No phytotoxicity to any of the trees in the experiment was seen, and only noticed for a short
time on the two dock species and Buckhorn plantain.
As the hulls provided less weed control and only controlled weeds that were also controlled by
the shell mulch, there probably was little or no weed control resulting from the juglone. This may be due
to the fact that the hulls were collected after harvest and had been allowed to decompose for at least a
month. Also, the hulls were wetter than the shells, causing some dry-down, and the hulls probably
decomposed quicker and released more plant nutrients than the shells, resulting in more weed growth in
the walnut hulls. Perennial plants and well-established annuals at time of mulch application were able to
grow through both mulches, with the exception of Malva neglecta, which did not reemerge through either
mulch.
It is questionable that any juglone was even present in the walnut hull mulch. Besides breakdown
of juglone by microorganisms or temperature, it is possible that the juglone was in an inactive form
during the span of the trial. Although reports contradict each other as to the conditions in which juglone
is active, low pH and anaerobic conditions seem to increase the amount of active juglone in the
environment (Schlien, 1990). Although the mulch was wet for most of the experiment, it had a high pH
which may have destroyed the juglone if it was not already destroyed by decomposition.
Since the hulls did not appear to cause any phytotoxicity to the trees, walnut hulls may be safe to apply in
orchards. The nutrient content of the hulls is similar to manure or compost, and could supply a very
significant amount of plant nutrients even if applied at lower rates than used in this trial. The hulls also
provided weed control comparable to a herbicide spray when applied under the right conditions. The
mulch did lower the temperature by about 1-2 degrees Fahrenheit, which could have implications in
young orchards where maximum root growth is important.
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