Poa is coming: Are you ready?
It may not seem like it now with large sections of the southeast grappling with heat and humidity but autumn is only a few weeks away. In addition to being excited about the arrival of football season, autumn is a time for turfgrass managers to implement annual bluegrass (Poa annua) control programs on golf courses, sports fields, and lawns. It seems like each year brings turfgrass managers a new hurdle in annual bluegrass control. Whether its abnormal weather patterns or ever increasing resistance issues, there is no arguing the fact that Poa is one of the most problematic weeds during winter dormancy. To prepare for the upcoming Poa season, it’s important to reflect on was learned last year to develop the best plan to implement this autumn.
“A goal without a plan is just a wish”
— Antoine de Saint-Exupéry
Delayed Germination Flush
In east Tennessee, annual bluegrass typically begins to germinate from late August through the middle of September. In 2016, this was not the case as extreme drought crippled much of the region last autumn. For example, Knoxville, TN only received 1.6 inches of rain during September-October 2016, well below the historical average of 5.7 inches for the same time period (Source: National Oceanic & Atmospheric Administration. National Weather Service Forecast Office — Morristown, TN). This not only stressed many turfgrass stands throughout the region but also significantly delayed annual bluegrass germination. A major rainfall event on November 30th (4.85 inches) facilitated a mass of annual bluegrass germination in early December.
Non-Selective Options Were Limited
The drought of autumn was followed by abnormally warm temperatures for the duration of the winter. Across the entire state of Tennessee, daily mean air temperatures were 7 to 9 degrees above normal in January and February 2017 (Source: National Oceanic & Atmospheric Administration. National Weather Service Forecast Offices — Memphis, Nashville, Morristown, TN). This resulted in many bermudagrass stands not fully entering winter dormancy thereby limiting the use of non-selective herbicides such as glyphosate (e.g., Roundup) or glufosinate (e.g., Finale) for postemergence annual bluegrass control. Removal of these options limited those who did not have an effective preemergence program in place to using only ALS inhibitors (e.g, Revolver, Monument, etc.) for postemergence control in spring. While these herbicides are effective options versus Poa (assuming resistance is not a factor), they cannot be used around creeping bentgrass putting greens. This forced many golf course superintendents to apply non-selective herbicides despite turf not being fully dormant, which caused delayed spring green-up at several locations.
Top Performing Programs
Each year our research team conducts annual bluegrass control trials at multiple locations across the state of Tennessee to evaluate the performance of herbicide programs for annual bluegrass control that vary mode of action to mitigate resistance risk. We encourage all turfgrass managers to approach Poa programmatically — much like they would disease management on putting greens. With herbicide resistance issues becoming so prevalent, there is a need to be thinking 2 to 3 years ahead when building programs to manage Poa in the southeast. Have a plan for the current season, a contigency in place should it fail, and different plans and contingencies for the years ahead. Diversity is key.
Top performing programs from our 2016–2017 annual bluegrass control trials are presented below. Top performing programs are defined as those that were in the top statistical category at all trial locations.
Preemergence Programs (PRE)
Delayed germination compromised the efficacy of many traditional preemergence herbicide programs last season. Many programs implemented at correct PRE timings in autumn offered limited control of annual bluegrass germinating in December. Exceptions to this were as follows:
Specticle at 10 fl oz/A
Specticle at 6 fl oz/A followed by Specticle at 6 fl oz/A in December
Specticle at 6 fl oz/A followed by Princep at 1 lb ai/A in October
Specticle at 9 fl oz/A followed by Tribute Total at 3.2 oz/A in March
Late PRE/Early Postemergence (EPOST) Programs
Many turfgrass managers chose to implement a late PRE/early postemergence program for annual bluegrass control from mid-October to early-November. Applications made at this timing control any plants that may have emerged from soil (but are still relatively small in size) and offer residual control of those that will attempt to emerge throughout winter and early spring. Historically, we have seen applications of simazine (e.g. Princep) perform well at this timing as well as ALS inhibitors such as Monument. This was not the case in 2016.
Extreme drought across much of Tennessee delayed annual bluegrass germination until December 2016. To that end, very few annual bluegrass plants had emerged when EPOST programs were initiated and these applications essentially served to provide PRE control only. Therefore, programs needed extended residual activity to perform optimally.
Top performing EPOST programs in 2016 were:
Kerb (0.75 lb ai/A) + Dimension (0.50 lb ai/A)
Kerb (0.75 lb ai/A) followed by Kerb + Dimension (0.75 + 0.50 lb ai/A)
FreeHand at 200 lb/A
Dormant Programs
Although abnormally warm weather conditions introduced some risk with using non-selective herbicides in 2016, from a Poa control standpoint, nearly all dormancy programs performed well last season. With glyphosate resistance in Poa on the rise, turfgrass managers who have relied on dormant glyphosate applications in the past should explore several of the alternative dormant programs presented below.
Roundup Pro at 32 fl oz/A
Finale at 6 fl oz/A
Reward (2 pt/A) + Princep (2 lb ai/A)
Sureguard at 12 fl oz/A
Spring Postemergence (POST) Programs
As has been the case in previous years, spring applications of ALS inhibiting herbicides provided excellent control of annual bluegrass in 2017. Top performing treatments included:
Revolver at 17.4 fl oz/A in March
Monument at 0.53 oz/A in March
Katana at 3 oz/A in March
Tribute Total at 3.2 oz/A in March
Turfgrass managers should be advised that resistance to these herbicides has become very common throughout the southeast. A common trend in the spring of 2017 was the struggle to control annual bluegrass on ultradwarf bermudagrass putting greens due to ALS resistance issues. Applications of Kerb in spring were effective in controlling these populations at select locations in 2017. However, more research is needed to better understand the weed control efficacy and turfgrass safety of Kerb applications to ultradwarf putting greens in spring
It is critically important that turfgrass managers not overuse ALS inhibiting herbicides for postemergence annual bluegrass control in spring. Instead, emphasis should be placed on developing Poa management plans that incorporate alternative application timings and herbicidal modes of action.
Diagnostics Increasing
After officially opening in fall 2016, the UT Weed Diagnostics Center saw an influx of annual bluegrass samples this spring, with the vast majority of individuals requesting that plant material be screened for resistance to ALS inhibiting herbicides such as Revolver or Monumnet. Support from a United States Golf Association Turfgrass & Environmental Research Grant has facilitated the development of an agar-based assay to screen annual bluegrass samples for resistance to ALS inhibting herbicides. This technique used at the UT Weed Diagnostics Center can provide turfgrass managers information about their plant material in a matter of a few days. Diagnostic testing can be a valuable tool to determine why a particular program may have failed to provide optimal control and to assist in developing better Poa management programs for use in the future. Our hope is that turfgrass managers will continue to make use of the UT Weed Diagnostics Center in the future.
#PoaDay — Don’t Miss Out!
In March 2017, the University of Tennessee hosted an online Field Day, termed #PoaDay, to showcase statewide research evaluating programs for managing annual bluegrass in warm-season turf. Thanks to support from Aqua-Aid, the event was a huge success, with online streaming facilitating content reaching over 600 individuals. Those who missed the event can watch the broadcast below.
Our team will be hosting #PoaDay in March 2018 with more details soon available through our website.
