Whole orchard recycling (WOR), the practice of chipping trees, spreading the chips on the orchard floor and incorporating them into the soil has proven to be a viable option for many almond growers, but walnut WOR has its challenges.

UCCE Orchard Systems Advisor Luke Milliron has been working on solutions for walnut growers who are pulling out older orchards and planning to replant with walnuts. Trial sites are in Yuba and Sutter counties. Cliff Beumel, president of Agromillora California Nursery, has also been involved in a WOR trial in the Sacramento Valley since 2021.

“We’re interested in seeing how the new trees perform, how the ground is prepared. We wanted first-hand knowledge about this entire process so we can help our customers,” Beumel said.

“There is a lot of grower interest in WOR, but growers in the Sacramento Valley still have burning as an option,” Milliron said.

The size of mature walnut trees means a huge amount of chips to be spread and incorporated into the orchard soil. Ensuring a uniform mix of chips and soil and a suitable planting environment for new trees were two challenges.

Beumel said the chipping process in the trial resulted in 136 tons of chips per acre. Even after accounting for the water in the wood chips, there was still 91 tons of dry matter, about a seven- to nine-inch-thick layer of chips, to be incorporated. Beumel said they used the largest stubble disc available, one typically used in rice, which resulted in a mixture of wood chips and soil in the top eight inches of the orchard floor.

Last spring, Beumel said it was not possible to push a shovel down into the soil due to the wood chips. One year later, he said that isn’t the case.

Fertilization of the new trees is a key component in WOR as the chips can potentially consume nitrogen as they decompose. Beumel said they applied highly soluble calcium nitrate and micronutrients weekly and/or every other week in small amounts by hand and then irrigated them in (though fertigation would work just as well.) Those nutrient applications rapidly move into the root zone and are not tied up in the chips, making them readily available to the new trees.

One year after planting, Beumel said he is encouraged about the growth of the new walnut trees. “There has been excellent, uniform plant growth, chips have broken down well. I’m anxious to see how they do in the next few years.”

Walnut growers are watching the trial, he said, as they make decisions about taking out older, unproductive orchards.

CDFA has added WOR to its Healthy Soils Program, and grants to offset the costs may be possible. Sequestering that amount of carbon in the soil as chips are incorporated may also provide an opportunity to receive credits in the carbon market.