Key Takeaways

• Achieving firm putting greens is more complicated than just withholding water. Firmness is affected by organic matter accumulation, topdressing characteristics and grass species, among other factors.
  
  
• Firmness measurements are not valid for comparison from course to course. The focus of collecting this data should be on improving firmness and evaluating playability on a given golf course.
  
  
• Providing consistently firm conditions requires long-term programs that promote healthy putting greens and dilute organic matter at the surface.
  
  
• Measuring firmness and monitoring organic matter percentage illustrates the impact that maintenance programs have on these parameters and allows you to correlate them with playing conditions.
  
  
• There is no “one-size-fits-all” approach to firmness. Different combinations of maintenance programs can provide firm greens, but if you do not measure, you do not know.
   

Two important concepts related to firmness should be understood: “terminal firmness” and “terminal softness.” Terminal firmness is the point at which a putting green will not get noticeably firmer with any short-term maintenance practices because of inherent physical characteristics including organic matter percentage and characteristics of the construction mix and topdressing sand. Withholding water may lead to more stress on the grass and a green that looks firmer, but the change in ball reaction will be minimal. Terminal softness lies at the other end of the spectrum where adding additional water minimally affects firmness. Basically, the green is as soft as it can get. You see this condition after a day of rain, for example.

Terminal firmness and softness are not static values. They can be affected in the long term with changes in aeration strategies, topdressing frequency and the topdressing sand that is used. In the short term, changes to these maximum and minimum values through daily maintenance are small.

The importance of terminal firmness and softness cannot be overstated. When the terminal softness of a green is reached and the greens are still relatively firm, it takes less time to regain firm conditions compared to a green that gets very receptive after rain or irrigation. Greens with a higher firmness value at terminal softness will generally also have a higher terminal firmness and vice versa. When the terminal firmness of a green does not provide desired playability, golfers and superintendents can get frustrated because there really are not any short-term solutions.

When firmness is discussed, ball reaction or “bounciness” is really the subjective criteria being evaluated. Quantitative firmness values do not “travel” from golf course to golf course due to agronomic differences. The “average firmness” number from soil-based Poa annua greens is not likely to provide the same ball reaction as that same value on sand-based creeping bentgrass or bermudagrass greens. When managing firmness, the focus needs to be on measured values at an individual golf course and the ball reaction that is produced at that value.

In this article, we will take a look at the key contributors to firmness, how they interact with each other, and how to improve firmness in the short and long term. Having measured firmness on greens for the last 18 years preparing golf courses for USGA championships, let me tell you that if the long-term fundamentals of firm conditions are not in place, there is only so much that can be done a week or two before a big event to make the greens firmer.  

Organic Matter

The intuitive thought regarding organic matter and firmness is that less is better. However, putting greens with very low organic matter levels in the top 2 centimeters of the soil profile do not always produce the best ball reaction in my experience. There is an elasticity to organic matter that adds bounciness to the surface so long as the organic matter is not excessive and is diluted with sand.

A balance between sand topdressing and organic matter at the surface is a critical component of firmness. A high percentage of undiluted organic matter in the top 2 centimeters will be a “soft” green even under dry conditions. When the USGA Green Section was working with researchers and superintendents to develop a standardized method of putting green organic matter sampling and testing, we collected data from hundreds of greens with the goal of correlating organic matter percentage with turfgrass health and putting green performance. Not surprisingly, the top 2 centimeters of the soil profile is the most dynamic with respect to organic matter percentage, and common sense tells us that conditions closest to the green’s surface affect firmness most. This underscores the importance of organic matter testing to evaluate maintenance programs and the relationship between organic matter levels, measured firmness and observed ball reaction.

Sand Selection

Sand used for putting green construction and topdressing can play a big role in firmness. Particle size distribution and particle shape are both important. At the time of putting green construction, mixes can be engineered to promote firmer conditions by incorporating specific amounts of soil, organic matter and other amendments while still maintaining good drainage and other physical properties.

Topdressing sand must be compatible with the existing rootzone material. Strictly based on soil physics, the rule was never to use sand with a finer texture than the existing soil for fear of creating a layer at the top of the profile that would hold water and keep surfaces wet. Superintendents often limit or forgo light topdressing because they have difficulty incorporating the sand into the turf canopy and the coarse particles negatively affect ball roll and mowers. However, research has shown that it is possible to topdress with sands that are less coarse than the rootzone mixture for routine topdressing to facilitate incorporation and promote firmness. The Green Section Record article “Light and Frequent Topdressing Programs” summarizes some of the information that should be considered when selecting topdressing sand based on ongoing USGA-funded research at Rutgers University.

One final and important, but often overlooked, sand characteristic is shape. While characterizing sand shape is not an exact science, angular sands will produce firmer conditions than round sands. Think about the stability of a pile of bricks compared to a pile of marbles. This represents the extreme at each end of the sand shape spectrum but illustrates the important role that sand shape can play in achieving firm conditions.

Grass Type

The grass species on the greens affects perceived firmness. Putting greens established with creeping bentgrass, Poa annua or a combination of these grasses tend to be more receptive at similar firmness values when compared to ultradwarf bermudagrass or seashore paspalum. Interestingly, warm-season grasses typically have higher organic matter concentration in the upper rootzone than cool-season grasses, but they can still play firmer.

