Tips From The Pros

ROLLING SPORTS FIELDS

Proper rolling is an underutilized tool in the sports field managers’ field prep toolbox in my opinion. You don’t hear much about the benefits of rolling natural grass sports fields. Instead, we hear a lot of cautionary tales. It seems counterintuitive to put a heavy roller out on our fields when they are already getting trampled heavily in many cases. But with a little experience, rolling can be a valuable tactic in improving the playability and appearance of our sports fields by carefully firming and smoothing the playing surface. This may improve ball roll and skip for sports like soccer, baseball/softball, lacrosse and tennis. This is probably why golf greens have been routinely rolled for over 100 years, a practice that has spread to fairways also to some degree. Rolling may improve the surface stability or “footing” on sports fields. When you walk out on to a high-performance natural grass sports field that is well managed, you will immediately notice two things; one obvious, one less so. Perhaps the first thing you notice is how closely the grass is mowed compared to most lawns, parks and other less intensively used and managed turfgrasses. You may also notice, more subtlety perhaps, how the surface feels firm and smooth. This, in part, is because most high-performance sports fields are properly rolled on a regular basis.

On a grass field, I don’t believe it’s ever a good idea to use the vibratory option that comes on most asphalt rollers. I would think the root damage may be severe. It’s also very important that you do some test-riding of your rented asphalt roller on a parking lot or equal before taking it onto your field. This way, you can make sure there are no hydraulic or other leaks. Before you go onto the field, inspect the drums and clean them as necessary. There’s a good chance they were last used on freshly lain asphalt.

However, as with many turf management cultural practices, there is a right and wrong way to do things and your results will vary accordingly. Mistakes can damage or even ruin a field. The first and perhaps most important consideration to rolling a sports field is to know your soil type. Different soil types will react quite differently to rolling. In general, native soil fields are more prone to compaction damage from rolling than sand-based fields. As such, we can generally roll a sand field more often and more heavily than a native soil sports field. Over compaction of the surface should be monitored when rolling a sports field in my opinion so as to ensure that the surface does not become too hard. This is done by routine monitoring of hardness or “impact attenuation” using an appropriate tool as described in ASTM F-355, F1936, and/or F1702 for grass fields. Penn State’s Center for Sports Surface Research has a good YouTube video about GMax (Click here to watch).

In one small, anecdotal experiment of mine, a 3-ton tandem roller only increased the GMax on a Clegg Impact Soil Tester by about 5g’s immediately after rolling with one pass. Again, every field varies, and I’m sure that if you don’t dial-in your roller, procedures and soil types, you can easily make a field too hard and unsafe with improper rolling. It is not unheard of to core aerate or otherwise cultivate a field after rolling to counteract any unwanted compaction if the situation calls for it. From my experiences, the firming effect on the surface only goes an inch or two deep, depending on several factors.

How long the firming effect lasts on a field can vary quite a bit from what I’ve seen. I have found that actively growing, healthy turfgrass surfaces will “spring-back” to some extent within several days, especially sand based fields. Again, it varies quite a bit surface to surface. The smoothing effect will last much longer than firming, generally until the surface is cut-up again by play.

Rolling intensity.

The next thing to consider is the intensity of your rolling. There are several different levels, or intensities in rolling on sports fields. We can classify turf rolling intensities into light, moderate and heavy, but quantifying those parameters is challenging in a practical sense. Generally we look at pounds per square inch (PSI) that is being applied to the surface. This is a more difficult thing to calculate on a pliable grass field with a concave roller. Most of the research I can find on turfgrass rolling has been done on bentgrass golf greens with lighter weight rollers relative to typical sports field rollers with much smaller drum diameters. But we can tease out a few basic principles and apply them to sports fields.

