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Official Blog of the UC Master Gardener Program of Contra Costa County
by Stephen I Morse
on October 1, 2014 at 1:57 PM
An interesting update from the Garden Professors BLOG on walnut alleopathy and the use of walnut chips for mulch...  
see <> the blog from Linda Chalker-Scott dated October 1st...calls to question the actual impact of walnuts... specifically what and when the impact is a problem...
by Ty
on April 19, 2016 at 4:01 PM
Will a black walnut tree effect potted plants or only the ones planted in the ground?
Reply by Stephen I Morse
on April 19, 2016 at 6:33 PM
Not a lot of information on this potential impact that I could find, but black walnut always stands out for its allelopathic impacts on plants and is a potential impact on many central CCC homes sitting in old walnut orchards.  
Those comments about allelopathy are almost always related to root/soil pathways. On the other hand, some rather skimpy information I found would seem to indicate that the leaf and litter (i.e., something close to "mulch") from allelopathic plants can have similar impacts, especially black walnut and probably eucalyptus.  
One reference I found "Impact of Mulches on Landscape Plants" at stated in part  
Properly applied landscape mulch will not have allelopathic effects on established landscape plants. It is most likely to injure newly planted or shallow rooted plants. A short period of composting and correct application of woody mulch will prevent damage."  
Realizing that not all plants exhibit allelopathic effects, I think that to be on the safe side you should consider the quote above and keep the potted plants clear of black walnut leat litter, etc. and if mulch is needed use from another source other than the black walnut. However, it does appear, used cautiously, well-composted mulch should minimize and/or eliminate the problem. Considering that potted plants don't usually require a lot of mulch and/or compost, I would use some other source of mulch and/or compost. Possibly the use of "other" mulch could also protect from the allelopathic black walnut leaves from accumulating on the soil and potentially causing impacts.  
Hopefully, that answers your concerns. If not, please do not hesitate to submit more questions.  
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