Hickory [Carya] rarely ever causes a problem and neither does Persian Walnut [Juglans regia] also known as Carpathian or English Walnut. They do produce juglone but to a much smaller degree than Black Walnut and Butternut.

The roots and nut hulls of Black Walnut (Juglans nigra L.) and Butternut (Juglans cinerea L.) produce a substance known as juglone (5-hydroxy-alphanapthaquinone). Persian (English or Carpathian) walnut trees are sometimes grafted onto black walnut rootstocks. Many plants such as tomato, potato, blackberry, blueberry, azalea, mountain laurel, rhododendron, red pine and apple may be injured or killed within one to two months of growth within the root zone of these trees. The toxic zone from a mature tree occurs on average in a 50 to 60 foot radius from the trunk, but can be up to 80 feet. The area affected extends outward each year as a tree enlarges. Young trees two to eight feet high can have a root diameter twice the height of the top of the tree, with susceptible plants dead within the root zone and dying at the margins. The juglone toxin occurs in the leaves, bark and wood of walnut, but these contain lower concentrations than in the roots and nut hulls. Juglone is poorly soluble in water and does not move very far in the soil.

The Ohio State University Extension and the American Horticultural Society have reported that R. periclymenoides, formerly R. nudiflorum, Pinxterbloom Azalea, and Exbury Azaleas Gibraltar and Balzac will grow near Black Walnut and Butternut trees. They also list many other plants that will grow in the root zone of these trees.

Conversely, some rhododendrons will kill plants growing near them. That is one thing that is so insidious about R. ponticum in England. The ponticum runs rampant and kills nearby plants. This property of one plant to secrete a chemical that affects other plants to help it gain an edge in plant competition is called "allelopathy".