Is regular use of Roundup around apple trees poisoning apples?

Expert response from Allan Felsot

Professor and Extension Specialist, Washington State University

Friday, 18/09/2015 16:42

Roundup has been used for quite a long time to control weeds in many orchards. It is used near the base of the trees as a directed spray. The trees and their productivity are not harmed by such an application, and careful attention to spray operational practices that avoid drift also results in a prolific growth of mixed vegetation in the row centers, which prevents soil erosion and encourages a proliferation of insect natural enemies that can help with biological control of insect pests. Thus, it is possible to use a herbicide with noted broad spectrum properties successfully in an orchard of mixed plant species.

This seeming paradox is understandable when one considers how environmental chemistry in the soil affects bioavailability to plants. When Roundup is sprayed on the soil surface surrounding trees, the glyphosate in the formulation does not penetrate into the soil because the herbicide binds to the soil in a process called sorption. Thus, glyphosate does not move to the absorptive root mass and therefore cannot move into the tree sufficiently to cause phytotoxicity. 

Indeed, the only way the Roundup can control weeds is to be sprayed on growing vegetation because studies show it is not active from the soil owing to the soil binding phenomenon. An older study (see citation below) published in the scientific literature had addressed the issue empirically of whether glyphosate applied to the lower trunks of trees would result in residues in fruit. The authors did not find any glyphosate residues in other parts of the tree using sensitive methods involving radiotracers. 

Thus, if glyphosate from the Roundup cannot move into the tree, there is no logical reason to think it would move into the apples. Finally, we know the glyphosate does not move into the tree because the trees do not exhibit any signs of phytotoxicity and the apples grow normally.

Putnam, A. R. (1976). Fate of glyphosate in deciduous fruit trees. Weed Science, 425-430.