Answer: Black walnut (Juglans nigra) has a reputation for being allelopathic--that is, for inhibiting the growth of other plants. Small amounts of a substance called juglone are released by the tree roots, but a far greater amount of this growth-inhibiting chemical is found under the canopy of the tree. Some plants that are tolerant to juglone include: arborvitae, cedar, catalpa, clematis, daphne, elm, euonymous, forsythia, hawthorn, hemlock, sycamore, astilbe, begonia, bellflower, orange hawkweed, hosta, pansy, phlox, marigold, primrose, snowdrop, sweet woodruff, trillium and zinnia. Citrus is reportedly sensitive to juglone so unless you can remove the soil and decaying roots of your walnut tree, you might want to plant your citrus in a raised bed rather than directly in the ground where decomposing roots could release juglone. The idea of planting in a raised bed is that by the time the citrus roots reach the soil where the walnut tree roots once grew, the juglone should have all dissipated.
Best wishes with your landscape!
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