Cultural Management of Selected Ultradwarf Bermudagrass Cultivars

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Richard H. White, Professor, Trent C. Hale, Former Graduate Research Assistant, David R. Chalmers, Extension Specialist, Mark H. Hall, Research Associate, James C. Thomas, Senior Research Associate, and Wallace G. Menn, Turfgrass Specialist; Soil and Crop Sciences Department, Texas A&M University, College Station 77843-2474

Abstract

Ultradwarf bermudagrass cultivars are widely used for putting greens due to their high shoot density, low mowing heights, and high-quality putting surface. However, successful management may require cultivar-specific annual N application rates and thatch control measures as compared to those used for the cultivar ‘Tifdwarf.’ This study evaluated the effects of N fertility, topdressing and vertical mowing on thatch development, shoot density and turf quality of five dwarf bermudagrass cultivars: Tifdwarf, ‘Champion,’ ‘Floradwarf,’ ‘Miniverde,’ and ‘Tifeagle.’ The study was conducted using a strip-split plot design and extended over a 63-month period. All treatments were replicated three times. Suggested optimum N fertilization rates for all five grasses in the study are 10 lbs of N per 1,000 ft²/year. This N rate should provide good-quality turf -- a high shoot density without excessive thatch accumulation. Floradwarf was most susceptible to high soil pH, followed by Champion. Careful soil pH management is needed when growing these cultivars. Frequent light vertical mowing was advantageous during the first year, but was detrimental to mature stands of ultradwarf cultivars when bermudagrass decline was active. Topdressing treatments used in this study were not sufficient to control thatch accumulation. When averaged across all treatments, thatch depth equal to or greater than 0.5 inch occurred by 42, 30, 42, 38, and 38 months after planting for Tifdwarf, Champion, Floradwarf, Miniverde, and Tifeagle, respectively.

Introduction

To provide the highest quality putting surfaces, many golf courses in the southern United States are growing one of the new dwarf bermudagrasses [Cynodon dactylon (L.) Pers. × C. transvaalensis (Burtt-Davy)]. These grasses, commonly called ultradwarfs, tolerate low mowing heights, have high shoot densities, high wear tolerance, and fast recovery rates (1,7). These characteristics result in high-quality surfaces and fast ball roll speeds which have led to their rapid acceptance.

Because these cultivars are relatively new, little data are available concerning proper cultural management for optimal long-term performance. Instead, managers have had to experiment on their own or try techniques that have worked for others. In many cases, these grasses have not performed well over time and entire greens have had to be replanted. Some problems have been linked to improper management for thatch control (1,3,7). Gray and White (2) report 5 to 11.4 times greater thatch accumulation for ultradwarf cultivars than ‘Tifdwarf’ bermudagrass. However, science-based recommendations for thatch control in ultradwarf greens are not available. Based on field observations of superintendents growing ultradwarf bermudagrass cultivars, O’Brien and Hartwiger (5) suggest a cultural program that includes the following: daily mowing, no vertical mowing, daily brushing or grooming twice per week, light topdressing on a 1-to-2-week interval, applying 6 to 8 lbs total fertilizer N/1,000 ft²/year, biannual core aerification using 0.5-inch-diameter tines, and monthly aerification using thin solid tines during the summer.

Our study was conducted in College Station, TX to determine the effects of management of N fertility, vertical mowing, and sand topdressing on thatch development, shoot density, and turf quality of four ultradwarf bermudagrasses and one dwarf bermudagrass.

Study Design

Tifdwarf, ‘Champion’, ‘Floradwarf’, ‘Miniverde’, and ‘Tifeagle’ were sprigged on 15 April, 1997 at 12 bu/1,000 ft² in 50-by-20-ft plots in concurrent studies on a putting green constructed according to United States Golf Association (USGA) recommendations (6). Physical properties of the sand-based root zone mixture are shown in Table 1. This root zone was selected as being representative of non-calcareous sand-based root zones commonly used in modern putting green construction throughout the United States. Champion, Floradwarf, Miniverde, and Tifeagle were chosen as being representative of recently released ultradwarf cultivars that, due to their high shoot density and stoloniferous growth habits, may require altered cultural management practices as compared to management practices that have worked well for Tifdwarf bermudagrass (4). For each cultivar, a strip-split plot experimental design was used to determine the effects of N, vertical mowing, and sand topdressing on thatch depth, shoot density, and turf quality. Main plots were annual cumulative N treatments of 6, 10, 14, and 18 lbs of N per 1,000 ft²/year which were representative of those commonly being used by turf managers at the start of this study. N treatments were applied bi-weekly throughout the year. During the first 27 months of the study the N source was a coated form of 18-4-18 (Lesco, Inc., Strongsville, OH). Beginning at 28 months after planting (MAP) and continuing through the remainder of the study, N applications alternated between ammonium sulfate and 18-4-18 as the source materials.

