Although quack grass will grow luxuriantly up to the trunks of both black walnut and butternut trees, I know, from things I have seen myself, that the roots of the latter and probably of the former have a deadly effect on members of the evergreen family.  I have seen northern white pine and other pines, too, suddenly lose their needles and die when, as large trees, they have been transplanted to the vicinity of butternut trees.  To save as many of these transplanted trees as possible, it was necessary for me to sacrifice almost one hundred fine butternut trees by cutting them off close to the ground and pruning all the sprouts that started.

Other instances have also demonstrated to me this deleterious power of butternut trees over evergreens.  For years, I watched a struggle between a small butternut tree and a large Mugho pine.  Gradually the Mugho pine was succumbing.  At last, when the pine had lost over half its branches on the side near the butternut, I decided to take an active part in the fight.  I cut off the trunk of the butternut and pruned off all of its sprouts.  The butternut surrendered and died.  The Mugho pine took new heart, lived and again flourished.

At another time, I transplanted several thousand Montana pines, about thirty or forty of which came within the branch limits of a medium size butternut tree.  Within a year, these thirty or more trees had turned brown and were completely dead, while those immediately outside the branch area were dwarfed and not at all thrifty.  The trees farther from the butternut were unaffected and grew consistently well.  A similar condition, although not to the same degree, developed under a white oak where more Mugho pines were growing.  Another instance occurred when a planting of several thousand Colorado blue spruce were lined out and fell within the area affected by two butternut trees.  The spruce were all dead within a few months.

Many people have observed the detrimental effect of trees of the walnut family on alfalfa, tomatoes and potatoes, resulting in wilting and dying.  It is the root systems of the walnut which are responsible for this damage.  Apparently, there is some chemical elaborated near the surface of the roots, and sensitive plants, whose roots come in contact with either roots or ground containing this factor, are injured and sometimes killed by it.  One must therefore be very cautious about trusting these trees as protectors of many of the ornamental and garden plants.  I am certain, from my own observations, that their influence on evergreens is strongly antagonistic.

On another basis is the association between catalpas and chestnut trees growing adjacent to one another.  Constructive symbiosis apparently develops when a young chestnut tree is planted within the radius of the root system of a catalpa.  The latter very definitely influences the chestnut tree to grow more vigorously than it otherwise would.