Whether a common weed killer will destroy a desirable shrub is a valid concern for any gardener, and the answer is not a simple yes or no. The outcome depends entirely on the chemical composition of the product, the method of application, and the physical characteristics of the bush itself. A mature woody plant possesses natural defenses that a tender weed lacks, but these can be bypassed by the wrong chemical or careless application. Understanding how herbicides are classified is the first step in protecting your landscape.
Understanding Herbicide Classification
Herbicides are defined by two primary characteristics: their selectivity and their mode of action. These distinctions determine whether a product targets only certain plants or kills nearly everything it touches. Non-selective herbicides are the primary danger to bushes because they eliminate almost all vegetation they contact. For example, glyphosate works by disrupting an enzyme pathway plants need to produce essential proteins.
Selective herbicides are designed to target specific plant types, such as broadleaf weeds, while leaving others, like grasses, unharmed. Herbicides are also categorized by how they move through the plant. Systemic herbicides are absorbed by the foliage or roots and travel throughout the plant’s vascular system to reach the root tips and growing points. This makes them effective at killing the entire plant, including the underground structure.
Contact herbicides only damage the parts of the plant they physically touch, typically causing rapid localized burn or wilting. These are less effective on established woody plants because they cannot translocate deep enough to kill the root system or protected stems. For a mature bush, a contact herbicide may brown the leaves but usually fails to inflict lasting damage, allowing the plant to recover.
Factors Determining Damage to Bushes
The physical structure of a woody bush provides natural resistance that herbaceous weeds lack. Mature shrubs have thick, corky bark and a waxy cuticle on their leaves, which act as a physical barrier preventing herbicides from penetrating the plant tissue. These protective layers mean that foliar sprays are often ineffective on fully established bushes.
Younger bushes, or those with thin bark and new, tender growth, are much more vulnerable to accidental exposure. Herbicides are also readily absorbed through the roots, meaning soil saturation or runoff near the base of a bush can be fatal. The extensive root system of a mature bush can quickly take up chemicals that leach into the soil, translocating the poison throughout the plant.
The most common cause of damage is accidental application, often through spray drift. Particle drift occurs when fine spray droplets are carried on air currents to non-target plants, causing symptoms like twisted, discolored, or distorted leaves. Plants are less susceptible to herbicide damage when they are dormant, as their metabolic processes are slowed. Exposure during active growth, especially during spring leaf-out, can result in severe damage.
Targeted Methods for Woody Plant Management
When the goal is to intentionally kill a bush or small tree, specialized techniques are required to bypass the plant’s natural defenses. Simply spraying the foliage is usually ineffective on a mature woody plant due to the protective bark and waxy leaves. The most reliable method for removal is the cut-stump treatment, which involves applying a concentrated systemic herbicide directly to the freshly cut surface of the stump.
This application must be done immediately after cutting, ideally within five minutes, to ensure the herbicide is absorbed by the vascular tissue before the plant seals the wound. For larger stumps, the chemical is painted onto the outer ring of living tissue, called the cambium and sapwood, which actively transports the herbicide down to the roots. Another method, the basal bark treatment, uses oil-soluble systemic herbicides applied directly to the lower 12 to 18 inches of the intact trunk, allowing the chemical to penetrate the bark and travel to the roots.
To protect desirable bushes near a treatment area, precision is the overriding concern. When using any spray, use physical barriers, such as cardboard or a tarp, to shield the foliage and young bark from accidental contact or drift. Where weeds are growing close to the base of a bush, it is safer to use a non-spray method, such as a granular selective herbicide or carefully spot-treating the weed with a small applicator brush. Other chemicals may be mobile in the soil and require caution to prevent root absorption.