Actual ground pressure, mass over area, may not be as much as you may think. We know the weight of the roller we are using fairly easily, even if we are adding water to a ballast for added weight. The hard thing to determine is the area that is impacted as the roller travels over the playing surface. I once did a small experiment to try to determine the area on a 5-ton tandem asphalt roller with 4-ft wide drums about 3-feet in diameter each. I left the roller to sit on a relatively unimportant part of the field for about one hour and then measured the “footprint” left behind for each of the two roller drums. The indents matched the curved rollers, so it was a bit of a guess. But I found the footprint to be about 12-iches wide by the 48-inch width of the drum. For two drums that would be 1,152 square inches. 10,000 lbs. (5 tons) over 1,152 sq. in. comes to about 8.7 pounds per square inch (PSI). That was far less than I expected and I considered this a moderate-heavy roll. I’m sure my numbers were erroneous to some extent because of the curved, or concave, shape of the drum. Someone smarter than me could probably come up with a way to better calculate the down-pressure on the surface when rolling turf that could be used to help rate roller intensities.

When you just want to compare the relative rolling intensities of two different rollers, you might use a simple calculation called the Roll Factor (RF).

Again, this doesn’t really tell us anything about the rolling intensity relative to the effect on the turf, but is a way to compare different rollers to each other. This same formula can also be used to compare the rolling to other maintenance equipment using the same equation with tires substituting drums. Find the weight of your mower reels and you can use RF to compare a typical mowing to whatever roller you are using.

From experience and common sense, rolling thinned turf or completely worn-away turf tends to compact the soil more than when rolling thick, healthy turf.

Rolling frequency.

The intensity of the rolling should also match the frequency you expect to roll the field. In other words, you may consider a higher intensity roll on the field for a one-time corrective measures after a football game in the rain or snow, and the long-term compaction damage may be minimal. On the other hand, rolling lightly in intensity but often (higher in frequency) may also damage by over-compaction.

With all these considerations, it may be best to approach rolling on an incremental-increase program where you start with a lighter intensity roller at lower frequencies and build up the program as needed.

Types of rollers.

For sports fields, we generally use one of two types of rollers but all must have smooth drums. You never want to use a toothed or “Sheep’s foot” type of roller for obvious reasons. Pull-behind single rollers can be purchased from many turfgrass equipment distributors. These are not the smaller walker drums you can rent for home-sod projects, but a little larger drums that are pulled by a utility cart or tractor. Pull-behinds cost less because you aren’t buying a drive-train or steering. I have seen several good ones that have been fabricated by field managers. As with most rollers, they have a way to add water to a tank to adjust the weight to the needs of the task at hand. Some have the edges of the drum being beveled to reduce the lip created when rolling a swath.

One consideration with a pull-behind roller compared to a tandem asphalt-type roller is that a pull-behind can’t be simply reversed when designing your rolling pattern. So unless you have some kind of hydraulic lift for a pull-behind roller, you really can only pattern ovals on the field with large sweeping turns and this will create several areas that will have to get double rolled or more as you will have to overlap them as you go.

This is why I prefer the right sized asphalt tandem roller for most applications to sports fields, where appropriate. Asphalt-type tandem rollers need to be carefully steered as you approach and roll the field. Make sure all surfaces on the approach to the field can handle the roller you are using. These tandem rollers usually are rear-roller steered or an equal pivot-type steering arrangement that can power-slide over grass on strong turns (similar to a 4-wheel drive) and tear or otherwise damage the grass. Most tandem asphalt type rollers were designed to run well in reverse. On a sports field, this allows you to avoid potentially turf-damaging turns by using a back-and-forth pattern; simply making a small, gentle turn at the end of your swaths to align the next swath, which goes in reverse. It is usually a good idea to overlap your swaths at least a few inches to avoid small gaps, leaving a visible lip. At the end, roll out any lips created at the ends of swaths with a few perpendicular “clean-up” swaths.

Roller drums.

Like all aspects of sports field rolling, this is an area ripe for good research. The width of the drum is one thing, but I imagine the diameter would more greatly affect the ground pressure and any potential traveling shear-zone more than the width of the drum. I have also seen fabricated, pull-behind roller drums that have beveled edges to minimize any lipping or ‘mohawking’, especially on turns. Typically, a scraper-bar of some type is installed on the drums that will scrape off any debris that may stick to the drums, keeping them smooth. On riding tandem rollers, one or both drums are the drive-wheels. So ease into your operational ground speed when rolling or you can literally “peel-out” and damage grass, especially when leaf wetness is present.