Sub-plots were vertical mowing treatments of (i) light, non-invasive, bi-weekly vertical mowing May through September (LFVM), and (ii) severe invasive vertical mowing once during spring transition and once immediately prior to overseeding in October (IHVM). Sub-sub-plots were topdressing treatments of (i) 0.02 inch of sand applied bi-weekly May through September followed by a 0.20-inch sand application at overseeding (FLTD), and (ii) 0.15 inch of sand in June and 0.20 inch in October at overseeding (IHTD). All treatments were replicated three times and occurred in all possible combinations. N, vertical mowing, and topdressing treatments were initiated in August 1997 after all grasses were fully established. During grow-in, the experimental area was topdressed and groomed as needed to smooth the surface.

Irrigation and mowing were uniformly applied. Plots were mowed daily at 0.125 inch during the summer and 0.156 inch during winter and early-spring. Insecticides were applied on a curative basis for fall armyworms [Spodoptera frugiperda (J. E. Smith)], sod webworms [Parapediasia teterrella (Zincken)], and southern mole crickets [Scapteriscus borellii (Giglio-Tos)]. Insect activity was similar among all treatments. Herbicides and fungicides were not applied. Severe bermudagrass decline [caused by Gaeumannomyces graminis var. graminis (Sacc.)] symptoms (Fig. 1) appeared in many plots by 24 MAP. To help control this disease, aggressive management of soil pH was begun at 28 MAP. Soil pH was maintained near 7.5 by using ammonium sulfate to supply half the total annual N and making an annual mid-summer application of 69 lbs gypsum and 3.4 lbs elemental S per 1,000 ft² followed by 2 to 3 inches of irrigation.

Fig. 1. Bermudagrass decline symptoms including characteristic thinning and chlorosis in Champion observed at 38 MAP.

All plots were overseeded with Poa trivialis (L.) at 12 lbs/1,000 ft² each fall. Verticutting and topdressing treatments were as described above for the respective treatments. At the same time each spring and fall, all plots were evaluated for bermudagrass quality, soil cores were collected, shoots were counted, and the uncompressed thatch depth was measured. Due to conditions beyond our control, turf quality ratings could not be obtained for 30 MAP. Quality ratings were made based on a scale of 1 to 9 (1 = brown, seemingly dead turf; 5 to 8 = acceptable quality; 9 = best quality). All data were subjected to an analysis of variance (ANOVA). Tukey’s Studentized Range Test was used for mean comparisons. Differences among treatments in the text are reported at P = 0.05. Because the grasses were planted in strips rather than being truly randomized, it was inappropriate to statistically compare measurements between grasses. Therefore, grass was omitted from the model as a main effect and the analysis of variance was run. Significant interactions were found between MAP and other variables. Therefore, the data were pre-sorted by MAP and grass, and then an ANOVA test was run on each individual MAP-grass combination.

Tifdwarf Bermudagrass

Thatch depth for Tifdwarf ranged from 0 inches at 14 MAP to 0.81 inch at 60 MAP (Table 2). Nitrogen application rates had no effect on thatch depth on any given date except for 19 MAP. At this date the two highest N application rates had higher amounts of thatch. There were increases in shoot density with increasing N at 14, 26, and 30 MAP (Table 3), but shoot density was similar for N treatments at 19, 38, 42, 53, and 63 MAP. Shoot density decreased in response to N application at 49 and 60 MAP due to increased competition from the overseeded grass and poor transitioning. Turf quality at 14 MAP increased with increasing N, but this was not the case at the other sampling dates (Table 4). At 49 MAP, turf quality decreased as N increased because shoot density also decreased with increasing N along with an increasing incidence of bermudagrass decline.

Thatch depths were similar for both vertical mowing treatments at all dates in Tifdwarf (Table 5). Shoot density was similar for both vertical mowing treatments except at 49 and 60 MAP when IHVM resulted in greater shoot densities compared with FLVM (Table 6). Generally, turf quality of the FLVM treatment was superior to the IHVM treatment near the beginning of the study (14, 19, and 26 MAP) (Table 7). Severe defoliation and mechanical disruption caused by the IHVM treatment resulted in less overseeding stand uniformity and lower turf quality compared to the FLVM treatment. As the study progressed, the trend reversed and by 49 and 53 MAP the IHVM treatment provided better turf quality than the FLVM treatment. This was related to greater severity of bermudagrass decline in the FLVM treatments resulting in lower shoot density. When bermudagrass decline was again brought under control, the FLVM treatment again provided superior quality at 63 MAP.

