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By Larry DiVito

“What height are you mowing that infield at now?”

For all of us involved with baseball in the sports turf industry, this is a question we are asked quite often, from high school baseball all the way up to the big leagues. The question comes not only from players and coaches. With nearly everybody having mowed a lawn at some point, many in society feel knowledgeable on the subject. Thus, the question on height of cut is directed at us from administrators, executives, media members, fans and parents. It seems to be a straightforward question, and you can provide an answer any way you would like. Some possible responses:

“About an inch and a quarter.”

“One and five-sixteenths inches.”

“Higher than last season.”

“None of your business.”

“Twenty-four millimeters.”

That last response is the one I like to use here at our ballpark. It is more or less the height we have been at since 2010; it’s an accurate answer and makes people think a bit. Twenty-four millimeters is .94 inches by the way. Baseball is interesting with its tradition of not regulating the height of cut for turfgrass in any way. At the major league level, stories have long been told of managers conspiring with groundskeepers to speed up or slow down infield ball speed by manipulating the height of cut. Sometimes this has to do with the makeup of the team.

In other situations, the physical dimensions of the ballpark will influence the team’s approach for the speed of the infield. In a small ballpark where the ball carries well and hitters are favored, management may want a slower infield to help out the pitching staff. While soccer appears to be more interested in monitoring mowing heights (the English Premier League Handbook states simply that the height of the pitch grass shall not exceed 30mm), baseball continues without any requirements. The subtle variations in turf and infield management at each ballpark, due to climate or team preferences, are an appealing part of the game for serious baseball fans. Of course, as turf managers we are well aware that height of cut is not the single determining factor in how fast, slow or consistently your field plays on a daily basis. So consider what your infield is made of, and how your cultural practices can influence the speed and playability of your turfgrass.

Having worked for 20 years in professional baseball, my current view is that it is the job of the turfgrass manager to execute a maintenance program meeting the needs of the players and coaching staff. For our discussion here, we will focus on the speed and pace of ground balls. In baseball, we do not have a technical device such as the Stimpmeter used on golf greens. What we do have is an opportunity daily to watch hundreds of ground balls in batting practice, both from fungoes and live hitting. This can serve as a good, preliminary informational resource for the turf manager. From there, you can watch game situations and seek input from key players and members of the coaching staff. Finding the right mowing height/ball speed relationship is something that evolves at the ballpark. Your job is to look to accommodate your team, find what works for them, and manage the turf from there.

Perhaps you have had the opportunity to construct or renovate your infield turfgrass recently. Consider your rootzone material. The physical structure of your rootzone will have a real and noticeable impact on how your infield plays. We value sand for its compaction resistance, drainage capacity and aeration properties. We also know that an ideal rootzone has nearly equal parts of air and water filled pore space. When building or renovating, it is critical to have particle size testing done on your rootzone.

A helpful document to acquire is the ASTM’s “Standard Guide for Construction of High Performance Sand Based Rootzones for Sports Fields.” This guide has detailed information to help you understand the details within your test results. Two things to consider are particle sizing and Coefficient of Uniformity (Cu). It is recommended that you avoid any extreme percentages of angularity or roundness in your sand particle sizing. Related to that is the Cu calculation, which measures the uniformity in particle sizing. The ASTM range for Cu of sand based athletic fields is at 2.5-4.5, with the lower end of the Cu range meaning a more uniform particle size and better resistance to compaction.

So what does all that mean for your ground balls every day? My opinion is that a Cu near or above 4.0 is higher than you would want for a baseball infield. You do not want the sand or soil below your turf to help accelerate the speed of a ground ball. While you need a certain amount of firmness for footing, a rootzone that compacts too easily can tend to play a little harder than you would like. I feel that a Cu near 3.0 is a nice target number. With a Cu higher than 3.5, compaction has the potential to prohibit the rootzone matrix from absorbing the force of the batted ball.

Type of grass

Your type of turfgrass is of course an important consideration. Be it Bermuda, Kentucky bluegrass or perennial ryegrass, the key point here is to grow a dense, healthy stand of turf. I recall an excellent talk I attended at the STMA Conference in 2012 by Dr. Karl Danneberger from Ohio State University. The lecture was titled “Solar Radiation is the Driving Force of Sports Turf Management.” It sounds simple, but after 20 years of doing this, I continue to be astounded by the positive impact of bright sunlight on turfgrass.

