Learn / Ask The Landscape Professional
Last week's question about oak leaves got me wondering about black walnut trees and leaves. There is a black walnut tree at my church and the nearby garden is struggling. Is the black walnut tree toxic?
That is a great question. Black
walnut trees have been written about for thousands
of years. The Greeks and Romans used the fruit
to treat intestinal ailments, and other cultures
throughout time have relied on black walnuts to
build physical strength, treat skin diseases and
even treat dog and scorpion bites. Today, the
fine-grained, very hard wood is used in high-quality
woodworking and the nuts are desirable for their
antioxidants and omega-3 fatty acids.
On the flip side, the toxic effects of walnut trees
have been observed since Roman times when it
was believed they were toxic to all plants. While
that has proven not to be completely true, walnut trees are detrimental to many plants and landscapes.
Knowledgeable landscapers know that it is difficult, and sometimes impossible, to grow plants under
or near black walnuts. Research from the mid-1800's into the early 20th century identified the cause
of the toxicity as a chemical called "juglone" that is produced by the walnut tree. Juglone is present in
all parts of a black walnut tree including the bark, leaves and especially the roots. The black walnut
tree can grow to be 100 feet tall and the zone of toxicity can extend around the tree to a radius of 80
feet, expanding as the tree grows each year. Plants that are susceptible to juglone will show signs of
yellowing, wilting leaves and will eventually die. There are a number of plants and trees that are resistant
to juglone including some species of birch, beech and maple. If you have a black walnut tree, it
would be best to segregate the leaves and do not use them to mulch your garden plants or add to your
lawn as organic matter. Studies have shown that the toxic effect of walnut leaves can last a month or
even longer if they are composted.
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About Linda Lillie
Linda K. Lillie is the President of Sprigs & Twigs, Inc, the premier
landscape design and maintenance, tree care, lawn care, stonework, and carpentry
service provider in southeastern Connecticut since 1997. She is a graduate of
Connecticut College in Botany, a Connecticut Master Gardener and a national
award winning landscape designer for her landscape design and landscape installation work.