Decisions on use of Ethephon to advance maturity in your walnut crop depend on your goals.
This plant growth regulator product is applied at walnut maturity or soon after to accelerate hull cracking and separation and increase lighter color kernels. It can also tighten harvest readiness to avoid second shakes.
Robert Beede, UCCE farm advisor emeritus, and UCCE farm advisor Janine Hasey wrote in the UCCE publication Sacramento Valley Orchards that depending on the season and the variety, harvest can be four to seven days earlier with use of Ethephon.
If the goal is to avoid a second shake and tighten harvest readiness, an application of Ethephon is made five to seven days after 100% of the nuts in the orchard have reached packing tissue brown. This does not accelerate harvest; it only sets the orchard up for a single shake.
If a grower wants to advance harvest to improve kernel quality, spread out harvest of different blocks of the same variety or avoid insect damage, this is done by applying Ethephon when 100% of the sample nuts reach packing tissue brown (PTB). This is when the tissue surrounding the kernel changes from white to oak color. After an application, the harvest will be moved up 7 to 10 days with a second shake about two weeks later.
Sampling nuts to know the stage of development is essential to meeting goals with Ethephon use. This should begin at least two weeks before PTB is expected. This can be done by collecting at least 100 nuts walking diagonally across a block and cutting them in half. Only nuts with complete browning of the packing tissue, including the area near the stem end, are considered at full PTB. Treatment should not begin until 100% of collected nuts are at PTB.
Treatment before 100% PTB can result in loss is kernel weight, nut quality and difficulty removing hulls. Applying five to seven days after 100% PTB still improves percent nut removal and improves chances of a single shake.
Packing tissue brown occurs first in the upper canopy. Water-stressed trees develop PTB sooner. Ethephon applied to trees stressed from under or overirrigation, disease or pests may cause leaf drop prior to and after harvest. Different walnut varieties have different responses to Ethephon applications.
Ethephon must be applied when temperatures are below 90 degrees F. It must hit the nut to be effective.
