Reverse resistance

In recent years, herbicide resistance among Poa annua populations in turfgrass systems has increased.

Figure 1 - Poa annua, commonly referred to as annual bluegrass.
Photo: Dr. Shawn Askew, Virginia Tech

Among the most challenging annual weeds for many professionals to control is Poa annua or annual bluegrass. Although sometimes grown as a desired species, Poa annua is frequently classified as one of the most common and troublesome weeds in managed turfgrass systems due to its adaptability, invasive biology and propensity toward herbicide resistance. Poa annua competes with desired species, disrupts aesthetics and functionality, and may increase maintenance costs.

Herbicide resistance is generally defined as the inherited ability of a plant to survive an herbicide application that would typically kill a normal population of the same species. Resistant plants are selected through repeated use of herbicides with the same site of action. As these surviving plants reproduce, resistance within a population increases.

In recent years, herbicide resistance among Poa annua populations in turfgrass systems has increased, creating new challenges for lawn care operators and other turfgrass professionals.

About Resist Poa

In fall of 2018, an interdisciplinary team of scientists spanning 14 academic institutions came together to address the herbicide resistance epidemic in Poa annua in managed turfgrass systems.

This project was funded by the United States Department of Agriculture-National Institute of Food and Agriculture (USDA-NIFA) as a Specialty Crops Research Initiative. Over the past four years, the project has accomplished many of its objectives to advance knowledge around the nature, extent and management of herbicide resistance in this troublesome weed.

Contemporary Challenges and Opportunities for Improved Lawn Weed Management: Insights from U.S. Lawn Care Operators. As part of the ResistPoa effort, a series of five virtual focus groups was conducted in 2021 to gather perspectives from LCOs across the United States.

These focus groups sought to identify common themes as it relates to weed management concerns, best practices, opportunities for the advancement of integrated weed management and the more effective management of Poa annua, in particular.

This article features highlights from the study. The complete study, published in Outlooks on Pest Management (volume 33, number 3), can be accessed for free on the ResistPoa website by searching available literature under resources:

Concerns Expressed by Study Participants

Participants in the five LCO focus groups were asked to express concerns around lawn weed management and herbicide resistance.

Several themes from this discussion include:

• Many participants suspect that herbicide resistance is a challenge in lawns and landscapes they manage, particularly for troublesome grassy weeds.

Poa annua, in particular, is uniquely challenging due to growing herbicide resistance issues.

• There is a perception that governmental pesticide regulations make it challenging to manage turfgrass weeds properly.

• This is compounded by a feeling that there are too few herbicide options available to manage weeds in turfgrass lawns.

• Many participants indicated that opportunities for information sharing between operators are too limited in the lawn and landscape industry.

• Some participants also felt that opportunities for misinformation among the public may generate unfounded concerns about fertilizer and pesticide use, leading to fear among clientele that can impact the ability to implement best practices.

Best Practices Identified by Participants

Perhaps unsurprisingly, many LCOs indicated that the most effective weed control programs involve both chemical and non-chemical measures. The degree to which an operator relies on non-chemical measures appears to be influenced by the extent of regulations in their region as well as client preferences.

Among non-chemical measures, cultural control practices that improve the competitiveness and health of turfgrass stands were most valued by study participants. Examples provided by participants included appropriate mowing or irrigation practices. As the old adage goes, “The best defense against weeds is (still) a healthy, dense stand of turfgrass.”

This can present a unique challenge to LCOs, as perhaps more so than some other turfgrass managers, they often have limited influence over key management practices such as mowing or irrigation that can significantly influence turfgrass health and pest proliferation.

Therefore, many participants emphasized that communication with homeowners and clientele is key to encouraging cultural practices that will not undermine the efforts of the operator.

In addition, it will be essential for practitioners to develop and implement integrated weed management strategies that reflect their local geographies to ensure programs are appropriate and well-suited to unique climates, soil types and lawn species.

Figure 3 - Poa annua growing in managed turfgrass.
Photo: Shawn Askew, Virginia Tech

Implications and Opportunities Going Forward

The study reinforces that herbicide resistance is perceived by many professionals as a growing problem in professionally managed lawns and landscapes. This supports that LCOs and similar professionals will be increasingly challenged to be proactive and purposeful in their approach to weed management.

Further, the study highlights opportunities to strengthen existing communication networks and coordinated management among landscape professionals and other key players, including additional contractors (mowers, irrigation specialists) and clientele.

This may require the landscape industry to consider new organizational schemes for information sharing that facilitate improved interactions between stakeholders including trade organizations, suppliers, universities and policy makers.

While future research is needed to identify more successful strategies for mitigating herbicide resistance, LCOs and other green industry professionals can consider the following action items:

• Develop regionally appropriate integrated weed management strategies that include both chemical and non-chemical control measures.

Figure 2 - P. annua treated with pronamide four weeks prior. These are herbicide-resistant plants among susceptible plants.
Photo: Scott McElroy

• Ensure that herbicides are used at the appropriate rates and timings. Inappropriate use of herbicide products, including applying at reduced rates, can increase the likelihood of resistance challenges in the future. Always read and follow label directions.

• Rotate and/or combine herbicide sites of action to prevent herbicide resistance from developing at the sites you manage.

• For those management decisions you are not responsible for, consider strategies for communicating best practices to homeowners and other clientele when it may improve turfgrass health and competitiveness and promote the success of your pest management program:

– Share extension and other science-based resources.

– Develop one-page flyers or short videos that promote best practices

• Similarly, consider how to communicate science-based information to clientele that support the management decisions that you/your company make related to fertilizer and pesticide applications.

• Look for opportunities to strengthen relationships and communication both with fellow operators and with experts that can help you stay up-to-date on trends and scientific advancements related to weed management and related topics.

– Participants in this study found university scientists and extension professionals to be the most trusted information source, but industry trade groups and chemical suppliers were also highlighted as valuable sources of knowledge.

Readers can learn more about the project and find helpful resources by visiting

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