Answer: Planting under or near walnut trees can be a challenge. Walnut trees are allelopathic; they exude a substance which inhibits the growth of other plants. The substance is juglone. Additionally, established walnut tree roots will compete with the roots of other plants for moisture and nutrients. Finally, the roots of walnut trees can take up enormous amounts of water, which can leave plants in the vicinity with extremely dry soil, even if they are watered regularly. While I can't say for certain that the walnut trees are responsible for the death of your dappled willows, there is a strong likelihood that they were.
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