black walnut root toxicity

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I was interested in the comments about problems with siberians under the black walnut tree because I'd never heard of it.  Here's a site, but no mention of irises, only the plants I already knew were a problem.  I wonder if it was the walnut that was the problems, or if something else was also at work.

http://www.extension.umn.edu/yardandgarden/ygbriefs/h407blkwal-tox.html

Margaret Boehm
now in Florida and no longer growing irises, sigh.