Key Takeaways

• Successfully managing a mixed stand of Poa annua and creeping bentgrass on putting greens requires superintendents to have an in-depth understanding of the strengths and weaknesses of each grass.
• A quality playing surface can be delivered with a sound agronomic program where both grasses coexist or practices can be put in place to shift grass populations.
• Herbicide options exist to suppress Poa annua in favor of creeping bentgrass or vice versa, but it cannot be overstated how important it is for expectations to be clearly defined should a strategy such as this be put into action.
• Collecting putting green performance data and benchmarking it against maintenance practices can lead to a better understanding of how putting greens are performing so more-informed decisions can be made.
   

Brushing

Brushing is an often-utilized surface management strategy where the goal is to encourage more upright growth of creeping bentgrass, which leads to smoother ball roll. The keys to brushing are determining how frequent and how aggressively it needs to be performed to produce a quality putting surface without jeopardizing turf health. Keep in mind how each grass has a different tolerance for how much mechanical wear it can withstand. Poa annua has a higher wear tolerance and the importance of monitoring how the turf responds cannot be overstated. Also, weather conditions will influence the brushing program since both grasses will have a lower tolerance for how much abrasion they can withstand when more stressful weather conditions are experienced.

A wide variety of options are available for brushing putting greens. While conducting Course Consulting Service visits, USGA agronomists often observe drop-down brushes mounted in front of the mowing basket, rotary brushes installed in line with the front roller, and pull-behind brushes that are typically used for incorporating topdressing sand. Different brushing techniques can be used to manage the amount of stress that is placed on the turf. Manufacturers even offer brushes with different levels of stiffness, so brushes can be changed depending on conditions.

Research on brushing greens has been conducted at both Ohio State University and Penn State University. The two-year study conducted at Ohio State revealed that when brushes are set at an even height with the rollers, the turf showed no negative effects, while an improvement in leaf texture was observed. However, the two-week study conducted at Penn State found that all brushing strategies being evaluated – soft bristles, stiff bristles and a counter-rotating brush – decreased turf quality and ball roll distance.

When looking at both research projects, the take-home message is that it is important to find a brushing strategy that fits the needs of a particular golf course so turf health is not jeopardized and playability improves. How frequent brushing is performed will play a major role in the success of this practice. Aiming to brush three to five days a week provided the weather cooperates is good starting point if this practice is being added to an agronomic program.

Grooming

Grooming is another surface management practice that can be performed to encourage upright growth of creeping bentgrass and improve putting green smoothness. Depending on the groomer and how it is set up, this practice can be performed weekly during mild weather. A more aggressive setup will require a longer timeframe between grooming events. A less aggressive setup would be adjusting the groomers to 15%-25% below the height of cut whereas a more aggressive setup would be setting the groomers at a depth greater than 25% below the height of cut. The most common options for grooming putting greens are a triplex set up with groomer cassettes or groomers mounted in line with the front roller of the mowers.

Vertical Mowing

There is often a debate around the difference between grooming and vertical mowing since these practices are similar. Grooming is used to manage leaf texture and encourage more upright growth while the primary purpose of vertical mowing is to remove organic matter from the mat layer. Vertical mowing will also encourage upright growth, but due to the aggressive nature of this practice it is typically performed in the shoulder seasons to minimize the risk of turf injury and reduce the temporary negative impact on ball roll.

Both vertical mowing and grooming are effective at thinning the canopy. If there appears to be too much turf on the putting greens, one of these practices can be used. Groomer blades will be thinner and less aggressive than the thicker vertical mowing blades. Be sure to choose the setup that will deliver the intended results.

The timing of vertical mowing can influence the balance between Poa annua and creeping bentgrass on greens. Vertical mowing in the late spring will favor creeping bentgrass while early spring or fall vertical mowing tends to promote Poa annua populations. In fact, aggressive vertical mowing when creeping bentgrass is growing slowly is an effective strategy for creating a competitive advantage that favors Poa annua over creeping bentgrass.

