walnut trees

LOGAND logand at msdos.ensam.inra.fr
Wed Nov 15 04:45:19 EST 1995

Bill wrote:

>Hi: I've read that certain plants won't grow near mature black walnut
trees. I
>had the pleasure of witnessing this phenomenon this summer. Most of the
>in my vegetable garden grew to a couple of feet tall and then wilted. The
>garden is 10 to 20 feet away from a 50 foot walnut. Apart from digging up
>new area for a garden or cutting the tree down (I could use the wood for
>making cabinets) I was wondering if there was a way to counteract the
>the tree has on vegetable growth. I cannot find any information on what
>exactly is the process involved. Is it a pH thing, a toxin, does it come
>the roots or decomposing husks? Any help would be greatly appreciated.

The allelopathic (biochemical interactions between higher plants) effects
of the black walnut Juglans nigra has been known since the first century
AD. The toxin involved is suggested to leach from both leaves and roots and
has been identified as the 4-glucoside of 1,4,5-trihydroxynaphthalene which
on hydrolysis and oxidation is converted to the napthoquinone - juglone. It
only becomes active when leached from the plant. The only way I can think
of to obviate the problem is to have an exlusion zone - ie. there will be
effects radiating out from the trunk to the distance covered by the canopy
and root architecture - though this is not a true remedy. Some plants show
resistance to juglone (or tolerance) - I only know the two mentioned in the
book I am referning to and they are not garden plants!

(Information from Introductgion to Ecological Biochemistry, fourth edition,
J.B. Harborne, Academic Press (should be available in all good science

Hope this is of interest if not actual practical help!

David C. Logan

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