Except for 49 and 60 MAP when IHTD provided superior thatch control, thatch depth was similar for Tifdwarf under both topdressing regimes at all observation dates (Table 8). Tifdwarf shoot density and turf quality were similar for both topdressing treatments at all dates (Table 9 and Table 10).

Champion Bermudagrass

Thatch depth for Champion bermudagrass ranged from 0.00 inch at 14 MAP to 0.61 inch at 30 MAP (Table 2). The only significant effect of N on thatch accumulation occurred at 19 MAP, at which time increased N application rates resulted in increased thatch depth. The intense pressure of bermudagrass decline likely limited the production of plant tissue at all N levels and effectively masked N effects on thatch accumulation at subsequent dates. Initially (14 and 19 MAP), shoot density and turf quality increased with increasing N application rates (Table 3 and Table 4). However, increased N either had no influence or a detrimental influence on shoot density and turf quality after 19 MAP.

Across all N treatments, shoot density ranged from 19.1 shoots per inch² at 49 MAP to 212.3 shoots per inch² at 19 MAP (Table 3). Shoot densities in the spring-time (38, 49, and 60 MAP) were low due to poor transitioning. By the end of the summers (42, 53, and 63 MAP) shoot density was increased compared to the springtime measurements. Bermudagrass decline symptoms were evident in Champion from 26 to 30 MAP and became more pronounced between 38 and 42 MAP. Although shoot density increased from 49.8 to 56.0 shoots per inch² at 38 MAP to 99.9 to 126.9 shoots per inch² at 42 MAP, turf quality ratings were less than acceptable during this period (Table 4).

Vertical mowing regimes had similar effects on thatch depth for Champion (Table 5) bermudagrass at all sampling dates. The FLVM caused less shoot density than IHVM at 30 and 42 MAP (Table 6). However, shoot density was similar for vertical mowing treatments on the remaining eight observation dates. The low shoot densities measured for both treatments at 49 and 60 MAP were indicative of poor spring transition. At 42 and 53 MAP, FLVM caused a substantial reduction in turf quality (Table 7) that was associated with an increase in bermudagrass decline severity compared to IHVM. This is the cause for the associated decrease in shoot density at 42 MAP and non-significant decrease in shoot density at 53 MAP. Thus, frequent, light defoliation of Champion by vertical mowing caused an increase in bermudagrass decline symptoms, loss of stand, and decreased turf quality (Fig. 2).

Fig. 2. Overview of one replication of Champion bermudagrass illustrating treatment effects and bermudagrass decline symptoms 42 MAP. Center vertical line divides infrequent heavy (left) and frequent light (right) vertical mowing treatments. Vertical dashed lines divide frequent light topdressing (outside columns) and heavy infrequent topdressing (inside columns) treatments. Nitrogen treatments from front to back rows are 6, 10, 14, and 18 lbs of N per 1,000 ft² per year.

Topdressing regimes had a similar effect on thatch depth in Champion except at 42 MAP (Table 8). Frequent light topdressing (FLTD) at 42 MAP was less effective in controlling thatch accumulation than was infrequent heavy topdressing (IHTD). Topdressing regimes had similar effects on shoot density of Champion at all observation dates (Table 9). Except for 19 MAP, there were no differences in turf quality between topdressing treatments (Table 10).

Floradwarf Bermudagrass

In general, thatch depth in plots planted with Floradwarf increased as N increased for the first 30 MAP. After 30 MAP, thatch depth between N treatments was similar (Table 2). Shoot density increased as N increased at 14 and 26 MAP (Table 3), and reached a maximum value at 42 MAP. At 38 MAP, overall shoot density ranged from 59.3 to 82.4 shoots per inch² but increased to 143.2 to 160.8 shoots per inch² at 42 MAP. The dramatic increase in shoot density at 42 MAP for Floradwarf can be attributed to suppression of bermudagrass decline by soil pH management. Shoot density, however, was extremely low in spring (49 and 60 MAP) because of poor transitioning similar to that observed in other grasses. Density increased to 84.5 to 115.2 shoots per inch² in late summer (53 and 63 MAP).

