Walnut kernels are mature and of lightest color and highest quality when the packing tissue between the kernel halves turns brown. To maximize kernel quality and minimize insect and mold damage, harvest as close as possible to the time when the most nuts have reached the packing tissue brown stage. The problem often encountered is that hull dehiscence (separation of the hull from the nut) occurs later than kernel maturity, and hot weather can further delay this process. To speed up hull dehiscence, accelerate maturity, and promote fruit abscission, ethephon is often used. The use of ethephon on the earlier-maturing varieties avoids the late season walnut husk fly and navel orangeworm flights.
Walnuts are either harvested with one or two shakes. When two shakes are planned, ethephon advances hullsplit and allows harvest to be conducted closer to the time when most nuts are at the packing tissue brown stage and of highest quality.
For a two-shake harvest, ethephon is applied when 100% of the nuts have reached packing tissue brown. Harvest can usually begin 7 to 10 days earlier than the normal harvest date, followed by a second shake about two weeks later.
For a one-shake harvest, ethephon is applied 10 to 14 days before the normal harvest date. With this application timing, nut removal is increased and a second shake is not needed.
Notes on the Use of Ethephon
- Harvest is normally 14 to 23 days after application; hot weather will cause it to be longer. It is recommended that test shakes be made to determine when the trees are ready to harvest.
- Do not spray low-vigor, stressed, or diseased trees, and do not apply when temperatures are below 60°F or above 90°F.
- Apply the spray within 4 hours of mixing the application.
- It is essential that you cover nuts thoroughly. Ethephon is not translocated from leaves to nuts.
- Only spray at one time the acreage that you can harvest in a reasonable amount of time.
- Recommended rates of ethephon may cause slight yellowing of leaves and some leaf drop on healthy trees.
- Make sure ethephon sprays only hit the target walnut crop. Since ethephon is a plant growth regulator, drift onto nearby fruit crops such as kiwifruit can have unintended consequences, including fruit drop.