Herbicides remain one of the most widely used and effective tools for managing weeds in various settings. Whether you’re dealing with invasive weeds on a large agricultural operation or in your own backyard, herbicides can be a great tool due to their efficiency and cost-effectiveness, especially when used correctly. On farms that span hundreds or even thousands of acres, herbicides are often among the few viable options for broad-scale weed control.

A common question that is always asked is: “Why didn’t the weeds die after I sprayed them?” Understanding why herbicide applications sometimes fail is key to improving their success. Several factors can contribute to herbicide failure, including improper weed identification, incorrect herbicide selection, poor application timing, wrong application rates, inadequate placement and herbicide resistance.

In addition to these user-controlled variables, abiotic factors or non-living environmental elements, play a critical role in herbicide performance including temperature, rainfall, humidity, wind, sunlight, and soil conditions, all of which are outside the applicator’s control but must be carefully considered.

For post-emergence (foliar) herbicides, the best results occur when weeds are actively growing and at the correct growth stage, typically small and under six inches tall. Actively growing plants absorb and translocate herbicides more efficiently, increasing the likelihood of successful control. Larger, more mature weeds are generally harder to kill and may require higher application rates or multiple treatments.

Proper weed identification is one of the first and most important steps in achieving effective herbicide control. Misidentifying a weed can result in selecting an ineffective herbicide or applying it at the wrong timing or label rate.  There are some weeds in California, like marestail and hairy fleabane, that are resistant to commonly used foliar herbicides such as glyphosate and glufosinate.  Always consult herbicide labels and other weed management resources, such as the UC ANR Integrated Pest Management website (ipm.ucanr.edu), to ensure you're using the right herbicide for your target weed species.

Abiotic factors can influence herbicide effectiveness. High temperatures can increase herbicide volatility, leading to off-target movement (drift) and reduced efficacy. Low temperatures can slow plant metabolism, decreasing herbicide uptake and translocation. High humidity can improve herbicide absorption through leaf surfaces, while low humidity can cause spray droplets to evaporate too quickly, reducing uptake.

Rainfall is another critical factor as shortly after application, rain can wash off foliar herbicides, reducing their effectiveness. Conversely, excessive rainfall can leach some herbicides out of the root zone, potentially contaminating groundwater. Wind can also be problematic: strong winds can blow herbicides off target; while spraying during temperature inversions or still air can lead to vapor drift of volatile herbicides.

Herbicide failures are rarely caused by a single issue. More often, they result from a combination of poor timing, incorrect application, and unfavorable environmental conditions. Successful herbicide application begins with proper weed identification, selecting the correct herbicide, following label directions, and paying close attention to weather and soil conditions. Understanding and managing these variables is important for effective, safe, and responsible herbicide use.