Mower Setup

Fine-tuning the mower setup can reduce the impact the differences in growth rate and growth habit have on the performance of mix stand putting greens. Placing the prism on areas of the putting greens where both grasses are present helps with mower setup decisions and to evaluate potential mowing quality differences between each grass. If a significant difference is observed, adjustments will be necessary. Using a more aggressive setup with a grooved roller may be the ticket to maximizing performance. Conversely, going with a solid roller and adjusting the aggressiveness or frequency of brushing or grooming may help improve smoothness in this scenario. Keep in mind it is best to make small adjustments and evaluate the impact rather than make a dramatic change. Every golf course is different and just because one approach leads to success at one course does not mean it will deliver the same results at other courses in the area.

Rolling

Along with mower setup, superintendents managing a mixed stand often find themselves trying to predict how frequently putting greens should be rolled. Like every surface management practice that is performed, rolling frequency is site specific and adjustments will have to be made throughout every golf season. Remember that creeping bentgrass and Poa annua have different wear tolerances. When soil temperatures are low and creeping bentgrass is semidormant, excessive rolling can result in turf injury because the turf is not growing and cannot recover. Ultimately, if voids are created from too much rolling the opportunistic Poa annua is likely to invade.

Water Management

Water management plays a key role in how putting greens perform, whether a monostand or mixed stand is being maintained. Poa annua and creeping bentgrass have different moisture requirements and therefore decisions must be made to determine how the playing surfaces are irrigated to maximize performance of both grasses

A moisture meter should be used to benchmark where turf health and playability are balanced for both grasses. Since Poa annua is more susceptible to moisture stress, thresholds will have to be established based on the needs of the Poa annua while at the same time avoiding overwatering the creeping bentgrass.

Ideally, deficits are corrected through hand watering because this makes it possible to be more precise with irrigation practices. This is especially critical when managing a mixed stand because of the different moisture requirements of each grass. If labor is limited and sprinklers are used more often, run times should be adjusted based on the water requirements for individual putting greens. Applying the same amount of water to every putting green is a dated irrigation strategy that often leads to underwatering or overwatering.

Water can be managed to shift the grass populations. By managing moisture based on the needs of creeping bentgrass, Poa annua will be stressed and create a situation where creeping bentgrass populations can increase. This will create bumpy conditions, but if the long-term goal is to increase creeping bentgrass, this is one strategy that can be effective.

Plant Growth Regulators

Sound use of plant growth regulators (PGRs) will play a significant role in producing quality putting conditions on greens with a mixed stand of bentgrass and Poa annua. Since these grasses have different growth rates, managing growth with PGRs will help maintain a quality playing surface, especially from a smoothness standpoint. If growth is excessive, putting quality will decline throughout the day and golfers who play later in the afternoon will not have a comparable experience to those who play in the morning. Not to mention, PGRs help the turf conserve carbohydrates (energy) used for growth.

When managing a mixed stand, the question becomes which PGR or combination of PGRs should be applied. With higher populations of Poa annua, trinexapac-ethyl or prohexadione-calcium are likely the best options because they do not suppress Poa annua like paclobutrazol and flurprimidol. Both paclobutrazol and flurprimidol are good options when creeping bentgrass populations are high and the goal is to create a competitive advantage for creeping bentgrass populations to increase.

Considering how much of an impact Poa annua seedheads have on ball roll, the PGR ethephon should be used for seedhead control. However, suppressing seedheads will also improve the health of Poa annua. If Poa annua populations are low and creeping bentgrass is the preferred species, it may be best to forego the use of ethephon so the Poa annua plants expend carbohydrates to produce seedheads and then use PGRs like paclobutrazol and flurprimidol to increase creeping bentgrass populations.

Research performed by Dr. Shawn Askew has proven that a late fall or early winter application of ethephon followed by spring applications of ethephon and trinexapac-ethyl are highly effective at controlling Poa annua seedheads. In areas like the Pacific Northwest, where seedheads can emerge in the fall, previously mentioned surface management practices like brushing and grooming can be used to mechanically remove them.

PGR applications should be timed using growing degree days. USGA-funded research has shown that tracking growing degree days makes it possible to keep turf regulated and avoid rebound growth or overregulation. Growing degree days should also be used to time spring applications of ethephon and trinexapac-ethyl for controlling seedheads.

Fertility

Nitrogen applications also play a key role in putting green quality. The goal should be to consistently maintain healthy turf with adequate nutrients, so the turf recovers from stress, ball marks and traffic, while avoiding excessive growth.

