8/27/2024 - By Dr. Jeff Atkinson

It’s early September and throughout much of the United States nighttime temperatures are beginning to fall and daytime temperatures are hot, but don’t seem to stick around for quite as long. Preemergence herbicides applied long ago in spring are hanging on, and in some cases, weeds are beginning to break through their barrier of control. If you are observing an increase in crabgrass, goosegrass, or other problematic summer weeds, you are not alone. Here are a few reasons why weeds overcome preemergence herbicide applications during the late summer period and considerations to improve control in the coming seasons.

Active Ingredients Break Down Over Time

As a rule, preemergence herbicides are soil-active herbicides. Whether sprayed or applied as a sparged ingredient on a fertilizer carrier, the objective of a preemergence (PRE) herbicide application is to treat soil rather than existing turf. Depending on the mode of action of the active ingredient, PRE herbicides work by inhibiting root formation or shoot formation of new seedlings, such as crabgrass.

All active ingredients, regardless of the mode of action, have a ‘half-life’ value which describes how long the active ingredient will remain intact in soil under normal conditions. Half-life is defined as the amount of time it takes for an active ingredient to degrade by half. For example, if one pound of active ingredient is applied per 1000 square feet, the half-life is the amount of time required to degrade the active ingredient to the point where 0.5 pounds of active ingredient per 1000 square feet remains. In general terms, the longer the half-life, the longer the PRE herbicide will remain effective.

It is not as simple as picking the PRE herbicide with the longest half-life, however. Turfgrass safety, weed control spectrum, and preventing the development of herbicide active ingredient resistance are all considerations that must be made when selecting which PRE herbicide to apply.

Preemergence Herbicide Longevity is Influenced by the Environment

Half-life is not a fixed number, it is influenced by environmental factors such as soil moisture, temperature, and microbial activity. As a rule of thumb, PRE herbicide half-life is reduced as soil temperature, moisture, and microbial activity increase. If PRE herbicide efficacy seemed to fall off earlier this year compared to past seasons, it is worth considering how this year compares in terms of temperature and rainfall to years past.

Applying Preemergence Herbicides Early Puts a Strain on Longevity

Historically, a single spring PRE application was all it took for weed prevention throughout the growing season. Over the last several years, a general trend has been for applicators to apply PRE herbicides earlier in the year than traditionally to prevent the establishment of crabgrass and other grassy weeds, which seem to germinate and establish earlier each year. Are temperatures warmer during Spring months compared to years past? Perhaps. In response, the industry is applying PRE herbicides earlier in the year than the historical standard.

Applying PRE herbicides earlier in the season puts a strain on late-season PRE weed control efficacy. Considering again the half-life of an active ingredient, the earlier a PRE herbicide is applied in a year, the more degradation of the active ingredient that can be expected later in the season. In many areas of the United States, a second PRE application mid-season is becoming the standard to ensure PRE weed control efficacy throughout the growing season.

A second PRE application mid-season must be balanced with the expectation for turfgrass seed establishment in the fall since the same active ingredient that prevents grassy weed establishment can also negatively affect desirable turfgrass establishment from seed.

Breaking the Barrier of Preemergence Herbicides Leads to Weed Breakthrough

Preemergence herbicides work by creating a barrier in the upper layer of the soil surface between the weed seeds and the air above. In most cases, PRE herbicides allow weed seed to germinate, then kill the seedling after the newly germinated seedling has absorbed the PRE active ingredient. This works great until the barrier has been broken.

A common example involves the use of edging equipment on curb and sidewalk edges. Along with increased temperatures in these areas which reduced half-life, edging equipment often digs into the soil along sidewalk and curb edges removing the PRE barrier and allowing for weed seed establishment. Other mechanical disruptions of a soil surface such as mid-summer aerification or dethatching can also disrupt a PRE weed control barrier.

Excellent Preemergence Herbicide Weed Control Starts with a Quality Application

Preemergence herbicides are only as effective as the quality of the application. In general, PRE herbicides can be applied in one of two ways, either spray applied as a liquid or applied as a ‘sparged’ material on a fertilizer carrier. For more information on the sparging process, check out this video.

Applying a PRE herbicide sparged on a granular fertilizer carrier is an effective way to optimize labor inputs, allowing for two inputs to be applied at once. To maximize efficacy, the appropriate amount of fertilizer per area must be applied, often referred to as ‘spread rate.’ The spread rate is most often described as pounds of fertilizer per 1000 square feet or pounds of fertilizer per acre. If the spread rate is too low, then distribution uniformity of the PRE herbicide (and fertilizer) will be inadequate to provide consistent weed control. Ensuring the spread rate is adequate to allow even PRE herbicide distribution is key to a successful PRE herbicide application on a granular fertilizer carrier.

Your Harrell’s Rep is Here for You

Every situation is unique and often, multiple factors contribute to the success or failure of a PRE herbicide program. A Harrell’s Territory Manager is an excellent resource to optimize your PRE herbicide program to result in higher turf quality, fewer complaints, and satisfied stakeholders. To learn more, contact your local Harrell’s Representative.