Winter Chill, Dormancy and Walnut Management

Budbreak on heated tree scaffolds in spring 2021.

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. Scientists expect such winters to be more frequent in the future.

Though lower-chill varieties are in development, the industry needs tools to support varieties that are currently in the ground for the next 20 to 40 years. Many products have been shown to compensate for inadequate chill in other crops and other countries but need to be tested in California conditions. With funding from the California Walnut Board, a team including myself and UC Davis’s Maciej Zwieniecki and Giulia Marino have begun testing dormancy breaking treatments to help California’s walnut growers sustain economic production in the lower-chill winters to come. Though this is shaping up to be an adequate-chill winter for walnuts, it’s good to understand what tools may be in the toolbox in case of future needs.

Why Trees Need Chill
Symptoms of inadequate winter chill have been seen in orchards in multiple recent springs (e.g., 2014, 2015 and 2020), including delayed budbreak, scattered or prolonged budbreak and buds on southern sides of branches never breaking. Prolonged bloom in many cases resulted in a wider variety in nut size, more small nuts and multiple shakes. Prolonged bloom could also result in the need for more sprays for blight control or husk split pests.
Minimum winter temperatures are critical to winter chill accumulation. California’s statewide winter minimum temperatures have been increasing since 1970. This warming has coincided with a trend towards delayed budbreak in California walnuts since the mid-1990s, indicating that lower winter chill accumulation is already impacting orchard phenology. By the 2060s, Central Valley winter minimum temperatures are projected to average 3 degrees F above minimums in the 1990s. In other words, in the next 20 to 40 years, Central Valley walnut orchards will get 14% to 20% less winter than in the 1950s when many of our grandparents were farming.

Anecdotal experience suggests the chilling requirement for ‘Chandler’ is around 60 to 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 1 out of 10 years in most of the Central Valley in the coming decades. While breeders are working to develop new cultivars with lower chilling requirements that retain the other desirable traits that growers and buyers want, the industry needs tools for orchards that are currently in the ground or will go in while waiting for new varieties to be developed. Numerous dormancy-breaking products are documented as capable of partially compensating for inadequate chill. However, these products have not been systematically compared in California growing conditions on walnuts in particular to test how much chill compensation is possible as well as ideal rates and timings.

What’s Been Studied
The trouble with studying dormancy breaking treatments is oftentimes trees will respond differently to these treatments depending on whether they have gotten enough winter chill or not. So, we could not just spray a bunch of products on trees after any old winter, see what the outcome was and assume that outcome would be consistent across lots of winter conditions in the future. Instead, we set up an experiment that forced trees to experience warmer winters by building clear-sided open-top chambers around fifth-leaf Chandler trees and pumping in hot air to increase temperatures during the winter.

These trees were coupled with unheated trees that got sufficient winter chill and then the same dormancy breaking treatments. Tented trees were heated from early November through February. The goal was to limit winter chill accumulation to 50 chill portions, approximately 30% less than the historical average chill accumulation, but within the range of what can be expected of warm winters in the southern San Joaquin Valley in the next few decades. In the winter of 2020-21, sensors in each tree told us ambient chill trees experienced 59 to 65 chill portions, whereas heated trees experienced 46 to 56 chill portions. In 2021-22, there were 65 to 70 chill portions in ambient trees and 56 to 60 chill portions in heated trees.

Once we had heated the trees all winter, four scaffolds were selected in each tree for treatment. Treatments were applied at rates and with adjuvants according to recommendations of the companies that provided them, applied approximately 30 days before the date of average historic budbreak. Chemicals were applied using a backpack sprayer from a pruning tower just up to the point of mixes dripping (equivalent to 150 gallons per acre). Scaffolds that were not being treated were wrapped in plastic drop clothes to avoid drift.

