Things are different in 2020. Social distancing, mask mandates, and sanitizing equipment were not part of the lexicon prior to this year but have become the norm in response to COVID-19. The pandemic has also challenged many in the turfgrass industry to adjust (or reduce) staffing and operate under budget restrictions that will likely remain for some time. Despite all of the changes brought forward in 2020, one constant remains the same in the turfgrass world — there will be a need to control Poa annua.
My staff at the University of Tennessee jokes that it is “Poa365” in our lab because we are constantly researching Poa annua to assist turfgrass managers in controlling this weed on golf courses, sports fields, lawns, and sod farms. Knowing that summer is coming to a close and many will soon be making product decisions via early-order programs, the objective of this post is to share lessons learned over the past 12 months to aid with decision making this season.
Why is Poa Annua so Difficult to Control in the Transition Zone?
1) Emergence is a moving target
Many turfgrass managers aim to control Poa annua infestations in the transition zone (and southern United States) with applications of preemergence (PRE) herbicides in early fall. One limitation to this approach is that unlike other annual grassy weeds (e.g., crabgrass), Poa annua can germinate across a wide range of environmental conditions which makes timing a PRE herbicide application quite difficult.
A graduate student at the University of Tennessee (Ms. Dallas Taylor, @utdallasgrass) has been tracking Poa annua emergence on a weekly basis since January 1st, 2019 and has learned quite a bit in doing so. In 2019, she observed initial emergence after 12 cooling degree days (CDD, base 21C) accumulated following the summer solstice (Image 1).
In 2020, she first observed annual bluegrass emergence on August 5th— when 4 CDD had accumulated. Many may wonder how this is possible with daily high temperatures still approaching 90F? Daily low temperatures in early morning hours are key. Below is a breakdown of climatic conditions for Knoxville, TN in August 2020 as of this writing (Image 2). Notice the number of days where low temperatures were close to (or below) 70F (21C).
While initial emergence may have already occurred for a small percentage of seedlings, the majority of plants will emerge from soil later in autumn. In 2019, this emergence flush occurred between the 42 and 46th week of the year (Oct. 14 — November 17th) in Knoxville. At present, we don’t have a reliable benchmark to predict when that will occur in 2020 or in locations outside of Knoxville. To that end, sole reliance on a PRE herbicide can be risky as results will be poor if the treatment is applied after plants have emerged (but might not be visible above the turf canopy).
2) Might Not Be Annual?
Traditional thinking is that Poa annua is as the name implies — a winter annual weed that germinates in autumn and dies in spring after setting seed. This paradigm could be changing though as we are observing mature Poa annua plants on Tennessee golf courses well into the summer (Image 3).
“Perennial Poa” is most often a term used to describe biotypes on putting greens in cooler climates. These selections often have high shoot density, lateral growth, and produce few seedheads. While those traits are useful for describing grasses suited for putting greens, they offer no information about life cycle. Many managing stands of Kentucky bluegrass or perennial ryegrass in northern climates can attest to the fact that Poa annua infestations at higher mowing heights persist throughout the summer; yet, these biotypes don’t fit the working definition of “perennial Poa.” This begs an intriguing question — are all Poa annua selections perennial? When one looks at turfgrasses within the Poa genus, many of the species are perennial including Poa pratensis, Poa trivialis, Poa supina, and Poa compressa. Do certain environmental conditions facilitate (or hamper) survival?
A graduate student at the University of Tennessee (Ms. Devon Carroll, @turfgirl24) has embarked upon a PhD project to understand environmental conditions on golf courses that facilitate Poa annua survival in the transition zone. She is regularly monitoring plants at different locations across golf courses to better understand microclimates that can lead to plants surviving summer. This information could have a significant effect on management strategies moving forward. For example, does a combination of fugitive irrigation water, fungicide applications, and shade facilitate Poa annua survival inside greens complexes? Will shade and compromised drainage allow Poa annua to survive in fairways (Image 4)? Does aboveground Poa annua foliage die back to the crown (beneath the turfgrass canopy) in summer only to return in the fall when conditions are more favorable for growth? Answering these questions will become essential to building effective management programs for the future.
3) Resistance is Real
A recent publication in the peer-reviewed journal Crop, Forage, and Turfgrass Management presents data on the scope of herbicide resistance within Poa annua populations on Tennessee golf courses — and it’s alarming! We’ve learned that within randomly collected plants from fairways and roughs:
64% had some degree of resistance to glyphosate (e.g. Roundup)
58% had some degree of resistance to prodiamine (e.g., Barricade)
21% had some degree of resistance to foramsulfuron (e.g., Revolver).
25% of those resistant to simazine (e.g., Princep) were also resistant to foramsulfuron or glyphosate
4% of plants were resistant to ALL HERBICIDES tested
There is currently a national effort to generate similar data for all sectors of the turfgrass industry including golf, sports turf, lawn care, and sod production. This is a multi-faceted research collaboration involving researchers from 14 different states. More information on that project can be found at resistpoa.org.
What Does This Mean for You?
1) Herbicide Mixtures Are the New Normal
Inability to predict emergence, potential for perennial plants to survive the summer, and widespread resistance issues have rendered herbicide mixtures the new normal in the transition zone and points southward. The ability to combine herbicides with pre- and postemergence activity in the same application will help with issues pertaining to emergence timing and perenniality, whereas as resistance can be mitigated by mixing herbicides from different mode of action groups. In statewide trial work conducted in 2019–2020, all of the top performing treatments were those that delivered multiple active ingredients in mixture (Image 5). For the past several years, mixtures applied in October/November have been the top performing treatments across Tennessee.
2) There May Be a Need to Try a New Approach
Those battling resistance issues have learned the hard lesson that aiming to control Poa annua AFTER resistance has evolved can be quite challenging. Knowing that using the same approach will only select for resistant plants over time, diversity is key! If a given herbicide is still working effectively, then don’t be afraid to try something new this season. This could mean changing active ingredients, application timing, or both. Those who have recently been challenged with resistance may also need to try something new — potentially targeting Poa annua with herbicides that temporarily injure desirable turfgrass. This is particularly true in the southern United States where desirable warm-season turfgrasses do not go fully dormant.
There are many biological reasons to care about effective Poa annua management but the most important reason may have nothing to do with biology at all. Effective Poa annua control is the gate key to a successful growing season in the transition zone. Facilities that make thoughtful choices about how to target Poa annua start the calendar year with clean turfgrass and positive momentum for the spring and summer that’s to come.