7/14/2022 - By Dr. Jeff Atkinson

Of the chronic challenges plaguing turfgrass managers, Poa annua eradication may be the most difficult of them all (Figure 1). Herbicide resistance, adaptability to a wide range of environmental conditions, and prolific seed production are a few of the reasons Poa annua continues to thrive. For these reasons, a Poa annua control program should have reasonable expectations as complete eradication is challenging to achieve.

Developing Your Poa annua Program

As the dog days of summer are here, it is hard to think about Poa management for the coming fall and winter months. Truth is, Poa season is right around the corner, and now is the time to work alongside your Harrell's Rep to develop an effective program. To maximize the effectiveness of a Poa management program in non-overseeded dormant bermudagrass, it helps to keep in mind key principles related to the biology of Poa, herbicide selection, and herbicide application timing.

Long Germination Window for Poa Annua

Research by McElroy et al. (2004) recognized that eight Poa annua ecotypes could germinate in a wide range of environmental conditions, including extreme heat, 102/84°F day/night temperatures, limited germination of most ecotypes, etc. Maximum germination occurred for all ecotypes in day/night temperatures of 66/50°F. Considering the mild winters often experienced by much of the southeastern United States and the optimum Poa germination window identified by McElroy et al., it is no surprise turf managers often observe Poa germination through the fall, winter, and spring months.

Further, the researchers were unable to detect a consistent germination response to light availability. However, seeds from each ecotype germinated in complete darkness, which the authors suggest is an indication that the old adage, "a dense turf canopy is the best weed management tool," may not entirely prevent Poa establishment.

Poa annua Control Options

So, what are our options in controlling Poa annua? Successful Poa annua management programs in non-overseeded dormant bermudagrass often incorporate both PRE- and POST-emergence herbicide chemistries. The first application of a program is traditionally a PRE herbicide applied in late summer or early fall, prior to Poa germination. More recently, turf managers are delaying the initial PRE application and combining it with a POST chemistry in mid-fall, due to the fact that the longevity clock of PRE applications begins to tick as soon as the material is applied. Considering this, alongside the long germination window of Poa annua, it is easy to see why many programs fail when they rely solely on an early PRE application.

The PRE + POST Approach to Poa Control

Applying a PRE + POST combination later than a traditional PRE application may seem like an odd approach at first pass. After all, this approach allows Poa to germinate! The objective is to control germinated seedlings at an early growth stage when they are easier to control while extending the longevity of the PRE chemistry.

An effective PRE + POST program is reliant on the correct chemistry selection, timely applications, and adjusting for herbicide resistance traits that may be present on the property. Research published by Brosnan et al. in 2020 found a few things in randomly selected Poa plants from Tennessee golf courses:

• 64% of the collections had some degree of resistance to glyphosate
• 21% to foramsulfuron (Revolver)
• 58% to prodiamine (Barricade)
• And 3% to simazine (Princep)

Preventing Resistance with Multiple Options

Resistance to multiple modes of action within a single plant was also documented. A list of common Poa annua control herbicides is below in Table 1. Alternating WSSA codes between applications and combining active ingredients with different WSSA codes is a research-backed method to delay and prevent the development of Poa herbicide resistance.

Timing of Herbicide Selection

When selecting a POST herbicide, timing is an important and often neglected variable. Many POST herbicide options lose efficacy during cooler months when Poa annua is not actively growing. Many herbicides, which are effective POST Poa control options, have environmental conditions in which they are optimally effective. Knowing which herbicide to apply during different seasons can prevent the headaches associated with an ineffective application.

For example, inhibitors of acetolactate synthase (WSSA 2) such as Revolver (foramsulfuron) and Monument (trifloxysulfuron) need Poa to actively grow to achieve control. WSSA 2 herbicides work by essentially starving the plant of essential amino acids. The depletion of amino acids will never materialize if Poa is not actively growing and thus not using amino acids.

A second important timing consideration is Poa life stage at application. As a rule, more mature plants are more challenging to control. As plants establish and mature in spring, control will become more erratic. Early, consistent, programmatic control is the most effective way to stay ahead of Poa control through the dormant bermudagrass season.

So, What’s the Answer to Controlling Poa?

A tremendous research effort, both historically and ongoing, has aimed to solve the riddle of Poa annua control in dormant bermudagrass turf. While there are effective strategies, given appropriate expectations, Poa continues to stay one step ahead. Adaptability to a wide range of environments, prolific seed production, and rapid development of herbicide resistance will continue to make Poa an annual challenge for turf managers. Because of the unique conditions at each location, a single Poa management program will not be effective under all scenarios. 

A Harrell’s Representative is an excellent resource for effective Poa management strategies to develop a sound management program. So don't be afraid to reach out! We are to help you develop the program(s) your turf needs to fight Poa.

References

McElroy, JS, RH Walker, GR Wehtje, E van Santen. 2004. Annual bluegrass (Poa annua) populations exhibit variation in germination response to temperature, photoperiod, and fenarimol. Weed Sci 52:47-52.
Brosnan, JT, JJ Vargas, GK Breeden, JM Zobel. 2020. Herbicide resistance in annual bluegrass on Tennessee golf courses. Crop, Forage & Turfgrass Management 6:1.