Use of Walnut Hulls for Weed Control

Weed control was cited as one of the primary problems for organic growers,
and ranked as the second greatest problem for organic fruit and nut growers, according to the 3rd Biennial National Organic Farmers' Survey. Weed control options for organic growers are few, and any additional weed control methods would be valuable. Walnut hulls used as mulch around perennial crops may be an effective means of controlling weeds, not only from the weed-smothering effect of the mulch,
but also from the allelopathic properties of the hulls. Walnut hulls are known to contain a substance called juglone, which acts as a respiratory inhibitor in many plants (Purdue pub. #H0-193, Black Walnut Toxicity). Walnut hulls also contain about 5% protein at harvest, which may contain sufficient amounts of nitrogen that would also cause them to be of value to organic growers as a fertilizer. Walnut hulls are typically not utilized for any purpose and are often a disposal problem for hullers, and many walnut processors have concerns that the disposal of walnut husks may be regulated in the near future. Often, husks and culls are dumped into piles outside, where they may contribute to nitrate contamination after decomposition.
Objectives: This trial will:
a. study the effectiveness of walnut hulls as a weed control tool;
b. evaluate any phytotoxicity to the trees treated with the walnut hulls;
c. determine if the weed control properties are due to the effects of juglone or from the mulching effect;
d. note any weeds tolerant to juglone at this particular site;
e. evaluate the walnut hulls' potential as a source of plant nutrients;
f. determine the effective life-span of hulls as a weed control agent.