As a gesture of appreciation, I thought I would offer a quick on-line
summary to refresh everybody's memories on this allelopathic effect of
walnuts. Most of this information is from the classic 1951 West Virginia
Univ. Ag. Expt. Sta. Bull. 347 by Maurice Brooks.
First, many of the other walnut family also have this effect, including
butternuts. Although the leaves, nuts, twigs, etc. all contain juglone,
the toxic effect seems to be only via direct contact between root systems.
Stone walls, etc. act as effective barriers. The pH tends to be higher
under black walnuts too.
There is a LONG list of species that are apparently tolerant to juglone.
Some even appear to thrive, such as black raspberry and Kentucky bluegrass.
Walnuts are beneficial in pastures because of their effect on a number of
forage and turf grasses. Others include: wild grapes, clovers, violets,
ferns, asters, mint, wild hydrangea, poison ivy, Virginia creeper,
composites such as asters, goldenrod, dandelion. Naturally occuring tree
associates include tulip poplar, white ash, basswood, black cherry, oaks,
hickories, red cedars (juniperus).
Documented toxic effects have been noted for: red pine, white pine, apples,
blackberry, alfalfa, cinquefoil, potatoes, tomatoes, and almost the entire
Heath family (rhododendrons, azaleas, etc.).
As to why I originally posted this query. A new amur maple and river birch
that I planted two years ago were not thriving, and I wondered if juglone
from a nearby black walnut was the cause. I have since dug up and
transplanted both, and discovered their puny state was more likely due to
the poor job I did of making sure their roots were not headed in circles
from being container-grown specimens.
Now that I know from Mr. Brooks that black walnuts are perfect companion
plants to turfgrass (maybe mostly due to timing and duration of canopy),
perhaps I will be more willing to tolerate their yucky fall webworm nests
and completely unglorious leaf senescence.
Thanks again folks.
Program on Ethics and Public Life
119 Stimson Hall
Cornell University, Ithaca, NY 14853