At a recent Missouri chapter of the Walnut Council field day we overheard growers say they "had always been warned about root grafting, but nobody seems to have seen it." Root grafting is a natural circumstance that occurs under the soil surface in the root zone. As roots multiply and cross over each other they tend to fuse and grow together. The result is that nutrients and water and any other materials moving through the roots are shared by the trees grafted together by their roots.
To "see it," one must observe the detrimental effects of one tree upon another on the surface. This can sometimes be seen when a tree is cut and its stump treated with a systemic herbicide to control regrowth. The herbicide also travels through the roots of neighboring trees that are root-grafted to the killed tree and they, too, die.
Yes, it has been seen. At HarperHill Farms at Butler, Mo. every other tree in a 12-year-old black walnut plantation was removed. The remaining trees were grafted to known cultivars. Originally planted on a 40- by 10-foot spacing, half the trees were removed to allow room for adequate growth of the remaining grafted trees.
Stumps of the removed trees were treated with an undiluted application of Tordon herbicide. The stumps averaged about 4 inches in diameter and thoroughly covered with the herbicide using a squirt bottle. The thinning and herbicide treatment were made in the spring of 1997. There was no regrowth on the stumps. However, in several instances, the neighboring tree that had been grafted soon sickened and died, showing classic signs of herbicide poisoning. These trees were 10 feet away from the treated stump.
Of about 100 stumps treated, only four neighboring grafted trees died.
The question comes to mind: Would this have occurred if the trees had been thinned at an earlier age before root grafting could take place?