The question of whether to cut weeds before applying a glyphosate-based product, like Roundup, is common for homeowners seeking maximum effectiveness. The general answer is that you should not cut or mow weeds immediately before spraying. This practice is counterproductive to the herbicide’s mode of action, which relies on maximum leaf surface area for uptake. Maximizing absorption ensures the systemic herbicide can eliminate the entire plant, including the root system.
How Glyphosate Works
Glyphosate is classified as a systemic, non-selective herbicide, meaning it is absorbed by the plant’s green tissues and travels throughout its entire vascular system. Once inside the plant, the chemical targets a specific biochemical process found only in plants and some microorganisms. It works by inhibiting the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), which is a component of the Shikimate pathway.
Blocking this enzyme prevents the plant from synthesizing the three essential aromatic amino acids: phenylalanine, tyrosine, and tryptophan. Without these building blocks, the plant cannot produce the proteins necessary for growth and survival. While growth stops within hours of application, visible effects, such as yellowing and wilting, take several days to appear as the herbicide translocates to the root tips and growing points. This systemic action requires successful absorption for a complete kill.
Why Cutting Reduces Herbicide Effectiveness
Cutting the foliage before spraying significantly reduces the total surface area available for the herbicide to be absorbed. Since glyphosate is a foliar-applied product, removing leaf tissue drastically limits the amount of chemical that can enter the plant’s system. A freshly cut plant is also under stress and may redirect resources toward healing the wound rather than actively growing. This stress slows down internal nutrient movement, or translocation, which is the mechanism glyphosate uses to travel to the roots. Applying the herbicide to a stressed plant often results in a superficial “top kill,” meaning the visible parts die, but insufficient chemical reaches the root structure, allowing perennial weeds to recover and sprout again.
When Trimming Might Be Necessary
There are specific situations where trimming weeds beforehand may be appropriate, though it requires a waiting period. If weeds are extremely tall, dense, or woody, the volume of material can prevent the spray from reaching the lower foliage or smaller surrounding weeds. In these cases, a light trim can allow better spray coverage and access to the target area. If trimming is necessary, you must allow the plant a recovery period before applying the herbicide. It is recommended to wait approximately four to seven days after cutting for new growth to emerge, as this actively metabolizing tissue is receptive to absorbing the systemic herbicide and ensures the chemical travels effectively to the underground roots.
Post-Application Waiting Period
After the herbicide has been applied, it is necessary to leave the weeds undisturbed for a specific duration to allow for full translocation. The first waiting period is the “rainfast” time, which is the minimum time needed for the chemical to dry on the foliage before rain can wash it off. This interval is often between 30 minutes and four hours, depending on the product formulation; always check the product label. The longer waiting period is before physically disturbing the weed through mowing, tilling, or pulling. Disturbing the plant too soon can sever the connection between the treated foliage and the root system, interrupting the herbicide’s journey, so waiting seven to ten days before removing the dead material ensures glyphosate eliminates the entire root structure.