Preparing for Poa

Visiting turfgrass managers in recent weeks has revealed one major theme: Poa control is the last thing on anyone’s mind right now. Sustained periods of hot and humid weather have most in the southeast focused on the not-so-simple task of maintaining turf quality until the fall. “Survive and Advance” has certainly been the theme of the summer.

Although it may seem like the 11th task on a top-10 list of things to accomplish, time spent preparing for the upcoming Poa season will be well worth it in the long run. Ever-emerging resistance issues, the ability of annual bluegrass (Poa annua) to germinate across a wide range of environmental conditions, and erratic weather patterns have resulted in Poa control becoming one of the more complex issues facing bermudagass and zoysiagrass managers in the southeast during the shoulder seasons.

Each year our program at the University of Tennessee conducts numerous studies focused on strategies for controlling annual bluegrass in bermudagrass and zoysiagrass turf. Our hope is that lessons learned from this research will be helpful to those in preparing for the upcoming Poa season.

Here is what we learned in 2017–2018:

Sole Reliance on a PRE Program Can Be Risky

A traditional approach to control Poa is to make a single application of a preemergence (PRE) herbicide such as prodiamine (e.g., Barricade) or indaziflam (e.g., Specticle) before Poa emergence in autumn. This can certainly be an effective approach to managing Poa in the southeast but sole reliance on a single PRE application can be risky for several reasons:

• Poa germination is not a singular event; there can be multiple germination events throughout autumn and early spring. Considering that microbial activity and moisture start degrading herbicide upon entry into the soil, a single application of a PRE herbicide at an appropriate timing may not offer enough residual activity to control germination events occurring later in the season — particularly if weather patterns are not optimal.

Annual bluegrass (Poa annua) germination and emergence from soil

• It is not uncommon for rainfall accumulation in spring to be quite high, therefore compromising the long-term control of a single application made in autumn. In the spring of 2018, rainfall accumulation across much of Tennessee far exceeded 30 year averages (Figure 1).

Figure 1 — Rainfall accumulation data during spring 2018 at several Tennessee locations

• Many Poa populations have evolved resistance to commonly used PRE herbicides, particularly dinitroanilines like prodiamine (e.g. Barricade) and pendimethalin (e.g. Pendulum). Even under optimal conditions, treating resistant populations with these herbicides will be unsuccessful.

While it is possible to successfully control Poa using PRE herbicides, turfgrass managers should plan on a split application program to remain Poa-free from September through April. We typically make two applications of PRE herbicides in spring to control crabgrass (Digitaria spp.) from March through September; why would we expect applications targeting Poa in the fall to be different? In situations where a split-application program isn’t possible, turfgrass managers should be prepared to control plants escaping PRE treatment with a postemergence (POST) herbicide in spring.

Mixtures are the Best Strategy for Effective Control

For several consecutive years, a main conclusion of our statewide Poa control trials has been that herbicide mixtures are an optimal strategy for controlling Poa in bermudagrass and zoysiagrass. Using a herbicide mixture offers several advantages including:

1. A mixture provides an opportunity to take advantage of both PRE and POST activity in a single application. For example, one of the top performing mixture treatments in our 2017–2018 trials was Barricade + Monument applied October 17th.
2. Using a PRE + POST herbicide mixture allows applications to be made later in the fall, increasing the likelihood that a single application will keep turf Poa-free throughout the spring. In Tennessee, an optimal time to control Poa with herbicide mixtures is late October. The POST portion of these mixtures controls plants that have emerged from soil while the PRE component can control plants germinating later in the autumn or early spring.
3. Combining two products typically involves bringing together two different herbicidal modes of action in a single application. This action will break selection pressure for resistance within a Poa population

Ultradwarf Greens Require Special Attention

There were many questions last spring about POST control of annual bluegrass on ultradwarf putting greens. When planning a Poa control strategy, these surfaces require even more focus than fairways or tees. Herbicide options for Poa control on ultradwarfs are limited to pronamide (i.e., Kerb; using a special 2ee label), foramsulfuron (i.e., Revolver), flazasulfuron (i.e., Katana), and amicarbazone (i.e., Xonerate 2SC). Each of these products has limitations when used for POST Poa control on ultradwarf greens in spring. For instance:

1. Resistance to ALS-inhibiting herbicides such as Revolver and Katana is becoming more common throughout the southern United States, particularly on ultradwarf greens. These herbicides offer the best combination of Poa control efficacy and turfgrass safety on ultradwarf surfaces. If ALS inhibiting herbicides such as Revolver or Katana are still effective on putting greens, superintendents should do everything possible to preserve this by diversifying their control programs and not continuing to rely solely on these products for Poa control. Keep in mind that a “rotation” from Revolver to Katana is meaningless from a resistance standpoint; these products have the same mode of action and will both select for ALS-resistance within a Poa population.

Image 1- Annual bluegrass (Poa annua) plants present under turf protected by a winter cover (right) are larger and more mature than those infesting turf outside the area protected by a cover (left)

2. Although applications of Kerb are labeled for use on putting greens, rates are reduced (compared to fairways/roughs) to ensure turfgrass safety. Additionally, Kerb efficacy for Poa control can be affected by plant size with larger plants often being less responsive to treatment. This can be an issue if implementing a Poa control strategy is delayed until March or April. It is common for winter protective covers used on ultradwarf surfaces to accelerate growth of seedling plants that may be present within ultradwarf canopies, particularly if greens remain covered for multiple days (Image 1).

3. Don’t be afraid to use mechanical techniques to control Poa on ultradwarf greens. There are many instruments to quickly remove Poa plants with minimal surface disruption. Maintenance crews can easily use these economical devices to keep greens Poa-free. We regularly use mechanical removal tools when collecting plant samples for research (Image 2). In situations where resistance is at its worst, mechanical control is the only option remaining to manage Poa. Implementing a mechanical control strategy now, even on a subset of greens, can help break continued selection for resistant plants; thereby preserving long-term use of powerful herbicides such as Revolver and Katana.

Image 2 — A Hound Dog Weeder (Ames Company) used to collect annual bluegrass plants for research

Diagnostic Testing Will Save Money in the Long Run

In 2016 The University of Tennessee opened their Weed Diagnostics Center to assist turfgrass managers with difficult weed management issues, particularly those related to herbicide resistance. While there may be some “sticker shock” when first inquiring about the cost of a test to confirm resistance within a Poa population, there is a long-term benefit to investing in this service. Applying an herbicide for Poa control to a population that is resistant to that mode of action is, in a word, wasteful. These applications waste time, money, and labor resources while offering little-to-no positive results. Diagnostic testing can help explain why a particular application failed (to prevent it from being repeated in the future). If conducted before applicaton, testing offers scientific information about how Poa plants at your location will respond to a given treatment.