Roundup, a common product containing the non-selective, systemic herbicide glyphosate, is widely used for weed control around homes and in landscapes. Since this herbicide is designed to kill nearly all plant life it contacts, homeowners must take precautions to protect valuable trees. Determining the safety of using glyphosate near trees depends entirely on understanding how the chemical works and carefully controlling its application. The potential for harm is determined by direct or indirect contact with the chemical, not merely its presence nearby.
Understanding Glyphosate’s Action and Tree Susceptibility
Glyphosate is a systemic herbicide, meaning that once a plant absorbs it, the chemical moves throughout the plant’s vascular system to reach growing points in the shoots and roots. The mode of action involves inhibiting the plant enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSP synthase). This enzyme is part of the shikimate pathway, which is essential for plants to produce specific aromatic amino acids required for growth.
Absorption primarily occurs through green, actively growing tissues, such as leaves, soft stems, and young, thin bark. Once absorbed, the chemical is translocated through the phloem and accumulates in the meristem tissues, the zones of active cell division and growth. When accidentally sprayed onto young trees, glyphosate can accumulate and move to the roots, causing symptoms like stunted growth or bark splitting the following spring.
Mature trees possess thick, corky bark that acts as a physical barrier, making them highly resistant to absorption through the trunk. However, the tree’s root system, which extends far beyond the visible canopy (drip line), remains vulnerable. While glyphosate is generally not readily absorbed by woody species’ root systems, it can be taken up through exposed surface roots, especially if they have been damaged or if the soil is freshly disturbed. Glyphosate also breaks down relatively quickly in the soil, which reduces the risk of long-term soil uptake by established roots.
Minimizing Direct and Indirect Exposure
The greatest risk to surrounding trees comes from direct spray contact or airborne drift, which can be mitigated through careful application techniques. To control drift, apply the herbicide during calm weather conditions, ideally when wind speeds are between three and ten miles per hour. Applying the spray when temperatures are below 80 degrees Fahrenheit also minimizes the risk of the liquid evaporating and turning into a vapor that can drift long distances.
Using a physical shield is a straightforward way to prevent direct contact with the tree’s bark or foliage. A piece of cardboard, plastic sheeting, or a spray wand shield can block the herbicide from reaching the trunk or surface roots. For weeds growing extremely close to the trunk, use a wick or sponge to wipe the herbicide directly onto the target plant, avoiding spray mist entirely. Using lower spray pressure and nozzles that produce larger, coarser droplets also significantly reduces the chance of fine mist being carried away by the wind.
Avoid spraying the herbicide over the tree’s root zone, which generally extends to and beyond the drip line. While glyphosate is minimally absorbed through undisturbed soil, applying it where surface roots are visible or where the soil has recently been dug up creates an opportunity for root uptake and damage. If you must treat weeds in this area, alternative methods like hand-pulling or mulching are safer options for the tree’s long-term health.
Assessing Risk by Tree Age and Type
The tree’s age and specific biological characteristics are major factors in determining its vulnerability to glyphosate exposure. Young trees and saplings are at a significantly higher risk because their bark is thin, sometimes still green, and lacks the protective, corky layer of mature trees. This thin bark allows for easier absorption, and the tree’s smaller size means a relatively small amount of chemical exposure can cause widespread systemic damage. Sublethal doses absorbed in the late season can be stored and then translocated the following spring, potentially causing damage that may persist for years.
Established, mature trees are generally more tolerant due to their thick, protective bark and larger mass, which can dilute a small accidental exposure. However, certain tree types present unique vulnerabilities that increase the risk regardless of age. Trees that naturally produce root suckers, such as sumac, black locust, or some fruit trees, are particularly susceptible. Since these suckers are actively growing clones directly connected to the main tree’s root system, treating them with glyphosate allows the herbicide a direct path into the parent tree. In these cases, it is safer to physically prune the suckers rather than use a systemic herbicide near them.