By Dan Hargey and Ben Wherley
From the transition zone southward, bermudagrass has become the Cadillac of warm-season turfgrass athletic fields. When provided ideal growing conditions and proper maintenance, bermudagrass offers wear tolerance and recuperative ability that is difficult to match. However, once the colder temperatures and shorter days of fall set in, growth slows and bermudagrass enters a dormancy period, often lasting 4 to 6 months, depending on the location. It is during these late fall and winter months that the cumulative effects of wear can become problematic. Whether for football, soccer, or baseball, turf managers are continually confronted with the challenge of maintaining safe, yet aesthetically desirable playing surfaces for much if not all of the year. Winter overseeding has long been one means of achieving this goal for many transition zone and southern turf managers.
Athletic field managers commonly elect to overseed with perennial or turf-type annual ryegrass due to their rapid germination, desirable color, and wear tolerance, which ultimately provides good physical protection to the underlying dormant bermudagrass. However, while these grasses can provide excellent quality and function during fall and winter, achieving good spring transition back to bermudagrass can sometimes be a challenge. This is often the case during years when persistently cool, wet, and/or cloudy spring conditions prevail. For perennial ryegrass, selective products such as the sulfonylurea class of herbicides have become an effective tool for assisting the turf manager in producing a timelier, consistent, and reliable transition back to bermudagrass year after year. It should be noted that herbicides are generally less of an option for aiding transition of annual ryegrasses due to their lack of sensitivity to sulfonylurea herbicides.
Given concerns with budget cuts and municipal irrigation water restrictions imposed in many areas of the southern US in recent years, some turf managers are finding it increasingly difficult to justify the practice of overseeding, while giving increased consideration to use of colorants during the dormancy period. Although a municipality may allow an irrigation variance during establishment, irrigating every 7 or even 14 days through the fall and winter might not be adequate for maintaining desired levels of density and growth due to excessive play or limited rainfall in many regions. Yet when not overseeded, months of wear and traffic on dormant bermudagrass can become particularly detrimental. The primary objectives of this study were to 1) evaluate and compare winter performance of overseeded perennial and turf-type annual ryegrass blends under limited irrigation and traffic, and 2) evaluate the benefit of fall colorant-treatment to bermudagrass and compare effects to overseeded or dormant turf.
Methodology
This study was conducted at the Texas A&M University Turfgrass Research Field Laboratory, College Station, from October 2013 through May 2014 on a stand of Tifway bermudagrass grown on a fine sandy loam soil. Studies were conducted under two different irrigation levels which were intended to simulate various stages of municipal water restrictions. One study received a single (0.8”) weekly irrigation and the other received no supplemental irrigation (rainfall only, with 8.6” of rain received over the November- May period). Overseeding was performed in early October with either perennial ryegrass (Futura blend, Pickseed, USA) or turf type annual ryegrass (Panterra SOS 400, Barenbrug USA) at a rate of 10 lbs. per 1000 sq. ft. Non-overseeded plots were either left untreated, or treated with a single early November application of turf colorant (Greenlawnger, Becker Underwood) just before dormancy (~50% green cover remaining in plots) at a rate of 7.5 gallons product per acre. Colorant was diluted to a ratio of 1 gallon Greenlawnger per 8 gallons water before application and applied using 8004VS flat fan nozzles. During the study, simulated traffic was applied to half of each plot using a Cady traffic unit at a rate of 4 passes per week, intended to simulate two football games between the hash marks from 40 yard line to 40 yard line. Overseeded plots were mowed to 1.25” weekly during the study with clippings returned. Monthly during the study, ratings were taken in plots. Data collected included turf quality and cover, percent wear, surface hardness, soil moisture, and spring bermudagrass transition differences in plots.