In 2021, we tested hydrogen cyanamide, often marketed as Dormex®, at 4%, a blend of nitrogen compounds marketed as Erger®, at 6%, and an analogue of the plant hormone cytokinin, marketed as Mocksi®, at 15 parts per million. In 2022, we tested Dormex® at 2%, Mocksi® at 20 parts per million, and calcium ammonium nitrate (CAN-17) at 20%.
Each year, there was also a water-treated control scaffold on each tree. None of these products are currently labeled for use as dormancy breakers in walnuts in California. Erger and CAN-17 are labeled as fertilizers. Dormex is awaiting California DPR approval which is expected soon, so check an up-to-date label. On each scaffold, shoots were flagged to track bud break progression and monitored every two to three days from late March through May. These recordings were used to calculate the timing of 50% budbreak, the duration of budbreak and the percent of buds that opened on a scaffold.

In 2021, among the terminal and lateral buds on trees that received sufficient chill, 50% timing was significantly earlier on scaffolds treated with Dormex but not significantly different among scaffolds treated with Erger, Mocksi or the water control. Dormex advanced terminal and lateral budbreak timing relative to the water control by an average of seven days and eight days, respectively, in the sufficiently chilled trees. However, on scaffolds on trees that did not receive sufficient winter chill, Erger also significantly advanced budbreak timing relative to the water treated control. In the insufficiently chilled trees, Dormex advanced terminal and lateral budbreak timing relative to the water control by an average of 16 days and 13 days, respectively. In the same trees, Erger advanced terminal and lateral budbreak timing relative to the water control by an average of five days and six days, respectively. In other words, 4% Dormex changed budbreak timing whether trees got enough chill or not, but 6% Erger only changed budbreak timing if the trees hadn’t gotten enough chill, and then only about half as much as 4% Dormex (Figure 1).

In 2022, we saw no responses in ambient chill trees to any of the treatments. However, on heated trees across both terminal and lateral buds, response to 2% Dormex and CAN-17 were not significantly different from each other, but both were significantly different from the water control, whereas Mocksi was not different from water. Terminal vegetative buds opened 19 and 17 days earlier with the Dormex and CAN-17 treatment than with water, respectively. Lateral vegetative buds opened 19 and 18 days earlier with the Dormex and CAN-17 treatment, respectively.

When trees lack sufficient chill, both Dormex at 2% and CAN-17 at 20% move budbreak timing to similar timing as that documented in the water-treated scaffolds of the sufficiently chilled trees. Dormex at 4% moved timing even earlier than the ambient water control, whereas Erger moved the timing but only about halfway between timing with water on heated trees and timing with water on unheated trees. In other words, at least in terms of budbreak timing, it appears Dormex at 2% and 4% and CAN-17 at 20% could prompt the heated scaffolds to behave like they had received enough chill, whereas Erger at 6% only partially compensates for lack of winter chill.

The questions of whether these products can change budbreak duration, which could impact number of blight sprays, harvest shakes, etc., and whether they can impact number of buds that break in the face of low winter chill accumulation has been harder to answer with this first round of experiments based on design and cost limitations. We have launched additional experiments in a greenhouse and grower fields to better answer these questions. But so far, we at least know three products that have some impact on low chill response behavior in walnuts: Dormex, Erger and CAN-17.

Is Action Needed This Winter?
This winter, the Central Valley is on track to accumulate as much chill if not maybe even a little more than the last two winters, which have gotten more than sufficient chill for walnuts. Looking at a sampling of 15 CIMIS weather stations using the UC Fruit and Nut Center chill calculator tool, on average, the Sacramento Valley and northern San Joaquin Valley have accumulated 47 to 48 chill portions to date (written January 9), about what had been accumulated by this time last year, and six chill portions more than this time of year in 2021.

In the southern San Joaquin Valley, an average of 44 chill portions have accumulated to date, two and three more portions than this time in 2022 and 2021, respectively. Both last winter and the winter before had accumulated more than enough chill by the end of February for walnut budbreak to progress normally. So, it’s not looking like this is a year that walnut growers will need to use dormancy-breaking treatments. That’s lucky given how much everyone is trying to cut down on inputs given walnut prices. Nonetheless, it’s good to be aware for future winters that there are options in the toolbox. 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!