It is commonly used to manage weeds in landscape settings because it is economical, controls a broad spectrum of weeds, and is easy to use. It can be unforgiving in that it is non-selective, and an accidental overspray to desirable plants can be devastating. This is especially true for young trees that have thin bark, and often have chlorophyll in their bark. Late season misapplications to these non-target trees results in sub-lethal damage, and the effects can last for years.
Glyphosate is a systemic herbicide that moves through the phloem and accumulates in the roots. That is why it "kills weeds, roots and all." It breaks down quickly in the soil. However, when accidentally over-sprayed onto the thin or pigmented bark of young trees, glyphosate accumulates in the phloem and can take years to break down. It is then translocated to the roots along with the sugars in the fall. When sap rises the following spring, it carries the glyphosate along with it, causing a variety of symptoms: witches brooms, cupping, stunted growth, chlorosis, and bark splitting. The injury can continue a number of years after the misapplication.
Close up of undersized foliage and buds that failed to open. Photo: S. Feather, Penn State
Dr. Hannah Mathers, an independent nursery crop/landscape consultant with Mathers Environmental Services, LLC, conducted research on bark splitting of nursery and landscape trees during her time at The Ohio State University. She found that drift from late season glyphosate applications was absorbed into young trees, damaging the bark structure and reducing their winter hardiness. This resulted in bark splitting that can open these trees to a variety of pathogens. Mathers also found that formulations of glyphosate that contain surfactants (products that increase its absorption into target weeds) increase the likelihood of damage.
Bark split that runs from the ground to the first scaffold of branches. This side of the tree never gets direct sun because of the wall behind it. Photo: S. Feather, Penn State
It is important to hand weed anything in close proximity to tree trunks and surface roots - or any other desirable plants - to avoid glyphosate damage. If you must spray near desirable plants, use a shield on the spray wand to minimize off-target damage. These can be purchased, or made from a two-liter soda bottle. Another option is to use a wick to wipe glyphosate directly onto target weeds.
Never use glyphosate to remove suckers growing from the rootstocks of grafted trees. They are directly connected to the tree's vascular system, and such applications will damage the tree, or possibly kill it. The best way to deal with suckers is to prune them off during the dormant season, or after new growth has hardened off, then apply a product that contains naphthaleneacetic acid (Tre-Hold Sprout Inhibitor A-112, Sucker Stopper RTU, Sucker Punch RTU and others).
Once trees have been damaged by glyphosate, the best course of action is to protect them from further stress. Provide irrigation during hot, dry weather, and control any insect or disease problems that occur. Fertilization is not the best course of action, because pushing growth might just add to the stress. Wait until the tree shows signs of recovery and then fertilize based on soil or tissue test results.