Question
QUESTION: I have a big walnut tree in my back yard and have always had trouble growing grass under it, I assumed that i didn't get the leaves and nuts raked up quickly enough but recently i was told that walnut tree roots also exude something that kills the grass and there is nothing you can do except cut the tree down. Is this true? is there anything I can sprinkle on the lawn to save it? A side note if it matters mine is an English Walnut grafted onto a Black Walnut rootstock.

ANSWER: Butch:

Yes, walnut trees emit a chemical called hydrojuglone which discourages neighboring plants and produces what is called the allelopathic effect.  The chemical comes from the leaves, roots, twigs, nut hulls and bark.  It is difficult, if not impossible, to overcome.

If you have fescue or bluegrass as a lawn, these do well under a walnut.  Other types may or do not do as well.  Try to make the soil neutral.  The walnut will try to make it acid.  Removing the leaves, nuts and twigs will reduce the chemical concentration under the tree.  Do not use this matter for compost, as the chemical is persistent.

I do not know that an English walnut had the same effect for sure.  However, since the rootstock is black walnut, the effect comes from the roots as well as drippings exuded from the tree.

Hope this helps.  Best wishes.


---------- FOLLOW-UP ----------

QUESTION: Thank You very much, other than keeping the leeves and such raked up is there anyhting I can sprinkle on the grass to keep the soil neutral?

Answer
Butch:

Sorry.  I did not answer this part of your question before.

Yes, there is.  The point of this standard of neutral pH soil is to encourage the breakdown of the acid causing the problem.  You may use lime, of course.  Compost and composting breaks down this compound effectively.  I would assume that applying compost to the area would not only keep the compound under some control, but also fertilize the lawn.  Compost is neutral by nature if made correctly.

Apply 3/8 inch of a high quality compost.  That level is good for grass.

Best wishes.