At 14 and 19 MAP, turf quality was low but increased with increasing N (Table 4). Floradwarf developed bermudagrass decline symptoms earlier than the other cultivars and turf quality remained poor throughout the first 53 MAP. After instituting aggressive soil pH management at 26 MAP, an improvement in turf quality was observed and by 42 MAP the Floradwarf exhibited nearly acceptable turf quality at 10 to 18 lb N per 1,000 ft²/year. At 49 MAP, turf quality of Floradwarf decreased with increasing N and increasing incidence of bermudagrass decline. By the end of the study the trend reversed itself and quality was increasing with increasing N application rates.

Vertical mowing regimes had similar effects on thatch depth in Floradwarf (Table 5) throughout the study. Shoot density was similar among vertical mowing regimes except at 53 MAP when the FLVM treatment had a reduced shoot density. FLVM reduced turf quality at 42, 49.5, and 53 MAP. This response is associated with an increase in bermudagrass decline severity similar to that seen for Champion bermudagrass.

Topdressing regimes had similar effects on thatch depth in Floradwarf except for 49, 60, and 63 MAP (Table 8). At 49 and 60 MAP, FLTD had greater thatch depth compared to IHTD. However, at 63 MAP, IHTD had a significantly greater thatch depth. Topdressing regimes had similar effects on shoot density throughout the study (Table 9). Except for 19, 42, and 60 MAP, turf quality between topdressing treatments was similar. At 19 and 42 MAP, the FLTD treatment produced higher turf quality compared to IHTD, while the opposite was true at 60 MAP.

Miniverde Bermudagrass

Thatch accumulation in Miniverde plots was rapid and increased with increasing N at 19 and 30 MAP (Table 2). From 0.65 to 0.77 inch of thatch was measured at 38 and 42 MAP. Shoot density also increased with increasing N through 30 MAP (Table 3), when it reached a maximum of approximately 218.3 shoots per inch² for the treatment that received 18 lbs N per 1,000 ft²/year. Most Miniverde treatments had acceptable turf quality at 26, 38, and 42 MAP. Turf quality increased as N increased for 14, 19, and 38 MAP (Table 4). However, turf quality at 42 MAP decreased as N increased.

Vertical mowing regimes had similar effects on thatch depth in Miniverde (Table 5) at most dates. However, at 53 and 63 MAP less thatch depth occurred in FLVM than in IHVM treatments. Shoot density was also similar among vertical mowing regimes except at 14, 26, and 42 MAP (Table 6). Shoot density was greater for the FLVM treatment at 14 and 26 MAP. However, shoot density was less for the FLVM treatment at 42 MAP. At 14, 19, 26, and 63 MAP, the FLVM treatment produced better turf quality than did IHVM (Table 7). However, at 42, 53, and 60 MAP, poorer quality was observed for the FLVM treatment due to increased bermudagrass decline severity.

There were no differences among topdressing regimes for thatch depth (Table 8), or turf quality (Table 10) for Miniverde. Topdressing regimes produced similar shoot densities except at 38 MAP when the FLTD regime produced a higher density (Table 9).

Tifeagle Bermudagrass

Thatch accumulation for Tifeagle increased with increasing N at 19 and 63 MAP (Table 2). From 0.51 to 0.69 inch of thatch was observed at 42 and 49 MAP. Turf quality of Tifeagle bermudagrass increased with increasing N when measurements were taken 14,19, and 63 MAP (Table 4). However, from 26 to 60 MAP, turf quality was unaffected by increasing N. Shoot density also increased with increasing N at 14, 26, and 30 MAP, but decreased with increasing N at 49 and 60 MAP (Table 3). Shoot density was as low as 29.7 shoots per inch² for Tifeagle at 49 MAP due to poor transitioning but recovered to relatively high levels of 119.2 to 139.9 shoots per inch² at 53 MAP.

Except for 26 and 63 MAP when thatch depth was greater in the FLVM treatment, vertical mowing regimes had a similar effect on thatch depth in Tifeagle bermudagrass (Table 5). Vertical mowing had a significant impact on shoot density at 42 MAP when IHVM produced a greater shoot density than the FLVM treatment (Table 6). However, at 49 and 60 MAP, FLVM had a greater shoot density than the IHVM treatment. Although vertical mowing regimes did not affect shoot density at 14 and 19 MAP, the FLVM treatment had superior quality than IHVM on these dates. Turf quality observations at 14 and 26 MAP reflect treatment effects resulting from heavy vertical mowing in fall which produced a non-uniform overseeding appearance that negatively affected turf quality the following spring (Table 7). At 42 and 53 MAP, FLVM decreased turf quality, largely due to increased bermudagrass decline severity.

Topdressing regimes had a similar effect on thatch depth in Tifeagle except at 42 MAP, when FLTD had greater thatch depth than IHTD (Table 8). Tifeagle shoot density was similar between topdressing regimes at all observation dates (Table 9). Except for 49 and 53 MAP, topdressing regimes had similar effects on turf quality (Table 10).

