Walnut toxicity

Q: Walnut Toxicity – I have a three year old walnut tree growing in a section of my garden, growing very vigorously. When I purchased and placed it in my garden I was not aware of its toxic impacts on some plant species.

I have several Australian natives plus two pistachios in its proximity. I have searched the internet looking for impacts on Australian native plants but can find nothing. I do not want to risk losing the other plants if there is any chance that the walnut will kill them as it grows. My instinct is to remove the walnut tree but I would hate to do it unnecessarily.

A:  It’s called allelopathy and is basically the walnut tree protecting it’s turf by exuding a chemical called juglone, and it applies mainly to the American Black Walnut. What we call the English Walnut (actually Persian, and the one most commonly grown locally) does produce the same chemical but to a far lesser extent. There are numbers of them growing among native trees, in the hills, without any affect. There is little evidence of any research that has been done in this regard.

Many other tree species, most notable among them various eucalypt species, produce chemicals which also have an allelopathic effect on other plants.

Not letting the leaf fall in autumn remain to compost will help, as it is the leaves that contribute particularly to the juglone in the soil.

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