Both black and English walnut species (Juglans spp.) contain a substance called juglone that's toxic to many plants, shrubs, fruits and flowers. While black walnut trees are generally more toxic than English walnuts, English walnut may be grafted onto black walnut rootstock, eradicating this difference. Trees grafted onto Persian walnut rootstock do not contain juglone, so anything can be planted beneath them. Planting around most walnut trees is divided into two zones to address walnut toxicity: inside the toxicity zone and outside of it.
Measure 50 to 60 feet from the walnut tree trunk in all directions for a mature tree. This is the toxicity zone, where you need to be most careful planting around the walnut tree. The toxicity zone of young trees may be twice the height of the tree; there's no way to accurately find out how large the zone is for young trees, because it expands each year.
Plant any garden plants outside the toxicity zone. You should not experience growth problems due to walnut toxicity. Plants that thrive outside of the zone and cannot be planted inside include tomatoes, lilies, hydrangea, lilac and mountain laurel.
Choose plants carefully when planting in the zone around the walnut tree inside the 50 to 60 foot radius you measured.
Choose plants that can grow around walnut trees, including zinnia, marigold, squash, beans, Virginia creeper, Eastern redbud and Japanese maple. Review Ohio State University Extension's list of trees, shrubs, vegetables and flowers that can and cannot grow under walnut trees (see Resources).
Dig holes for your plants that are twice the size of the plant's root ball. Remove rocks and weeds from the hole. Then pull your plants from their containers and massage the root ball to loosen it.
Place the plants in the prepared holes and spread their roots out in the soil. Backfill the hole with soil to finish planting. Plant all plants around your walnut tree in this manner.
Water until the ground becomes saturated and the soil compresses around your new plants.