Thank you for your question. Unlike walnut trees, no source indicates that Katsura trees are allelopathic, the scientific description of having an effect on other plants. So, there must be another reason plants grown in the mulch are not thriving. It could be that the mulch is piled too deeply, so water cannot get to the plants’ roots. Or that the mulch is removing nitrogen from the soil (typically lasting no more than a year after mulching), so the plants have chlorosis (yellowing, then brown leaves, then death). But, without a photo of the plants, we cannot diagnose the problem. Can you provide more information? Thanks!
Thank you, Kristena, for the quick response. You've answered all my questions. That the katsura isn't allelopathic was the most critical, and the answer I was having trouble finding. The depth of the mulch and its nitrogen depleting character is most likely the other part of the problem. I had planted zucchini, that simply aren't thriving. However now that I know the mulch isn't allelopathic, I will remove some mulch, spread it elsewhere, and give the zucchini a chance to do something before the season is over. At least it has kept the weeds at bay! Thanks truly!
More than welcome!
Here is an Extension article you will appreciate: http://cru.cahe.wsu.edu/CEPublications/FS160E/FS160E.pdf