Weed control is a major challenge for organic growers, ranking as the second greatest concern for organic fruit and nut producers, according to the 3rd Biennial National Organic Farmers’ Survey. Options for weed management in organic farming are limited, making new methods highly valuable. Using walnut hulls as mulch around perennial crops may effectively control weeds, utilizing both the smothering properties of the mulch and the allelopathic effects of the hulls. Walnut hulls contain juglone, a substance that inhibits respiration in many plants. Additionally, with around 5% protein at harvest, walnut hulls may provide nitrogen beneficial for organic growers. Currently, these hulls are often underused and present disposal challenges for hullers, with concerns looming over potential regulations on walnut husk disposal. Typically, husks and culls are piled outside, risking nitrate contamination through decomposition.

The objectives of this trial were to:
a. Evaluate walnut hulls for weed control;
b. Assess phytotoxicity in trees treated with hulls;
c. Determine if weed control is due to juglone or mulching effect;
d. Identify weed species tolerant to juglone at this site;
e. Examine walnut hulls as a source of nutrients;
f. Determine the effective lifespan of hulls as a weed control agent.

Walnut hulls were applied 3 inches deep in a mature conventional prune orchard to assess their suitability as mulch for weed control. No evidence of phytotoxicity from juglone was observed in either weeds growing through the mulch or on the trees. The hulls were slightly less effective as mulch compared to almond shells, which do not have known allelopathic properties.

In a similar trial funded by the Organic Farming Research Foundation, walnut hulls were applied to a one-year-old organic citrus orchard. Weed control was less effective in this rough orchard floor at the same application rate. Phytotoxicity was noted in two species not present in the earlier trial: Buckhorn plantain, Curly dock, and Cluster dock.