An integrated strategy for controlling Poa trivialis

It’s a lot like annual bluegrass (Poa annua), but with real stolons, none of the hype, and almost no effective chemical control strategies in cool-season turf.

Rough bluegrass (Poa trivialis) is a perennial cool-season grass that’s a problematic weed in cool-season turfs. The species fades sooner in summer than other grasses because of intolerance to heat, drought and diseases, but differences in color, texture and growth habit also make it noticeable to turf managers and golfers.

The species was introduced to North America from Europe and has become naturalized where cool-season grasses are well adapted. Additionally, improved cultivars have been intentionally planted in shaded sites or unintentionally planted by contaminated seed. Rough bluegrass has also been used in winter overseeding programs for warm-season grasses in the southern U.S.

Figure 1: Rough bluegrass regrowth from nodes on a stolon.

After its introduction to a golf course, rough bluegrass usually spreads by vegetative propagules during aeration (Figure 1). It’s a lot like annual bluegrass (Poa annua), but with real stolons, none of the hype, and almost no effective chemical control strategies in cool-season turf.

Despite research and outreach efforts, this troublesome grass still has a somewhat anonymous reputation. Working with rough bluegrass sometimes is tantamount to educating the public about its likelihood for alien abduction: Only those who’ve had a close encounter take notes, and everyone else thinks you should be medicated.

Figure 2: Rough bluegrass can be difficult to distinguish from other species. A) Rough bluegrass (left) and annual bluegrass (right). B) Untreated rough bluegrass (B1), rough bluegrass treated with paclobutrazol (B2), untreated perennial ryegrass (B3) and perennial ryegrass treated with paclobutrazol (B4).

But make no mistake; rough bluegrass is out there, lurking. The problem is that it can be difficult to distinguish from other bluegrasses or even perennial ryegrass (Lolium perenne) under growth regulation (Figure 2). Rough bluegrass often may not decline severely in midsummer if water is not limited and if preventative fungicides are applied when environmental conditions favor disease development. In these situations, it’s often the poorer mowing quality associated with the decumbent growth of stolons that disturb turf stands (Figure 3).

Figure 3: Patches of rough bluegrass growing in a perennial ryegrass fairway during summer that exhibit grainy growth (insert) and poor mowing quality.

In any case, rough bluegrass most often isn’t noticed or even considered before its establishment is widespread. The problem is — as with any weed — this is when control is most difficult. Multiple strategies are required for control because there is only one herbicide currently labeled for selective post-emergence control of rough bluegrass in cool-season turf, and because even its efficacy can be unpredictable.

Cultural control?

Cultural management and growing environments are most important to consider in controlling rough bluegrass. Controlling rough bluegrass in a shady, damp locale likely is impossible, as it will outcompete other turfgrasses. Well-drained soils that are irrigated only to prevent severe stress of desirable species will make rough bluegrass less competitive, and should reduce infestations with time. One isolated study has reported reductions in small infestations of rough bluegrass seedlings with foliar Mg and Fe fertilization, but because there are no other reports of similar results it’s safe to consider this strategy ineffective.

My colleagues and I have shown that mowing a new tall fescue (Lolium arundinaceum) sward at 3 inches or higher reduced rough bluegrass coverage up to 57 percent compared to mowing at 1.5 inches after three years, but obviously that’s not practical in fairways or tees on golf courses. Increasing seeding rates likely has no effect, either. In the aforementioned study, we seeded tall fescue at 4 lbs. to 12 lbs./1,000 ft2 (with 1 percent rough bluegrass seed by weight), but seeding rates ultimately had no effect on minimizing rough bluegrass. There is more work to be done to improve cultural control strategies for rough bluegrass.

Velocity

Velocity (bispyribac-sodium; Nufarm) is the only herbicide currently labeled for the selective control of rough bluegrass in cool-season turf. Many researchers have shown both its efficacy with multiple applications and the potential for rough bluegrass recovery that ultimately limits control.

Velocity may cause phytotoxicity to desired species, but it’s usually short-lived, with recovery typically seen by two weeks after the last treatment. Velocity’s efficacy usually increases at air temperatures greater than 75 degrees F, so we recommend treatment in spring to midsummer. According to the label, four applications at 6 oz./A on a 14- to 21-day interval are allowed. In previous research, four applications at 3, 4.5 or 6 oz./A offered similar control (at or greater than 73 percent), provided that warm temperatures followed treatment.

In this research, creeping bentgrass (Agrostis stolonifera) was seeded two weeks after the final application, which increased long-term control after rough bluegrass was thinned from treatment with Velocity. Others have investigated seedling safety following Velocity applications, with no damage to creeping bentgrass, Kentucky bluegrass (Poa pratensis) or perennial ryegrass if these species are seeded at least two weeks following treatment. Velocity also may be applied after seeding, and has been shown safe on spring- or fall-seeded creeping bentgrass as early as seven days following emergence when treated at rates up to 6.0 oz./A. Be sure to check the most current label for specifics before using Velocity or other pesticides.

It’s important to note that control with Velocity still is ultimately variable. We saw in recent research with colleagues that three consecutive applications at 6 oz./A controlled rough bluegrass 90 percent in one location, 75 percent in a second location and only 14 percent in a third location. We believe control varied among locations because of cultivar differences and differences in maximum temperatures following application.

Xonerate, Tenacity and PGRs

We also tested the efficacy of various rates and combinations of Xonerate (amicarbazone; FMC) and Tenacity (mesotrione; Syngenta). These herbicides and herbicide combinations were ineffective in controlling rough bluegrass.

