Katherine Jarvis-Shean, UCCE Orchard Advisor, Sacramento, Solano and Yolo Counties
Janine Hasey, UCCE Farm Advisor Emerita, Sutter, Yuba, Colusa Counties

Given the prolonged leaf-out window we saw in walnuts this spring, it is likely that there will also be a wide window in the timing of hull split and nut removal. This will likely result in the need for two shakes in some varieties. Ethephon, also known as Ethrel®, is a commonly used tool for advancing hull split and harvest-readiness in walnuts. Ethephon is an ethylene-based plant growth regulator applied at walnut maturity (100% packing tissue brown), or shortly thereafter, which accelerates hull cracking and separation from the shell. It’s important to remember that there are two potential timings of ethephon, based on different goals.

Ethephon to Tighten Harvest-Readiness
If your goal is to tighten the harvest-readiness window in a block, to avoid having to return for a second shake, apply ethephon 5 to 7 days after 100% of the nuts in the orchard have reached packing tissue brown (PTB), approximately ten days before you would otherwise harvest. This timing of ethephon application does not change the timing of harvest. Rather, it facilitates a one shake harvest.

Ethephon to Advance Harvest
If, on the other hand, your goal is to advance harvest, either to increase kernel quality or spread out the harvesting of different blocks of the same variety, the timing of application for this is different. To advance harvest timing as much as possible while still maintaining good oil content, apply when 100% of nuts reach PTB. This will move harvest up by 7-10 days, with a second shake approximately two weeks later.

Knowing the percentage of nuts that are at PTB is critical to meeting your ethephon-related goals. Check out more on ethephon use, including sampling to know when a block has achieved PTB (Photos 1 and 2).

Photo 1. Immature walnuts.

Photo 2. Mature and ready for ethephon treatment. Photos by R. Beede.

Considerations for Solano and Ethephon
Several growers in the Sacramento Valley have planted or are considering the newer variety Solano because of its early harvest timing, light kernel color, and vigor. With newer varieties, it takes time to learn how to manage them for optimum yield and nut quality. There have been years, 2014 and 2019, where some kernel color decline was observed where Solano harvest was delayed past optimum timing. Because of the apparent need to advance harvest timing in Solanos to retain light kernel color, they are a candidate for using ethephon for early harvest. Different varieties have different sensitivities to ethephon, e.g. it is more effective on Chandler and Howard than on Serr. Anecdotal evidence suggests Solano may be more sensitive than many common varieties to ethephon. Until we have ethephon use research on new walnut varieties such as Solano and Durham, it may be advisable to use lower rates (especially on younger trees).