Herbicides that can burn tissues like Gramoxone or Ignite usually only cause localised damage on green bark. But remember that all trunks grow small amounts of new bark each year as the girth expands. Most residual herbicides like Princep, Dual or Sinbar are less likely to cause injury to trunks. When we see injury from these herbicides, it has usually been taken up by roots or leaves. However, herbicides that are systemic, like Roundup and Amitrol, can be absorbed into the trees and moved to the roots or the growing points.
Trees become more vulnerable to permanent injury after mid-season when terminal buds set.
This is when trees start pushing stored energy to their roots, and may take the herbicide down with them. So confine your use of systemic herbicides to spring and early summer only. Another reason to be cautious about allowing systemic herbicides like Roundup into your trees has been discovered by accident in forestry research. And to make matters worse, these small doses appear to make trees more susceptible to disease. In reforestation trials, weedy scrub trees that were not controlled by low doses of Roundup, were found dead of diseases several years after the research was completed, even though they were not well-controlled by the initial Roundup treatment.