Turf Tips 101: Sports Field Rolling in Practice

Patterning your roll.

How you pattern the rolling on your field is important. Tandem-drum riding rollers are usually patterned in a back-and-forth fashion with small turns at the end of each swath to line up the next swath in reverse. Pull-behind type rollers are usually done in a “traveling oval” like a Zamboni does on a hockey rink between periods. Be aware that this type of patterning will require you to overlap and double-roll certain areas at both ends of the swaths. Unless you manage this, you can really overdo areas at the ends of swaths with your roller in this pattern. Don’t bite off too much in spacing your traveling oval rows as this will increase overlapping at the ends of swaths. Doing too tight of spacing between your traveling oval rows can increase chances of turn damage and tearing.

Try to pattern swaths with the longest runs possible, lengthwise on a rectangular field for example. This will minimize turn damage and overlapping with a pull-behind roller. Ideally, you have a service or warning track of some kind the can be used for end-of-swath turns, or at least enough room so that any overlaps are far beyond the field of play.

Not that it is your top priority, but be aware that rolling may leave strong striping in the turfgrass like mowing does and may affect ball roll in certain sports. Keep your swaths straight, you’ll get a more consistent result and it looks better.

Overlapping swaths. You want to overlap swaths a few inches at least to avoid creating an unrolled lip between swaths. Where a double-roll is indicated, you can either do the second roll perpendicular to the first, or you can do a ½-swath overlap which will equate to a double-roll.

Ground speed when rolling is very important. Rolling a field with too fast a ground speed, especially on a bumpy, beat-up field may “bounce” the roller a little causing more turfgrass physical damage and getting poor results. Rolling, and the depression made under the curved drum creates an advancing shear-zone underground as the turf surface deforms. This affect increases with higher ground speeds, according to this primer on asphalt rollers. (I am assuming this affect is also seen on grass surfaces). Regardless, rolling by its nature is not a good thing for the grass plant itself, in most cases, so it should be done carefully and unrushed. Every time you roll the field, you have to find the right ground speed. Rolling is done as a corrective and conditioning measure for the playing surface soil, not the grass plant. Consider rolling an added stress, especially when the grass is already under moderate or severe stress, like a bluegrass field in the middle of a long, hot summer for example.

Roll a clean surface with little to no organic or other debris when possible. Debris can stick to roller and get skinned off in small mats all over field. Rake-out or otherwise remove any loose soil, divots and dead grass shoots. When possible, roll a recently mowed surface, as you may not get a quality cut for a day or two after rolling. Roll a field that is not too moist or dry, at field capacity is what I look for.

Rolling does not correct significant surface depressions and grades issues, just minor surface issues. You can’t roll-out deeper ruts, lips or other abrupt changes in grade. Rather, rolling a sports field is an in-season maintenance item done to firm and smooth the surface.

Other uses for rolling:

Roll lightly after seeding and sprigging. A very light rolling is used to gently press the seed or sprigs into the soil to keep them shaded and moist, aiming for better and quicker establishment.

In some cases, the turf may have heaved or lifted from the winter’s ground frost cycles. A light rolling may be indicated to press the turf back to the soil and smooth the surface once thawed.

Newly installed sod is often rolled for the same reasons. This is usually a relatively heavier rolling, especially on the thick-cut sod sometimes used on sports fields for in-season repairs. I like to roll new sod lengthwise, in the same direct the rolls go. I believe this is better on the sod seams. (See discussion of advancing shear zone above). If a second roll is indicated, I may go perpendicular to the direction of the sod rolls, carefully.

Resources of the Month:

University of Tennessee Extension bulletin on rolling.

Cornell University bulletin on rolling sports fields.

End Quote

“There is not a sprig of grass that shoots uninteresting to me” Thomas Jefferson, in a December 23, 1790 letter to his daughter imploring her to write more often.