If you have a black walnut planted in your backyard, then you'll need to know that the tree itself could be hurting your plants. Every part of the black walnut tree contains juglone, which ends up in the soil and is harmful to plants that are sensitive to it. If you want to plant near a black walnut tree you'll need to work with plants that are tolerant of juglone and follow a few simple rules for planting.
Dig the soil to loosen it where you want to plant, avoiding areas which are directly next to the roots of the walnut tree. Remove approximately half of the soil in your bed area and replace it with commercial planting soil to lessen the juglone levels initially.
Set up a raised garden bed in your preferred location if breaking ground is hampered by dense roots. After setting up a frame, fill the bed with commercial planting soil and clear away dropped leaves, limbs, and walnuts as they fall.
Plant your juglone-tolerant plants according to their particular planting requirements and water as needed. If you are planting close to the base of the tree, select shade-tolerant plants such as hostas.
Watch your plants growing habits and if you notice any of your new plants struggling over the first year or two consider moving them farther away from the trunk of the black walnut and out from under the canopy, or drip line, of the branches.