The plant growth regulator Trimmit (paclobutrazol; Syngenta) also didn’t reduce the coverage of established rough bluegrass in this experiment. In the research of others, however, monthly applications of Primo (trinexapac-ethyl; Syngenta) reduced the cover of seedling rough bluegrass from 4 percent to 1 percent. This is the only report of rough bluegrass reduction with a plant growth regulator (PGR), and it’s unlikely that any PGR will control rough bluegrass on its own. However, we do think PGRs improve the mowing quality of rough bluegrass and darken its color similar to that of perennial ryegrass or Kentucky bluegrass, potentially masking its presence (Figure 2).

Nonselective control

Although it’s difficult to hear, rough bluegrass is most consistently controlled with glyphosate followed by reseeding. But even this is no easy task.

Treating rough bluegrass with a nonselective herbicide in late summer aligns well with recommended fall seeding for cool-season grasses, but spring applications offer the best control. In work in Nebraska and Kansas from 2011 to 2013, the most consistent control of rough bluegrass was observed with spring-applied glyphosate (69 percent to 99 percent control), whereas glyphosate applied in mid- or late summer controlled rough bluegrass 14 percent to 91 percent or 42 percent to 94 percent, respectively.

The best control was observed during one year in Manhattan, Kan. — the year with the hottest summer temperatures. This pattern fits with control from selective herbicides, in that we see the best control with applications prior to the onset of extended high temperatures. I recommend applying glyphosate in spring for optimum rough bluegrass control, followed by spot seeding if desired. This obviously isn’t practical on large areas, but it may be on select problematic tees or isolated areas in fairways.

Figure 4: The visual quality of rough bluegrass in Nebraska before treatment with a single glyphosate application in spring (A), midsummer (B), or late summer (C), and the control from each application when rated the following spring (below). Note reduced control when treated in late summer (C), even though visual quality was similar to that treated in spring (A).

Alternatively, use glyphosate as part of a renovation strategy in late summer to early fall. Understand, though, that even rough bluegrass that is fully green at the time of glyphosate application in fall may not be well controlled (Figure 4), and plan to implement control strategies over multiple years to control rough bluegrass.

An integrated strategy

Obviously, multiple strategies are required to effectively control rough bluegrass. In shaded areas, or in areas that receive frequent rainfall, control will be more difficult because the cultural mechanisms for control already have been restricted.

If rough bluegrass doesn’t severely decline during summer, consider nurturing it. When possible, improving drainage and limiting irrigation should favor other species over rough bluegrass if you want to control it. Our desirable species also are more tolerant to traffic than rough bluegrass, so don’t be afraid to get physical. Be aware that sanitation is important during aeration, vertical mowing and even mowing to limit unintentional propagation. Mow or cultivate infested areas last if possible. For large infestations, consider beginning a renovation with Velocity applications in midsummer, followed by multiple glyphosate applications in late summer before seeding. Then apply Velocity as soon as one week following seeding — at least with creeping bentgrass — to limit rough bluegrass reestablishment.

For smaller infestations, consider spot treating with glyphosate followed by reseeding or the physical removal of rough bluegrass followed by spot sodding. Alternatively, utilize multiple spot applications of Velocity beginning in spring to midsummer, and interseed with creeping bentgrass, Kentucky bluegrass or perennial ryegrass beginning no sooner than two weeks following the last treatment.

Rough bluegrass control is possible by persistently implementing a combination of these strategies. However, if you achieve successful control, don’t forget about Poa triv. Much like Arnold Schwarzenegger in “The Terminator,” it will be back, with sequels.

Cole Thompson, Ph.D., is an integrated turfgrass management specialist at the University of Nebraska-Lincoln. You may reach Cole at cole.thompson@unl.edu for more information.

Photos by: Zac Reicher (Figure 1), Cole Thompson (Figures 2,3 & 4)

References

Bell, G., E. Odorizzi, and T. Danneberger. 1999. Reducing populations of annual bluegrass and roughstalk bluegrass in creeping bentgrass fairways: A nutritional approach. Weed Technol. 13(4):829-834.

Hurley, R. H. 2003. Rough Bluegrass. Pages 67-75 in: Turfgrass: Biology, Genetics, and Breeding. In: Casler, M. D. and R. R. Duncan, eds. John Wiley and Sons, Inc., Hoboken, NJ.

Lycan, D. W and S. E. Hart. 2006. Cool-season turfgrass reseeding intervals for bispyribac-sodium. Weed Technol. 20(2):526-529.

Reicher, Z. J., L. W. Nees, and M. D. Levy. 2011. Roughstalk bluegrass contamination in creeping bentgrass seed lots. Appl. Turfgrass Sci. doi:10.1094/ATS-2011-0422-01-BR.

Rutledge, J., D. Morton, D. Weisenberger, and Z. Reicher. 2010a. Bispyribac-sodium, sulfosulfuron, and interseeding creeping bentgrass for long-term control of roughstalk bluegrass. Hort. Sci. 45(2):283-287.

Rutledge, J., D. Weisenberger, and Z. Reicher. 2010b. Response of seedling roughstalk bluegrass and creeping bentgrass to bispyribac-sodium or sulfosulfuron. Hort. Sci. 45(2):288-292.

Thompson, C., J. Fry, M. Kennelly, M. Sousek, and Z. Reicher. 2014. Seasonal timing of glyphosate application influences control of Poa trivialis. Appl. Turfgrass Sci. doi:10.2134/ATS-2013-0044-BR.

Thompson, C., M. Sousek, Z. Reicher, J. Fry, and M. Kennelly. 2016. Evaluation of selective herbicide combinations and paclobutrazol for rough bluegrass control. Crop Forage Turfgrass Manage. doi:10.2134/cftm2015.0213.