Of course, the other side of that is the infield tarp. Quite often, many of us at the college and professional levels have our infield tarp on during daylight hours on a game day. The threat of rain or light snow in the spring leads to a great deal of sunlight being denied to the turfgrass. By blocking light to the plant, the wear tolerance of the turf decreases significantly. Because of that, we like to focus on pre-stress conditioning by keeping potassium levels high and using biostimulants in our spray program. Tarping for extended periods of time also increases the disease pressure on the turfgrass. I find it useful to have the infield and other turf under the tarp on a separate fungicide program. Daily evaluation of the turf and anticipating problematic weather conditions will drive the use of contact and systemic fungicides.

Given that you are likely mowing at your preferred height of cut at least four times per week, keep in mind the goal is to maximize the density of the plant. Along with that, we try to avoid having the turf laying over due to continued mowing of the pattern with reel mowers. At the professional level, the expectation is usually for the turf to be mowed every game day at a consistent height during the season. If you are looking to speed up or slow down your infield, you can simply increase or decrease your frequency of cut over the course of a week without changing mowing heights.

Our preference here in Minnesota is to mow a neutral cut every third day during a homestand. Starting at first base and mowing to third base, we then mow back over the same path in the opposite direction. When the team is out of town, we mow in that same procedure from different angles. Frequent mowing at a lower end height of cut will help improve turfgrass density in cool season turfgrass. Improved seed varieties of both Kentucky bluegrass and perennial ryegrass will tolerate mowing heights between 7/8” and 1” quite easily. My experience with warm-season grass was during my four seasons at Dodger Stadium, where I had the benefit of working for Eric Hansen. As soon as the team was out of town, Eric’s program for the 419 Bermuda turf was to take the height of cut down a bit, maybe 1/8” or 3/16” and mow in three directions. This reduced the grain in the 419, eliminated any puffiness of the turf, and helped transition out the ryegrass by exposing more of the Bermuda to sunlight. With fertility and some time, the 419 would settle back into its game day condition in about a week. Again, remember your goal throughout the season is to increase turf density and maximize the leaf surface of the plant.

One challenge we face in baseball is timing of irrigation during home stands. With games daily for 7-10 days, and start times that vary greatly for TV, infield irrigation is not at all on a set program. Consider also that during a homestand the tarp may be involved quite a bit as well. For about the past 10 years or so, I have been exclusively hand watering the infield turf during home stands. This keeps our baselines, cutouts and dirt edges from being oversaturated and gives me time to evaluate the turf as well. If we are playing a day game, we never water the turf in the morning, preferring to do so the night before if necessary. Working around the schedule, and watching weather patterns, the goal is optimal rootzone moisture while still having time to dry down the upper canopy of the turf.

I use a Turf Guard sensor on the infield, which gives me moisture content readings at a 2” and 7” depth. Depending on time of year and expected weather, we may want to be 12% or 14% moisture on average for the two depth readings on a sand-based field. Regarding wetting agents on the turf, I prefer to use a penetrant type. If I am hand-watering the morning of a night game, I need to be sure that water is moving downward that morning. We don’t want to overwater at all, but also try and avoid situations where the rootzone would dry out too much during a game. My goal is for the turf and soil matrix to absorb the force of the baseball. This leads to truer bounces and better consistency.

There is no time to rest when a big league team heads out on a road trip. Cultural practices need to commence right away. In addition to your work plan, there may be other events scheduled for the field. Time of year and type of turfgrass will influence your plan for those weeks. With cool season turf, I like to be sure we slice seed the infield on two different occasions prior to June 1. Core aerification occurs when there is a proper window for recovery prior to our next game. Because our rootzone has a Cu slightly below 3.0, we pull cores on the infield as follows: 1. Topdress with USGA sand. 2. Core aerify 3. Hand rake cores. 4. Roll in two directions with the greens roller. This process eliminates any wheel indentation from the weight of the topdresser, and gives us a pool table effect on our flat infield turf after using the greens roller.

During the summer months and through the end of September, we take every opportunity we can to execute solid tine aerification. Dependent on timing, this can be with either needle tines or slicing tines. Any time we can get oxygen into the rootzone is a positive for us. I have also been very pleased by the benefits of having a lightweight greens roller the past few years. It gives an excellent finish after coring, particularly on newly sodded areas when you are coring to break up the sod layer in the upper rootzone. Again, our goal is a dense stand of turf, with a firm enough but not overly hard rootzone.

