Poa Is On The Way: Are You Ready?
“This is the worst Poa year I can remember”
“I’ve never seen it like this before”
“We’ve never had this much Poa ”
These were common refrains across Tennessee this spring as many turfgrass managers struggled with annual bluegrass control. One of the things our research team has learned over the years is that it’s critically important to have a well-thought out plan to effectively control annual bluegrass. It may be hard to believe, but those plans will need to be put into place soon to effectively manage annual bluegrass this winter. The post below outlines some of the key lessons learned in our 2018–2019 research that will hopefully aid turfgrass managers in controlling annual bluegrass this season.
Lesson #1: The Days of a “One-Shot” Poa Control Program are Over
One major take-away from our statewide trial work last year was that the days of one herbicide application (i.e., a “one-shot” program) providing season long annual bluegrass control are over. In trial work conducted in Knoxville, Nashville, and Jackson last year we evaluated 17 “one-shot” Poa control programs and only 3 provided >90% control at all locations.
There are several reasons why the days of a “one-shot” Poa control program are over. First, there is widespread resistance to some of the most common herbicides used for annual bluegrass control in our state (i.e., Barricade, Roundup, Revolver, etc). Second, erratic weather patterns make annual bluegrass emergence difficult to time and also compromise the residual activity of many products.
To that end, we are recommending that turfgrass managers move towards use of a “1–2 Punch” program for annual bluegrass control or consider the use of a “Zone Defense” initiated in October.
What is a “1–2 Punch” Program?
Our team has defined a “1–2 Punch” program as an approach to controlling annual bluegrass that involves making a minimum of two herbicide applications, ideally of products varying in mode of action. Last season, these “1–2 Punch” programs performed well in trials conducted across Tennessee, especially when compared to “one-shot” of a preemergence product (Figure 1).
What is a “Zone Defense” Against Annual Bluegrass?
For several years our research trials have highlighted that the best approach for controlling annual bluegrass in warm-season turfgrass is to apply a mixture of different herbicides in mid-to-late October. We’ve termed this approach a “Zone Defense” because when implemented correctly, herbicides in these mixtures are from different mode of action groups, much like zone defenses in football require players in different positions to work together to achieve a goal. Last year we evaluated six “Zone Defenses” against annual bluegrass and four of these provided 85 to 100% control at all locations. These included:
FreeHand (pendimethalin + dimethenamid-P)
Katana + Kerb
Monument + Barricade
Monument + Barricade + Princep
Appropriately designed “Zone Defenses” against annual bluegrass are an optimal resistance management strategy in that they break the cycle of sole reliance on a single herbicide. Moreover, when applied in late October in our trials, these applications have facilitated effective, season-long, annual bluegrass control in a single application. We will be studying several new “Zone Defenses” against annual bluegrass this season and will report our findings in 2020.
Lesson #2: Ultradwarf Greens Require Special Attention
Annual bluegrass control on ultradwarf putting surfaces continues to be challenging throughout Tennessee and beyond. Widespread resistance to acetolactate synthase inhibiting herbicides (e.g., Revolver, Katana, etc) has left golf course superintendents with few options for selective control on greens. One of these remaining options is Kerb SC, an herbicide labeled for annual bluegrass control on ultradwarf greens in addition to bermudagrass at maintained at higher heights of cut (e.g., fairways, roughs, etc). A key lesson learned last year was that there is no need to get aggressive with application rate when using Kerb SC for annual bluegrass control on the ultradwarfs. In our research on TifEagle, we saw no benefit to increasing Kerb SC rates beyond 1.8 pt/A with applications made in autumn. This year we will be studying varying approaches to deliver this 1.8 pt/A to an array of ultradwarf cultivars.
Although Kerb SC continues to be an effective option for annual bluegrass control on ultradwarf putting greens, sole reliance on Kerb SC will simply select for populations with resistance to that chemistry (**There is already a confirmed case of Kerb resistance in Georgia**). Additionally, it is well documented throughout Australia that continued use of Kerb selects for microbes in the soil that rapidly degrade the herbicide, thereby reducing its efficacy. We will be exploring alternatives to Kerb for annual bluegrass control on ultradwarf putting surfaces this season and will report our findings in 2020.
Lesson #3: Much Ado About Glyphosate
Questions about glyphosate have been abundant in 2019, in large part due to it being the focus of several high profile lawsuits that have attracted media attention. Weed scientists with University of Tennessee Extension developed a fact sheet to assist turfgrass managers in answering questions about glyphosate and its use in weed management programs. This factsheet can be accessed on our website.
Our research has consistently shown that glyphosate (e.g., Roundup Pro) is an effective option to control annual bluegrass in dormant bermudagrass turf at rates of 16 to 32 fl oz/A. However, sole reliance on this treatment has resulted in glyphosate-resistant annual bluegrass becoming common across Tennessee, particularly on golf courses. We anticipate that many turfgrass managers will explore alternatives to glyphosate for annual bluegrass control this season- be it for resistance management reasons or other external factors.
Alternative options for non-selective annual bluegrass control in dormant bermudagrass include glufosinate (e.g., Finale, Cheetah Pro, others) and diquat (e.g., Reward). In trial work conducted across Tennessee for the past several years, we have observed that 6 qt/A of Finale is required for consistent annual bluegrass control during bermudagrass dormancy. For example in 2019, annual bluegrass control with Finale at 6 qt/A ranged from 97 to 100% in East, Middle, and West Tennessee compared to only 37 to 93% for the 3 qt/A rate. This season we will be studying Finale rates within the middle of this 3 to 6 qt/A range. Regarding Reward, applications of this herbicide alone have proven to be ineffective in our trials. However, we have observed positive results with a mixture of Reward (2 pt/A)+ Princep (2 qt/A), particularly on annual bluegrass populations susceptible to photosystem II inhibiting herbicides.
It is important to keep in mind that these alternatives (Reward and Finale) are applied at much higher application rates than glyphosate. Glyphosate is commonly applied to dormant bermudagrass at rates of 16 to 32 fl oz/A compared to rates of 2 pt and 6 qt for Reward and Finale, respectively. Additionally, when comparing the acute toxicity of herbicides (i.e. oral LD50 values), Reward and Finale are substantially more hazardous than glyphosate.
Lesson #4: Working Towards an Emergence Model
Annual bluegrass is known to germinate across a wide range of environmental conditions which makes timing herbicide applications, particularly those targeting preemergence control, very difficult. Our team has been working in collaboration with data scientists at mesur.io to develop a model that predicts when annual bluegrass will emerge in warm-season turfgrass each year. While this project is still on-going, we have seen that over the past three seasons, annual bluegrass emerged between the 42 and 46th week of the year in Tennessee; this roughly equates to the time period from mid-October through mid-November. While our team has a lot more work to complete on this topic, keep in mind that this blog was posted during the 32nd week of 2019. That leaves only 10 weeks before we start see annual bluegrass across Tennessee. Are you ready?
