by Becky Wheeler-Dykes, UCCE Glenn Orchard Systems and Weed Ecology Farm Advisor

Recent research has shown many potential benefits of utilizing whole orchard recycling (WOR) when removing an orchard. Instead of burning the wood produced in orchard removal, WOR involves chipping the wood from pulled trees and redistributing it back into the field using a chip or amendment spreader. This management technique has been mostly used in walnut orchards being followed by new walnut orchards. The conservation of the carbon in the wood benefits the soil by increasing nutrients available for the next crop, improving the soil's water holding capacity and and enhancing microbial activity. After a recent evaluation of weed populations in a WOR trial in Yuba County, it appears that weed suppression in the first few years after orchard establishment at a WOR site may be yet another benefit to this practice.

A WOR trial in Yuba County, led by UCCE Yuba-Sutter Farm Advisor Clarissa Reyes, was initiated by removing and chipping a mature walnut orchard at removal in late fall of 2023. The chips were spread at 60 tons/acre in winter 2023 in a grid design to allow comparison of orchard performance between areas with chips and areas without chips (control). RX1 rootstock trees were planted in April of 2024 and budded to Wolfskill walnuts in September 2024. In the first three months after orchard establishment, weed suppression was observed in the plots that had been covered in wood chips. Weed populations were surveyed in each plot in the orchard in August 2024.

A representative three foot by twenty foot strip within the rows between two trees was evaluated for each plot within the trial. The percent of soil covered by weeds was estimated, and the dominant weed species were recorded. The plots treated with wood chips averaged 14% weed cover, while the control plots with no wood chips averaged 58% weed cover. This difference was significant and very visually apparent.

In addition to difference in coverage of weeds, different species were observed in the chipped versus control plots. Almost all plots, regardless of treatment, had common knotweed (Polygonum arenastrum) and littleseed canarygrass (Phalaris minor). Control plots also had established populations of crabgrasses (Digitaria spp.), hairy fleabane (Conyza bonariensis) and fringed willowherb (Epilobium ciliatum). It is interesting that hairy fleabane, a particularly difficult weed to manage with known herbicide resistance, was suppressed in plots with chips. The possible mechanism of suppression for this particular weed could be physical, in that the seeds cannot germinate or seedlings are unable to break through the thick layer of mulched wood chips. Hairy fleabane has also been shown to be susceptible to allelopathy from juglone, produced by walnuts. There is no specific evidence indicating that the juglone in the soil from the wood chips is the control mechanism, but it may be worth evaluating in future trials.

We expect that over time, the weed suppression will decrease and eventually there will be no difference between the chipped plots and the controls. However, reduced weed pressure in the critical first few years after orchard establishment may prove to be a valuable benefit to WOR. Annual weed surveys will continue in coming years of the Yuba County WOR trial.

Image 1. Photo of control plots that received no wood chip application (left) and plots that were treated with spread chips (right). The control plots pictured all had strong stands of established hairy fleabane, while neighboring control plots had no hairy fleabane.