Herbicide Application Challenges In 2020


For many landscapers, the arrival of the busy Spring season is a chance to achieve improvements and efficiencies over last year’s practices. In a turfgrass context, it could mean mapping better routes to visit a group of properties, or outlining the top five reasons for customer call-backs last year.

Herbicide Application weed control
(Photo: Getty Images)

Each January, our team in the University of Tennessee Plant Sciences Department meets to discuss what we learned in the previous season working with turfgrass managers on weed control issues. Essentially, the items brought forward are those that rose to the top in the midst of hundreds of different research trials, thousands of treatments, and many different weed species. This article provides an outline of what we learned in 2019 in the hopes that it will be useful in 2020.

Climate Curveballs

Abnormalities in climate are becoming more common and 2019 was no exception. For example, it only took the first 12 weeks of 2019 for rainfall accumulation in Knoxville, TN to equal 50% of the entire yearly average. This affected winter annual weed management programs, particularly in warm-season lawns, and made the process of initiating spring preemergence (PRE) herbicide programs difficult.

Additionally, the summer stress period that many cool-season lawns face each season was longer than ever before in 2019. Air temperature in our area during September, a month many consider to be the beginning of Autumn, was no different than June, July, or August last year. The 90-day period of Summer stress that cool-season lawns typically endure was extended to 120 days in Knoxville, Nashville, Chattanooga, and Memphis.

Climate curveballs from Mother Nature affected weed control in several ways including:

1. PRE programs were challenged more than normal. Extended heat created a growing environment favorable for summer weeds while simultaneously making turf less competitive against infestation. Additionally, these conditions favored soil microbial activity, the primary factor in PRE herbicide degradation. While this scenario is common during the summer, many turfgrass managers were faced with an extra month, or more, of these conditions in 2019. Hopefully conditions will be different in 2020 but if not, communicating this point to landscape customers will be integral in the coming year. Additionally, sequential applications of PRE herbicide will be needed for season-long weed control in these periods of extended heat.

Herbicide Application
Grasses at the University of Tennessee Plant Sciences Department.(Photo: Courtesy of J.T. Brosnan)

2. There was a limited window for Fall renovation. Typically, many cool-season lawns in the transition zone are renovated in the Fall. Projects are done at this time of year because cooler temperatures facilitate the germination and establishment of cool-season turfgrasses like tall fescue. With September temperatures mirroring those of July and August in Tennessee, this process had to be delayed into October or later. This delay not only challenged lawn care professionals to complete the process before the end of the season, but also left less time for new seedlings to mature prior to the onset of Winter. Similar to the discussion of PRE herbicide efficacy, communicating this point to clients will become important should conditions in 2020 be similar to last year.

3. Calendar based approaches may not be accurate. Historically, calendar-based approaches have been used to target annual weeds like crabgrass, goosegrass, and annual bluegrass. For example, crabgrass germinates in spring when soil temperatures warm to 55 degrees. Often lawn care professionals will apply a PRE herbicide at this time and then re-treat eight weeks later to target goosegrass, an annual weed known to germinate after crabgrass.

In 2019, we saw calendar based approaches fail to provide acceptable control because weeds don’t germinate and emerge based on a calendar. Thus, there’s an increasing need for predictive models to assist turfgrass managers with decision-making. Luckily, researchers have been actively working on this topic for several years. Dr. Matt Elmore and his team at Rutgers University in New Jersey have developed a model to predict goosegrass emergence from soil. Using this model, goosegrass emergence in Knoxville, TN measured 48% during the first week of May 2019. This was a time when many calendar-based programs delivered a sequential application of PRE herbicide targeting goosegrass, and it was no surprise that these programs performed poorly given the amount of emergence at application. Our team at the University of Tennessee is working on a similar model to predict emergence of annual bluegrass in the Fall and hopes to have it available in 2020.

What can turfgrass managers do until more models are readily available? One actionable step for 2020 is to monitor environmental conditions. Tracking air temperature, soil temperature, and rainfall can allow turfgrass managers to make better decisions about when to implement a weed control strategy rather than simply relying on a calendar. Doing so has the potential to make sure the investment in the time and money to make an application are optimally spent; moreover, this practice also has the potential to limit client call-backs over poor-performance. Installing a weather station at the main office or using online resources can help turfgrass managers in accessing these data. For example, data in Tennessee can be accessed here.