The exact “correction factor” for lack of a better term is hard to nail down. However, for championship preparation, even though warm-season greens may measure softer by value, golfer perception is that they play much firmer than this measurement suggests compared to cool-season grasses. The direct cause of this difference is not precisely known, but the stiffer leaf blades of warm-season grasses in conjunction with the dense mat of stolons and rhizomes just below the surface likely plays a role. The variations in firmness readings and ball reaction among different grasses, or even the same grass in different environments, is just another illustration of why comparing firmness measurements from one course to another is not useful.

Short-Term Programs to Promote Firmness

The ability to consistently maintain firm putting greens is not the product of short-term maintenance programs. By short-term, I mean practices designed to promote firm conditions a month or two before an event all the way through the event itself. Carefully reducing water inputs will have the most immediate impact on firmness, but if necessary management programs to promote firmness have not been implemented consistently over time, the greens will only get so firm (terminal firmness) and that level of firmness may not dramatically impact ball reaction, especially for accomplished players.

Daily programs such as rolling may increase firmness if they are not already part of regular maintenance. If greens are rolled three or more times weekly, the benefits rolling provides to firmness have already been realized. Additional rolling will help with speed and putting quality, but not firmness. Introducing frequent sand topdressing to match turf growth rate can begin firming greens in two or three months, but generally 12 to 18 months is needed to see a noticeable change in measured firmness and ball reaction.

Short-term firmness is negatively impacted by core aeration. Aeration relieves soil compaction, which promotes softer conditions at the surface. If one feels that aeration must be performed prior to an event, consider solid-tine aeration and filling the holes with topdressing sand. While still not ideal, the impact on short-term firmness will be less with solid-tine aeration. Every course is different, but experience suggests that any aeration beyond venting will require six to eight weeks before the greens return to peak firmness.

Long-Term Programs to Promote Firmness

Rebuilding putting greens using a mix that is engineered to promote firm conditions is a great way to firm-up greens. However, this approach is not practical unless a putting green renovation is already planned. Most situations will call for a review of cultural programs that help to manage the surface mat layer to promote consistently firm conditions and increase the level of terminal firmness that we’ve talked about.

Consistently following a program of light and frequent topdressing is an important part of diluting organic matter at the surface. The ideal rate of topdressing will depend on turf growth rate. For cool-season putting greens, topdressing frequency should be higher in the spring and fall compared to the heat of the summer. Warm-season greens will need more topdressing during the hot summer months. In some regions, topdressing will stop in the winter while others may require year-round topdressing applications. Every situation is different, and the key is to focus on programs that work within the variables of a given golf course.

Core aeration removes organic matter and allows it to be replaced with sand topdressing and is an important component of long-term maintenance programs. The importance of completely backfilling the holes with sand cannot be overstated. The rule of thumb has been that around 20% annual surface disruption is a good goal to maintain organic matter levels. Although limited research has been conducted investigating specific percentages of surface area disruption and the correlation with organic matter, the work that has been done generally supports this goal and superintendents have traditionally found hitting this target provides good results (Atkinson et al., 2012). However, there are courses that maintain firm conditions and manage surface organic matter without reaching this threshold. Additional options such as solid-tine aeration and high-pressure sand injection have benefits in reducing organic matter percentage in the surface layer and promoting firmness.

There are different programs and combinations of programs that can help or harm efforts to maintain firm conditions. If putting green firmness is important, data collection is critical to evaluate firmness and the impacts of programs that are being implemented to encourage firm conditions. Using the GS3^™ ball in conjunction with the DEACON^® management system allows firmness to be measured accurately and efficiently over time. Other tools for measuring firmness are also available. Regular measurements show the impact of daily maintenance practices, weather and water management, and the gradual impact of long-term programs. Visual evaluations of firmness based on ball reaction and ball marks will also provide some insight, but this subjectivity introduces extra variables and opinions into the evaluation.

In addition to firmness data, tracking clipping yields can provide insight into turf growth rate and the topdressing required to match that growth. Monitoring organic matter percentage, especially in the upper portion of the rootzone, can provide a better understanding of the amount of organic matter that optimizes firmness for a set of greens.  

Conclusion

Firmness is a simple concept, but maintaining firm conditions and healthy putting greens is quite complex. Withholding water is only one component of firmness. Consistently firm greens require directed programs over the long term. There are different ways to achieve firmness and there are also exceptions to any guidelines. The bottom line is that if you are not measuring firmness and the factors that influence it, there is no way to know how management programs are affecting your pursuit of firm greens. Putting green performance becomes guesswork and can be subject to recency bias and golfer emotions on any given day. Data allows for factual rather than emotional conversations to shape the firmness journey.

Reference

Atkinson, J.L., McCarty, L.B., & Bridges, Jr., W.C. (2012). Effect of core aerification frequency, area impacted, and topdressing rate on turf quality and soil physical properties. Agronomy Journal, 104, 1710-1715. doi.org/10.2134/agronj2012.0224