Rate of Thatch Accumulation

It is commonly accepted that a healthy bermudagrass putting green will contain 0.25 to 0.5 inch of thatch. However, more than 0.5 inch of thatch is considered excessive (8). Based on the measurements of thatch accumulation from this study (Table 2), 38 months were required for all N treatments of Tifdwarf, Floradwarf, Miniverde, and Tifeagle to reach a thatch depth of 0.4 inch. However, Champion bermudagrass reached a minimum depth of 0.4 inch by 26 MAP. This more rapid accumulation of thatch indicates the need for implementing thatch control measures sooner after planting for this cultivar. When averaged across all treatments, thatch depth equal to or greater than 0.5 inch occurred by 42, 30, 42, 38, and 38 MAP for Tifdwarf, Champion, Floradwarf, Miniverde, and Tifeagle, respectively.

Suggested Cultural Programs for Successful Management of the Ultradwarfs

Thatch depth, shoot density, and turf quality over all dates and treatments were regressed against N application rate for each cultivar (Fig. 3). These data indicate that a N fertilization rate of approximately 10 lbs of N per 1,000 ft²/year is adequate for all five bermudagrass cultivars studied. This N rate will provide good shoot density, a reasonable amount of thatch, and a marked improvement in turf quality over lower application rates. However, it must be remembered that this is based on a study which utilized a scheduled application rate spread out uniformly throughout the year on turf that was overseeded during the cool season and was subjected to very light traffic. Therefore, some changes to this application rate may be needed depending on site specific conditions including soil type, climatic conditions, irrigation program, traffic, and the expectations of the course officials and the golfing community. Also, the incidence and severity of Bermudagrass decline increased with increasing N probably because of more rapid thatch accumulation and subsequent mechanical damage caused by vertical mowing and scalping from mowing. Therefore, the initial use of high levels of N fertilization on the ultradwarf cultivars, while providing immediate results in terms of high shoot density, results in poorer long-term performance.

Fig. 3. Turf quality, shoot density, and thatch depth averaged over all vertical mowing and top dressing treatments as a function of N application rate.

FLVM appeared to be advantageous in young (< 26 MAP), healthy stands of all five cultivars used in this study. In such stands, FLVM produced higher quality turf, similar amounts of thatch, and similar shoot densities. However, FLVM should be used cautiously as greens mature, since it may promote the severity of bermudagrass decline symptoms. During periods of intense disease activity (30 to 53 MAP) FLVM frequently resulted in reduced shoot density and turf quality in mature stands of all five cultivars. It is thought that the increased amount of cut surfaces on leaves, stems, and stolons resulting from FLVM allowed easier entrance of disease organisms into the turfgrass plants.

Topdressing will likely be required more frequently than the schedule used in this study to successfully control thatch. FLVM or grooming should be used in conjunction with light frequent topdressing to reduce thatch accumulation, particularly on young stands. Although not included in this study, core aerification should also be used routinely to aid in rooting and to control thatch. Because of the many variables involved, cultural programs should be evaluated routinely and refined as growth and environmental conditions change.

Acknowledgments

This research was supported in part by the United States Golf Association (USGA), the Texas Turfgrass Association, and the Houston Golf Association. The able technical assistance of Mr. William Robinson is also gratefully acknowledged.

Literature Cited

1. Beard, J. B., and Sifers, S. I. 1996. New cultivars for southern putting greens. Golf Course Manage. 64:58-62

2. Gray, J. L., and White, R. H. 1999. Maintaining the new dwarf greens-type bermudagrasses. Golf Course Manage. 67:52-55.

3. Guertal, E. A., Hollingsworth, B. S., and Walker, R. H. 2001. Managing ultradwarf bermudagrass cultivars. Golf Course Manage. 69:49-53.

4. Guertal, B., and White, R. H. 1998. Dwarf bermudagrasses demand unique care. Golf Course Manage. 66:58-60.

5. O’Brien, P., and Hartwiger, C. 2001. Tips on designing an Ultradwarf bermudagrass putting green management program for the SE region. USGA Regional News Update, June 4, 2001.

6. United States Golf Association. 1993. USGA recommendations for a method of putting green construction. USGA Green Section Record, March/April 1993.

7. Westmoreland, J. W. 1998. Another option for southern greens. Golf Course Manage. 66:64-67.

8. White, R. H., and Dickens, R. 1984. Thatch accumulation in bermudagrass as influenced by cultural practices. Agron. J. 76:19-22.