1/12/2024 - By Dr. Jeff Atkinson

Poa annua is a dynamic weed. Herbicide selection, application timing, and environmental conditions at the time of application can all influence the effectiveness or ineffectiveness of a P. annua preemergence program.

Poa annua Has Arrived - What to Do Now?

To the misfortune of the turf manager, P. annua, which is not controlled by fall preemergence herbicide applications, is often not visible until the spring when they break through the turf canopy.

So, if you find yourself in the situation of managing P. annua breakthrough in the spring, there are important considerations that can help maximize postemergence control.

Poa annua Biology

The first step to any effective pest control program is understanding the pest’s biology. Although more resources are dedicated to Poa annua research in turfgrass programs across the United States than any other weed, there are still a lot of things about P. annua we don’t have concrete answers on.

However, through significant federal funding commitments via the Resist Poa - USDA-SCRI Annual Bluegrass Collective, and the dedication of university researchers across the United States, we are learning more about germination and establishment patterns, prevalence and distribution patterns of herbicide resistance, and effective control strategies.

One key to appreciating the difficulty of P. annua management is understanding the genetic ‘flexibility’ of the species.  In fact, P. annua can be found on all continents, including Antarctica! This extreme adaptability is a result of a large genetic library contained within each P. annua plant derived from the parent species Poa supina, which is a cool-season perennial species native to the cool, moist European Alpine region, and Poa infirma, which is a cool-season perennial species native to dry, Mediterranean climates in Europe (Mao and Huff, 2012).

For turf managers, this translates into a weed with a long germination window, is not easily controlled by adverse environmental conditions, and rapidly develops resistance to herbicides when frequently applied without rotation. Four key P. annua biology characteristics are important considerations when building a Poa annua management program:

·         The majority, but not all, Poa annua germinates in the fall in temperate regions.

·         Poa annua becomes less physiologically active as temperatures cool.

·         Poa annua is a prolific seedhead producer.

·         Herbicide resistance is real, and is not going away.

Here is a look into each of these a little closer:

Poa annua Germination

Observation suggests Poa annua has a wide germination window, however a debate continues about how much germination occurs in temperate climates during the fall months versus how much germination occurs in the spring as temperatures warm. Recent research at the University of Tennessee aimed to answer this question (Taylor et al., 2021).

The researchers found through two years of data collection in the Knoxville, TN region that P. annua emergence changed most rapidly between the 40^th and 43^rd week of the year when seven-day mean soil temperature and rainfall were 66F and 0.5 inches, respectively, even though initial emergence occurred on September 4^th in 2019 and August 5^th in 2020, a full month earlier.

Since we know when the majority of Poa annua germinates, control should be easy, right?  Wrong.

Unfortunately, the keyword is ‘majority’ as a percentage of P. annua can germinate in the warming spring months.  This extremely long germination window pushes the limits of the residual efficacy of fall-applied preemergence herbicides such as prodiamine (Barricade, Syngenta) and indaziflam (Specticle, Envu) (Cross, 2014).

Poa annua Becomes Less Physiologically Active as Temperatures Cool

Many systemic herbicides require physiological activity to maximize efficacy.  One common example of Poa annua management is the relatively poor control achieved by sulfonylurea (SU) herbicides when temperatures cool.  Examples of SU herbicides include flazasulfuron (Katana, PBI Gordon), trifloxysulfuron (Monument, Syngenta), and foramsulfuron (Revolver, Envu).  Sulfonylurea herbicides work by inhibiting a plant’s ability to produce essential amino acids which serve as the building blocks of proteins and facilitate other vital plant functions.

Without these amino acids, sensitive plants lose the ability to function over time and ultimately die.  However, under cool temperatures P. annua’s need for these essential amino acids is drastically lower, allowing time for the plant to detoxify the herbicide active ingredient, recover, and resume normal growth.  Sometimes this can be observed in the field as a growth regulation effect of P. annua, or other target weeds followed by resumption of normal growth when plant death was the expected outcome.

During this time, contact herbicides should be considered.  Active ingredients such as diquat (Reward, Syngenta), glufosinate (Cheetah Pro, Nufarm), and flumioxazin (Sureguard®, Nufarm) should be considered.  These materials do not rely as much on physiological activity to be effective.

However, they are non-selective and therefore should only be applied to dormant desirable turf.  In many regions of the transition zone, bermudagrass is fully dormant through the winter season.  Apply with caution to zoysiagrass, especially in more southern regions, as zoysia often appears dormant while further investigation in the canopy reveals green, non-dormant stolons and rhizomes.  These plant parts can be negatively affected by herbicide applications, leading to a delay in spring greenup at best or plant death at worst.

Poa annua is a Prolific Seedhead Producer

It’s true that in many areas of the Southern United States, unsightly Poa annua infestations will eventually go away as the summer season arrives.  As temperatures warm and mowing frequency increases, summer stress will cause the remaining P. annua plants to melt away. Following this strategy, however, only makes the problem worse for next fall.

Estimates suggest that annual P. annua biotypes are capable of producing up to 2,250 seeds per plant in a single season and that the surface layer of soil could contain up to 7,500 seeds per square meter (Holm et al., 1997 and Renney, 1964)! If Poa annua is allowed to grow, develop, and seed, the prolific seeding nature of P. annua is setting the stage for a more severe infestation the following year. For this reason, taking measures to maximize control before the major spring seedhead flush is an important step to managing future infestations.

Poa annua Herbicide Resistance is Real, and Not Going Away

Unfortunately, in part due to Poa annua’s flexible genetics and in part due to our overuse of single-mode-of-action herbicides, herbicide resistance is increasing in prevalence. In many cases, P. annua resistance to multiple modes of action is present in a single population, rendering several herbicide modes of action ineffective at controlling a population at a single location.

Models have demonstrated that herbicide rotation and combination of herbicides with different modes of action is an effective strategy to delay the development of herbicide resistance. Too often we as an industry become creatures of habit. We find something that works and stick with it. Proceed with caution using this mindset, as what works today will not work tomorrow if it is used year after year after year.

Poa annua Management

There’s no getting around it, nor a silver bullet for it. Poa annua management is tough. Your local Harrell's Representative is well-versed in effective P. annua control strategies for your area and is your best resource to improve your program using strategies that work on a local level.

Looking for more reading material on the subject? Consider these past blog posts:

Annual Bluegrass (Poa annua) Control Using Fertilizer with StayGuard 0.125%

Achieving More Consistent Control of Poa annua Seedheads

Poa annua Management in Dormant Bermudagrass -- What's the Answer?

References:

Cross, R. 2014.  Ecological, Physiological, and Molecular Characterization of Annual Bluegrass (Poa annua L.) Herbicide Resistance and Its Control on Golf Courses.  All Dissertations.  1431.  https://tigerprints.clemson.edu/all_dissertations/1431.  Accessed 8 December 2023.

Holm, L.G. 1997.  World Weeds: natural histories and distribution.  J. Wiley, New York.

Mao, Q., and D.R. Huff.  2012.  The evolutionary origin of Poa annua L.  Crop Sci.  52:1910-1922.

Renny, A.J. 1964.  Preventing Poa annua infestations.  Proc. Northwest Turf. Conf.  18:3-5.

Tayor, D.R., M. Prorock, B.J. Horvath, and J.T. Brosnan.  2021.  Modeling seasonal emergence of Poa annua in urban greenspace.