The challenge with a mixed stand is how each grass has a different nitrogen requirement. Creeping bentgrass greens might only need 1-2 pounds of nitrogen per 1,000 square feet per year, while Poa annua greens can require 3-4 pounds of nitrogen per 1,000 square feet per year to provide acceptable turf quality. Therefore, the total amount of nitrogen applied should be somewhere between the optimal ranges needed for each grass. If the goal is to increase creeping bentgrass, fertilizing based on its needs will help create a competitive advantage over Poa annua.

Applications of nutrients besides nitrogen should be based on routine soil analysis. A soil analysis should be completed at least once a year and it is common for this procedure to be completed in the spring and the fall.

New Techniques for Managing a Mixed Stand

Collecting putting green performance data is a growing trend in the industry. More superintendents are measuring and recording clipping volume and tracking all the practices that have an impact on putting green performance. The USGA Green Section recognizes that collecting and analyzing data can result in improved putting green performance and this was a driving force behind the development of the Surface Management feature found on the DEACON^_TM Platform. A series of graphs can be generated with the data that is entered to not only help make more-informed decisions, but also to serve as a communication tool.

Surface management practices are typically benchmarked against putting green speed because this is currently the most widely understood putting green metric. Some superintendents incorporate a bobble test, where they count the number of times a golf ball hops during green speed measurements to evaluate smoothness. This test is obviously subjective, but there are currently very limited options for measuring smoothness.

Collecting data is beneficial for maintaining a mixed stand because there is a record of when the putting greens were performing the best and which practices were completed to produce these conditions. A more proactive approach can be put in place based on the data to manage moisture, schedule topdressing and other cultural practices that promote surface smoothness, and apply PGRs and fertilizers to manage growth and turf health.

If you’re interested in learning more about how data collection can be used to improve putting green management, USGA agronomists participated in a panel discussion on data collection at the 2022 GCSAA Conference and Trade Show along with Dr. John Sorochan and Jared Nemitz, director of golf course and grounds at The Ford Field and River Club in Georgia.

Shifting Populations

There is often a desire to increase the population of one grass or the other when dealing with a mixed stand. We have already covered how the PGRs paclobutrazol and flurprimidol can be used to suppress Poa annua and increase creeping bentgrass populations. Using PGRs can be effective for shifting species populations, but this is a long-term strategy that can take years to yield significant results. Certain herbicides can be used to shift populations one way or the other more rapidly, but it cannot be overstated how important it is to understand the potential risks and for expectations to be clearly defined before one of these more-aggressive strategies is implemented.

Methiozolin (PoaCure) is a herbicide option to reduce Poa annua populations. Generally speaking, this herbicide has been used at facilities with high percentages of creeping bentgrass in the greens. However, some superintendents managing greens with greater than 50% Poa annua are still using this product to increase creeping bentgrass populations, they just need to be very careful with their approach. It is important to communicate how the putting greens will perform during the transition and how long it will take to realize the benefits.

Conversely, decision-makers at a facility may wish to increase Poa annua in their greens. Dr. James Murphy from Rutgers University has been exploring if fluazifop (Fusilade II) can be used to suppress creeping bentgrass and increase Poa annua populations. This strategy has been evaluated and put into action successfully at some facilities in the Northeast. It is not a one-and-done type of application, so it will be necessary to commit to this program for longer-term results.

Conclusion

Poa annua and creeping bentgrass are different in so many ways including their growth characteristics, responses to environmental conditions and optimal nutrient and soil moisture levels. When a sound agronomic program is in place to manage a mixed stand of Poa annua and creeping bentgrass, the impact of these differences can be minimized so putting green performance is maximized.

Given the uniqueness of every golf course, the agronomic program must be tailored to determine how, when, and which practices are performed. Frequent evaluation and fine-tuning will be necessary to maintain consistency between both grasses. It is also possible to shift grass populations in favor of one grass over time. Of course, the agronomic program will have to be changed to shift the populations and there will be short-term disruptions to realize the benefits.

Putting greens with mixed stands are found all over the world at highly regarded courses. When managed correctly, conditions can be produced that challenge golfers of all skill levels.

Zach Nicoludis is an agronomist in the Central Region.