4. Complicated postemergence control as well. Climatic abnormalities can also affect efficacy of postemergence herbicides, particularly those used for goosegrass control. For example, our team evaluated efficacy of several herbicides labeled for goosegrass control including foramsulfuron (Revolver), foramsulfuron + thiencarbazone + halosulfuron (Tribute Total), topramezone (Pylex), fenoxaprop (Acclaim Extra), and carfentrazone + 2,4-D + MCPP + dicamba (Speedzone). Goosegrass control was minimal when these herbicides were applied to plants growing in an environment of limited soil moisture (less than 12%). Although we haven’t determined the exact mechanism(s) leading to poor efficacy in the dry environment, we have learned that the growing environment is a key factor in weed control. In follow-up experiments with Revolver, goosegrass control was optimal when plants were treated in an environment conducive for transpiration (movement of water through the plant; these conditions include high air temperature, low humidity, etc.) and where moisture was available in the soil for transpiration to occur. As a result, turfgrass managers should pay close attention to conditions at application to make sure their investments in postemergence treatments are well spent in 2020.

Herbicide Application
A student getting an in-depth view of turfgrass conditions at the University of Tennessee.

Herbicide Resistance

Herbicide resistance has become a new challenge for many turfgrass managers, particularly in regards to controlling weeds such as annual bluegrass, goosegrass, and select sedge species. To date, resistance has been discussed on a case-by-case basis as there are few large-scale surveys documenting the extent of the problem. Last year, our team concluded a random survey of annual bluegrass on golf courses across Tennessee and learned that resistance was more widespread than expected, particularly for herbicides such as prodiamine (Barricade), glyphosate (Roundup), foramsulfuron (Revolver), and simazine (Princep).

What about resistance in the lawn care industry? There is currently a national project tackling this topic. Fourteen different universities received funding from the United States Department of Agriculture—Specialty Crops Research Initiative to address this problem collaboratively. More information on their work can be found at www.resistpoa.org.

Herbicide Application
Testing different patches of turfgrass at the University of Tennessee.


In 2019, many in the turfgrass industry learned there is a lot of client concern about glyphosate, the active ingredient in many non-selective herbicides, most notably Roundup. Reasons for this are beyond the scope of this article, but turfgrass managers should expect end-user questions to continue in 2020 as the herbicide continues to draw media attention. Those looking for resources to assist with answering customer questions should contact their local university Extension office for help. Many offices will have fact-based publications that can be useful in communicating accurate information about glyphosate. An example resource developed by University of Tennessee Extension can be found here. To summarize the UT Extension document: The U.S. Environmental Protection Agency (EPA) released a report in December 2017 concluding that glyphosate was not carcinogenic to humans; they reaffirmed this finding in April 2019. Currently, UT Extension recommends glyphosate in accordance with federal and state regulations.

Turfgrass managers should remember that their local Extension office is available for in-season guidance and support; this is the mission of land-grant universities in every state. This year will certainly provide new weed management issues that will become valuable lessons for the future.


The UT Turfgrass & Ornamental Weed Science program offers resources here. The page includes: an image based weed identification tool; an herbicide resistance guide for annual bluegrass control; and over 30 fact sheets on a variety of topics—such as maximizing herbicide efficacy, conversion factors for calibrating sprayers, and more.

BrosnanBrosnan is a professor in the Department of Plant Sciences at the University of Tennessee (UT). As leader of the new Weed Diagnostics Center, Brosnan’s research focuses on effective and economical strategies for broadleaf and grassy weed control in various turfgrass systems. Recent research has focused on exploring the issue of herbicide resistant weeds in managed turfgrass. He serves as an advisor to the Tennessee Turfgrass Association Board of Directors and is also actively involved in the Weed Science Society of America, Golf Course Superintendents Association of America, and the Sports Turf Managers Association.

BreedenBreeden has been a weed science extension assistant at UT since June 1996. He is responsible for managing all UT’s turfgrass and ornamental weed science field research projects, and is instrumental in the development of new turfgrass and ornamental weed science extension/educational materials. He received both his B.S. and M.S. degrees in Plant and Soil Science (Weed Science) from UT.

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