Answer: Black walnut (Juglans nigra) has a reputation for being allelopathic--that is, for inhibiting the growth of other plants. Small amounts of 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 are extremely sensitive to Juglone and others are quite tolerant. You might consider planting a groundcover rather than turfgrass. Ivy is tolerant of Juglone, as are Astilbe, Bee Balm, Bellflower, Bergamot, Ajuga, Coral Bells, Crocus, Shasta daisy, wild ginger, Lamb's ear, Lungwort, Morning Glory, Pansy, Phlox, Marigold, Primrose, Sweet Cicely, Honeysuckle and Zinnia.
Since your walnut tree is there to stay, try to plant one of the less-susceptible plants in the vicinity of the trees roots or canopy. Also, when planting under any tree, remember that feeder roots close to the surface of the soil will compete with the roots of other plants. As a result, you may have to water more frequently to keep the plants supplied with adequate moisture.
Q&A Library Searching Tips