8/29/2024 - By Dr. Jeff Atkinson

As the dog days of summer are here it is hard to think about Poa management for the coming fall and winter months. The truth is, Poa season is around the corner and now is the time to build an effective program. Keeping in mind key principles related to the biology of Poa, herbicide selection, and herbicide application timing are sure to maximize the effectiveness of a Poa management program in non-overseeded dormant bermudagrass.

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

Figure 1. Poa annua eradication in non-overseeded bermudagrass is challenging to achieve. All too often, superintendents find themselves with difficult-to-control Poa escapes the following spring.

Poa annua Long Germination Window

Research by McElroy et al. (2004) recognized that eight Poa annua ecotypes could germinate in a wide range of environmental conditions. Extreme heat, 102/84°F Day/night temperatures, brought limited germination of most ecotypes. 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.

Poa annua Management Options

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 before Poa germination. More recently, many turf managers are delaying the initial PRE application and combining it with POST chemistry in mid-fall, because the longevity clock of PRE applications begins to tick as soon as the material is applied. Considered 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.

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.

Effective Use of Poa annua Management Products

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. (2020) found of randomly selected Poa plants from Tennessee golf courses that 64% of collections had some degree of resistance to glyphosate, 21% to foramsulfuron (Revolver®), 58% to prodiamine (Barricade®), and 3% to simazine (Princep®). Resistance to multiple modes of action within a single plant was also documented.

I have included a list of common Poa annua control herbicides 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.

Table 1. Common herbicides for Poa annua control in non-overseeded dormant bermudagrass.

Poa annua Management - Herbicide Selection and Timing

When selecting a POST herbicide, timing is an important and often under-considered variable. The fact is multiple POST herbicide options lose efficacy during cooler months when Poa annua is not actively growing. Many herbicides that 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), Monument® (trifloxysulfuron), and Katana® (flazasulfuron) need Poa to actively grow to achieve control. WSSA 2 herbicides work by starving the plant of essential amino acids, which will never materialize if Poa is not actively growing and thus not using amino acids.

A second important timing consideration is Poa's life stage at the time of application. As a rule of thumb, 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 through the dormant bermudagrass season.

New Poa annua Management Tools are Available!

Harrell’s recently launched StayGuard® on Fertilizer which is an on-fertilizer formulation of flumioxazin, the same active ingredient found in SureGuard®. While SureGuard® cannot be applied to green bermudagrass, StayGuard® bypasses leaf tissue since it is on a granular carrier and can be safely applied to green bermudagrass with minimal injury potential. University research and turf manager experience has suggested that StayGuard is an effective tool not only for general weed control but for Poa annua management as well.

During the fall of 2023 through the spring of 2024, StayGuard® on Fertilizer Poa annua programs were evaluated at Auburn University. StayGuard® applied at 200 lbs./A on September 15^th, followed by StayGuard® at 200 lbs./A on November 15^th, followed by a spray application of Princep at 1 quart per acre and Tribute Total at 1 fl. oz. per acre on November 16^th provided 100% Poa annua control on April 10^th, 2024 (Figure 2). While the active ingredient in StayGuard, flumioxazin, is not a new active ingredient or a novel mode of action, fertilizer applications allow for it to be used in ways that were previously impossible. As Poa annua continues to adapt, using old active ingredients in new ways will be a key strategy in our attempt to control this troublesome pest.

Figure 2. Nontreated (left) vs. StayGuard (right) applied at 200 lbs/Acre on September 15^th followed by StayGuard at 200 lbs/A on November 15^th followed by Tribute Total at 1 oz/A + Simazine at 1 quart/A on November 16^th. Picture taken April 10^th, 2024, 145 days after initial treatment. Auburn, Alabama.

What is the Answer for Poa annua Management?

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. Luckily you have your Harrell's Territory Manager, perhaps your best resource for effective Poa management strategies and the key to developing a sound management program. Reach out to them today to get ready for dormancy!