Fertility

What about your fertility program? It is nice to write up a preseason plan for feeding your infield turf, but I prefer to monitor as I go through the season. Because of the tarp, poor timing with weather may lead to weeks without natural rainfall. Or you may get heavy rainfall when the team is out of town and your tarp is not on. We enjoy that natural rain, but it also can mean nutrients get flushed from a sand-based rootzone. We are trying to grow dense, healthy grass, but we do not want to overfeed it. Excessive nitrogen can lead to lush, slick and divot-prone cool season turfgrass. It may also slow the baseball down more than you would like. I prefer nitrogen to potassium ratios of 1:2 with granular products. For instance, I may apply a granular 6-0-12 product, all soluble nitrogen, at a rate of .25 lbs. N/ .50 lbs. K every 18-21 days. Should rainfall flush any of that application, I will tighten up the interval as needed. Along with the granular applications, spoon-feeding of the infield is supplemented by foliar feeding as well. This allows us in between home stands to apply micronutrients and biostimulants such as humic and fulvic acids, plant sugars, and sea plant extracts. We will also add about .10 lbs. N to our spray just prior to the home stand.

At different points in my career, my favorite infield conditions have been with turf that is primarily perennial ryegrass. I enjoy its uniformity at a lower height of cut and like the way the baseball moves across it. Also, as a finer bladed turfgrass, there is better overall nutrient uptake of foliar spray applications, which means more control of week-to-week fertility.

What about plant growth regulators? PGR use will give you an increase in turfgrass density as well as controlling clippings. Better lateral growth from PGRs improves the overall leaf surface of the plant, forming the dense mat and strong tensile strength you are looking for to get optimum playability. PGR use results in greener turf due to reduced leaf elongation, thus concentrating chlorophyll in the smaller blades. Along with thoughtful fertility and sound cultural practices, PGR use enhances the turfgrass through pre-stress conditioning.

The most commonly used PGR is trinexapac-ethyl. Research into the use of Growing Degree Day models to gauge intervals for applications of PGRs is becoming quite useful to turf managers. The product will break down more rapidly during times with high air temperatures. For the steadiest growth regulation of your turf, it is common to start with a half-rate application, followed by lower rates of trinexapac-ethyl applied more frequently to benefit the turf. If using a PGR, you may consider avoiding areas of extreme traffic where you need aggressive growth to combat wear in those isolated spots.

We have focused quite a bit on the turfgrass and rootzone of the infield. What about the skin areas? First of all, you can help or hinder the playability of your infield tremendously with how you manage the dirt area in front of home plate. That is often where ground balls begin. By keeping the area soft and consistently moist, you can take speed and topspin off the baseball. If it is too hard, it will produce acceleration and increased topspin. This not only speeds up the ground ball, it also can lead to a high bounce on the last hop for the fielder, due to the increase in topspin. To work the dirt in this area, you can drop eight to ten bags of calcined clay there and rototill it into the top three or four inches (stay a foot away from the foul line, where players leave the batter’s box). Level it out, roll it and soak it down. Give it at least a week to settle down. The increased volume of calcined clay will retain more moisture for you and help minimize the force of the ground ball off the bat.

More on dirt

What about your infield dirt in general? We always stress keeping good moisture in the infield skin area. You need to think about watering through the entire dirt profile going into a homestand. A heavy soaking needs to saturate the dirt and perc all the way through. The first key to good bounces on your dirt is having that moisture all the way though the profile. The time to do a heavy soaking during a homestand is right after a game, giving you the next day to start your prep work on the dirt. From there, your game day routine involves working the upper ¼” or so of the dirt with your nail drag, rakes, mat drag and walking roller. Pre-game watering should be a touch up for game conditions, taking into effect the sunlight, wind and dew point that day. Topdressing materials and quantities seem to vary due to personal preference. An abundance of calcined clay as topdressing will help hold moisture in the dirt, but it also changes the pace of the ground ball.

It is hard to describe in writing, but the correlation of speed of the infield grass and the infield dirt is a key factor in achieving optimum playability. For instance, a shortstop would generally not like an infield with tall, dense cool season turf and dirt that is a bit dry and fast. The pace of the ball will change in that case. My goal is for the infield to play a bit on the fast side, not excessively though, with the turfgrass evenly cut to form a dense mat, and the dirt to be firm and moist. Part of that program is mowing height and frequency, part of it is turf fertility and part of it is infield dirt topdressing, which we like to be consistent and sufficient rather than excessive in volume.

The other key factor is water management of both the rootzone and the infield skin. The goal with the pace of the ground ball is for it to be consistent from turf to dirt. This is especially important for middle infielders and their mental “game clock” when it comes to making decisions on plays. The consistent pace will lead to an increase in double plays turned and better overall play.

It is not my intent to offer one fixed and absolute way of doing things. Generally, better playability comes when the turfgrass and dirt have enough give to minimize the force of a batted ball. Ultimately, what is most important is to first gain an understanding of what the ideal speed and pace of the ball is for your facility. From there, take into consideration all of the variables discussed here. Put it all together and achieve the level of playability that makes you, the players and your coaching staff happy.

Larry DiVito is Head Groundskeeper for the Minnesota Twins.

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