Excessive organic matter accumulation (>3 to 4% by weight) compromises soil aeration, drainage, turfgrass health, and playability. Aerification and vertical mowing are traditionally used to manage shallow organic matter. However neither of these practices impacts the entire playing surface. Fraise mowing is an aggressive cultural practice that impacts 100% of the playing surface and removes all plant and soil material up to a 2-inch depth. Although fraise mowing was originally devised to mechanically control annual bluegrass in cool-season sports fields, the practice has gained acceptance for bermudagrass thatch management. Despite its growing popularity, fraise-mowing research is limited, and its effect on soil physical properties has yet to be thoroughly explored. The objective of this research was to evaluate the effects of fraise mowing on the soil physical properties of two soils beneath established ‘Tifway’ hybrid bermudagrass.
Trials were conducted in 2016, 2017, and 2018 on a sandy loam soil (loam) at NCSU’s Lake Wheeler Turf Field Laboratory in Raleigh, NC and a sand capped soccer field (sand) in Cary, NC. Four fraise mow depths 0.25 inch (shallow); 0.5 inch (intermediate); 1.0 inch (deepest); and an untreated control (control) were applied in mid-June every year with a Koro Field TopMaker (FTM) 1200. Turf quality and percent cover were visually assessed weekly. Field hardness and stability were measured every 14 days. Water infiltration (ksat) and retention of the sand were also measured. Studies concluded in September every year.
Impact on turfgrass quality and cover
Ideally, field conditions would return to an acceptable level as quickly as possible following fraise mowing. From a long-term perspective, fields should have better quality due to fraise mowing. In our studies, all fraise mowed treatments on both soils had unacceptable turfgrass quality and cover for various durations. Turfgrass cover decreased and recovery time increased with fraise mowing depth on both soils. On the loam, shallow, intermediate, and deep treatments had acceptable turf quality by 2, 4, and 5 weeks, respectively. In the non-treated turfgrass areas, turf quality and cover was unacceptable from 3 weeks after others were treated through mid-September as a result of excessive scalping. In 2017, recovery after fraise mowing was longer. Shallow treatments had acceptable turf quality at 4 weeks after treatment and intermediate and deep treatments were recovered at 6 weeks after treatment. In the non-treated areas turf quality and cover was acceptable on all dates except 4, 5, and 10 weeks after treatment (because of excessive scalping).
On the sand in 2016, shallow, intermediate, and deep treatments had acceptable turf quality by 2, 3, and 6 weeks after treatment, respectively. Non-treated areas maintained acceptable turf quality on all rating dates except 7 to 10 weeks after treatment. In 2017 on the sand, similar recovery times were observed after fraise mowing. Shallow and intermediate achieved acceptable turf quality at 3 weeks after treatment and deepest by 4 weeks after treatment.
Thatch removal with fraise mowing
At both locations and over both years, fraise mowing effectively decreased thatch levels of hybrid bermudagrass. Thatch content decreased with fraise mowing depth, and control treatments had twice the thatch content as the deepest treatment (8.3 vs. 4.1 g, respectively).
Fraise mowing influenced field stability
On the loam in 2016, only the shallow treatment had higher shear values (a.k.a., greater field stability) than the control and the difference between all treatments was small (<7 N m-2). However, in 2017, deep and intermediate treatments had the highest shear values. Control and shallow treatments had lower tensile strength than the deepest and intermediate treatments. More thatch in the shallow and control treatments likely lowered their stability. Removing the thatch to engage more of the high cohesiveness of the underlying loam likely increased the stability of the deepest and intermediate treatments in 2017. On the sand in 2016 stability decreased with depth of fraise mowing treatment. The non-treated areas had the highest shear values (105 N m-2), shallow and intermediate had similar values and the deepest treatments had the lowest shear strength (76 N m-2). In 2017, the non-treated areas and shallow treatments had similar values, which were higher than the intermediate and deepest treatments (~104 N m-2). The removal of reinforcing roots, stolons, and rhizomes and the poor cohesive strength of the sand rootzone likely resulted in the lower stability of the deepest and intermediate treatments.
Relationship with water infiltration (ksat)
It was not known if removal of the top organic layer would have a significant influence on water movement through a soil profile. Since both compacted soil and organic layers may impede water flow, we thought it may positively impact water movement. But our testing found that during both years intermediate and deepest treatments had slower water infiltration (ksat) rates than non-treated areas immediately after fraise mowing. The non-treated areas in 2017 had 200 to 400% higher ksat values than fraise-mowed treatments on the day of fraise mowing. These reductions dissipated by 4 weeks after treatment in 2016 and 6 weeks after treatment in 2017. Similar reductions in ksat have been recorded following aerification due to hardpan formation. Like aerification, surface crusting from repeated passes of the fraise mower may have decreased ksat of the deepest treatment.
Over both soils the deep and intermediate treatments were harder than the non-treated areas and the shallow treatments. However, hardness values for the deep and intermediate treatments were within the appropriate range of 65 to 100 gravities for professional soccer fields, while those for the non-treated areas and the shallow treatments were not. This difference in firmness was attributed to less thatch and organic matter in the intermediate and deepest treatments. Similar results have been seen with previous fraise mowing studies.
During both years, fraise mowing changed the soil physical properties of both soils. In both soil types, thatch content decreased (positive response) while turfgrass quality decreased and field hardness increased (negative responses) after fraise mowing. In the sand, infiltration rate and soil stability decreased (negative responses). Because of the cohesiveness of the loam, stability increased (positive response) after fraise mowing. The removal of shallow organic matter likely increased field hardness (negative response) of both soils. Despite the differences in soil physical properties and a general perception that more was negatively influenced than positively influenced, all fraise mowed surfaces had acceptable playability and significantly less thatch. Thatch reduction may supersede any temporary changes in soil physical properties. Additional soil physical properties were researched that were not presented in this article. These evaluations will be repeated in 2019 with the addition of a hollow tine aerification treatment structure.
We would like to thank Jimmy Simpson, CSFM, and the WakeMed Soccer Park Grounds Crew; Casey Carrick, CSFM, and the UNC Athletic Dept. Grounds Crew; Chad Price, CSFM, CFB, Carolina Green Corp., as well as Sam Green of Aqua-Aid North America for their contributions. This research was funded by the North Carolina Center for Turfgrass Environmental Research and Education.
Raymond K. McCauley is a graduate research assistant with the Department of Crop and Soil Sciences, North Carolina State University; Grady L. Miller, PhD, is professor of turfgrass science; and Garland D. Pinnix is a graduate research assistant in Raleigh. References available at www.sportsturfonline.com