Comparative performance of perennial vs. turf-type annual ryegrass
Of particular interest in this study was the comparative performance and quality of perennial ryegrass and turf-type annual ryegrass, especially under the context of limited irrigation and traffic stress. Our data indicate that the two offered similar levels of quality from December through April in both 1 day/week irrigation as well as un-irrigated studies. Under the 1 day/week irrigation, winter visual quality averaged 6.9 and 7.3 out of 9 (perennial and annual, respectively) in the absence of traffic. Under un-irrigated (rainfall only) conditions, perennial and annual ryegrass winter quality was also very similar (5.4 and 5.3 out of 9, respectively), just above minimally acceptable quality. It should be noted that while overall visual quality ratings were similar between the species, perennial ryegrass did exhibit somewhat darker green color compared to the annual ryegrass. However, slightly superior upright growth, density, and uniformity of the annual ryegrass offset this, contributing to its similarly high quality during the study. The two species also exhibited similar levels of traffic tolerance under the four passes per week traffic level. When averaged across the season in irrigated plots, traffic caused a similar (<10%) reduction in quality in both species. How the two would compare under more intensive traffic is also of interest, but could not be gained from this current study. Finally, in May ratings of percent bermudagrass transition, similar levels of bermudagrass were observed (~60%) in both annual and perennial ryegrass overseeded plots.
Performance of colorant-treated bermudagrass
We were also interested in evaluating the benefit and longevity of a single early November colorant application to bermudagrass. While colorant-treated plots held acceptable quality well into mid-January, ~8 to 10 weeks after treatment, mean seasonal quality of colorant-treated plots averaged 4.5 out of 9 in both irrigated and un-irrigated conditions, which was significantly better quality and appearance than dormant turf, but inferior quality to overseeded plots in both irrigation levels. Because the colorant effects had noticeably faded by April, no differences in bermudagrass greenup between untreated and treated plots were observed in April or May ratings. Finally, colorant application mitigated the effects of traffic only slightly, relative to injury sustained by untreated plots (8 and 13% quality decrease, respectively). Because the effects of the single colorant application were relatively shortly lived, we could speculate that a repeat application of colorant midway through the winter might have allowed for aesthetically acceptable turf during the entire bermudagrass dormancy period as well as facilitated more rapid spring green-up.
Overseeding effects on surface hardness
Surface hardness is an important indicator of surface performance as it relates to player safety. In this study, we were particularly interested in better understanding effects of irrigation level, traffic, and overseeding on surface hardness (Gmax). Perhaps not surprisingly, surface hardness levels measured using a Clegg Impact Tester were generally higher under un-irrigated conditions in this study. Across both irrigation levels, there were significant differences in surface hardness due to both treatment (overseeded vs. non-overseeded) and traffic. Surface hardness was noticeably reduced by reducing traffic as well as overseeding. On average, the hardest surfaces were detected under non-overseeded /trafficked treatments (77 Gmax), followed by overseeded/trafficked (67 Gmax), non-overseeded/non-trafficked (57 Gmax) treatments, and overseeded/non trafficked (52 Gmax). It should be noted that while differences were detected among treatments, to our knowledge, none of these levels would be considered high enough to be deemed a safety concern.
While winter overseeding with perennial ryegrass will continue to be commonly practiced within the sports turf industry, information on feasibility of alternative options for accommodating winter play in the context of water shortages will allow turf managers to make appropriate decisions in managing and protecting their turf during dormancy. Factors such as budget, irrigation/rainfall availability, and event schedules need to be taken into consideration. For situations where high traffic is received during bermudagrass dormancy, perennial ryegrass has been the standard, but with the development in improved turf-type annual ryegrasses, a more affordable option may be available for more limited budgets with little sacrifice in aesthetics. Turf managers with limited events may wish to consider fall colorant applications just prior to dormancy as an option to save time, maintenance, and resources. However, single applications appear to be relatively short lived, and repeat applications midway through the dormancy period may be necessary. All situations are different, and thus, various options should be considered by the turf manager.
Dan Hargey is a graduate research assistant in Turfgrass Science in the Department of Soil and Crop Sciences at Texas A&M University. Ben Wherley is assistant professor of Turfgrass Science and Ecology in the Department of Soil and Crop Sciences at Texas A&M University.