

Grower trials – Similar budbreak story, yield is more complicated
With generous collaboration from Crain Walnuts and the Nickles Soils Lab, we’ve spent two years so far comparing Dormex at both 2% and 4% and CAN-17 at 20%, monitoring budbreak timing, maturity timing, yield and quality. The two Chandler orchards we’re working in are fairly representative of the industry. One is a healthy orchard just a few years into its prime yielding years (10 th leaf) near the town of Glenn, and the other, planted in the hills above Arbuckle, is a few years past its prime, with some limb and spur dieback from tight spacing and Botryosphaeria.

So far, budbreak timing results have been consistent with what we saw in our tented tree trial. In the spring of 2023, after a high chill winter, budbreak was 2-3 days earlier across treatments when compared with the control. In the spring of 2024, after a good but not luxuriously high chill winter, budbreak was 5-6 days earlier at the Glenn site and 8-11 days earlier at the Arbuckle site. This year we also looked to see if earlier budbreak resulted in earlier maturity. We found at the Glenn site 100% Packing Tissue Brown occurred 10-11 days earlier in the treated trees than the control, with no difference between treatments. At the Arbuckle site, there was a numeric trend of 100% PTB occurring a few days earlier, but it wasn’t significantly different from the control. This points to a potential interesting side-benefit for these dormancy breaking tools – the ability to shift harvest timing for growers with too many acres of Chandler to harvest at once. That said, we need a few more years of data to get a better sense of how consistent these results can be.

Budbreak timing difference between Control, CAN-17 at 20% and Dormex at 2% and 4%.

Our yield results have been both interesting and surprising. The results are detailed in the table below. Last year, even when chill was more than adequate, we saw a significant increase in yields, 1,600 lb on average at the Arbuckle site associated with Dormex at 4%, and an increase in yields, though not statistically significant, in the other two treatments at Arbuckle (700-1,000 lb). We also saw an increase in yield in all treatments, though not significant, at the Glenn site (400 lb). This year, however, yields were the same or lower in our treatments than the control. Yields were 400-700 lb lower than the control at the Glenn site, statistically significant with CAN-17, and 200 lb less to 200 lb more than the control at Arbuckle, though none of these differences were statistically significant. This was surprising given that 2023-2024 was a milder winter than 2022-2023, so we would have expected a bigger return from using a dormancy breaker. That said, added up over two years, the cumulative yield was not different from the control at the Glenn site, and was still significantly higher (shown by different letters behind the numbers) with Dormex at 4%, at the Arbuckle site.

*n.s. indicates no statistically significant difference in yields among treatments. Different letters indicate statistically significant difference in yields.

All this together points to the notion that these tools, applied at the timing and rates we used, are likely not suited for every orchard every year. We tried a high rate of Dormex to look for a tipping point of “too much”, and with the data we have, it looks like Dormex at 4% is rarely worth the additional cost compared with 2%. In other countries, where they’ve been able to use hydrogen cyanamide for more than a decade on walnuts, 1.5% is a more common application rate in mature trees, and lower in younger trees. The big swing in yields we saw across both sites this year indicates to me that applying dormancy breakers after optimal chill winters like 2022-2023 to an orchard that is already thriving, like the Glenn site, is unlikely to warrant the cost in the long run. That said, they are likely to still be valuable tools following low and medium chill winters, have potential for encouraging additional budbreak in stagnant orchards like Arbuckle, and have exciting potential for moving harvest timing for growers with a lot of Chandler acreage.

We’ll keep working in these same orchards for a few more years to gather data after more, different chill winters. You can catch me talking about this in more detail at Tri-County Walnut Day, February 6th in Tulare, Sutter-Yuba Walnut Day, March 5th in Yuba City, North Sac Valley Walnut Day, March 6th in Red Bluff, Yolo-Solano-Sacramento Walnut Day, March 12th in Woodland, Quad County Walnut Day, March 18th in Modesto or reach out through kjarvisshean@ucanr.edu.