Experts and BHG readers answer.
Remember, the roots of a large black walnut tree can extend far beyond the reaches of its canopy, so there is no sure-fire safe distance for planting. Keep in mind, the further away, the better. If in doubt, experiemnt with cheaper plants. Some colorful perennials to consider planting near black walnuts include astilbe, bee balm, hostas, lamb's ear, sweet woodruff, lungwort, ferns, daffodils, hardy geranium, and bellflowers. These are some of the plants fellow gardenrs have grown successfully under of near black walnuts.