Answer: Although the majority of the toxin is released in the root zone which can extend about half again as far as thebranch spread, there is also a certain amount present in the fallen leaves and stems and nuts, so unfortunately, I don't think your plan will work in the long run. This is especially true because the shrubs in question as a group have shallow but very wide-spreading roots and do not do all that well in containers without specialized soil mixes and watering techniques. Would it be possible to move them to another location instead? I'm really sorry I can't be more encouraging.
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