Predicting emergence patterns for timing control programs

Seedheads reduce the aesthetics and quality of golf course turfgrass. TifGrand bermudagrass (Cynodon dactylon (L.) Pers. x C. transvaalensis Burtt Davy) and Sea Isle 1 seashore paspalum (Paspalum vaginatum Sw.) are popular selections for golf course fairways, tees and roughs. These cultivars produce seedheads throughout the growing season under various mowing heights and maintenance regimens. Diamond zoysiagrass (Zoysia matrella (L.) Merr.) is a fine-textured cultivar that may be used for greens, tees and fairways. This cultivar produces seedheads during the spring and fall but has limited-to-no seedhead production during summer months. Suppressing seedhead emergence of these grasses enhances aesthetics, functionality and ball-roll distances for long-term successful culture.

Controlling seedheads with plant growth regulators (PGRs) helps superintendents maintain turfgrass uniformity. The use of PGRs also may enhance tillering and root growth, which often is compromised during seedhead production. Growth suppression with PGRs of grasses prior to seedhead development delays inflorescence more effectively than applications after emergence. Most PGRs don’t provide acceptable control of seedheads after emergence at rates with limited injury potential to golf course turfgrass.

Field experiments were conducted at the University of Georgia Griffin campus from July 2012 to December 2015 to determine the influence of photoperiod and temperatures on seedhead production of TifGrand bermudagrass, Sea Isle 1 seashore paspalum and Diamond zoysiagrass. Photoperiod is defined as the number of hours of daylight per day. The experiments were initiated to help determine the environmental factors that trigger initial seedhead emergence of these grasses and the association with seasonal seedhead production. In addition, the grasses were mowed at various regimens to further evaluate the influence of mowing operations on seedhead production.

Figure 1 Plots of Sea Isle 1 seashore paspalum used for seedhead production experiments that were mowed at various heights from 2012 to 2015.

The bermudagrass and seashore paspalum fields included mowing at 0.25 inch two days per week, 0.5 inch two days per week, 1.5 inches weekly, and no mowing (Figure 1). Zoysiagrass was mowed at 0.25 inch two days per week, 0.5 inch two days per week and no mowing. Mowing treatments were made during active turfgrass growth to 5-foot by 10-foot plots arranged in a randomized complete block with four replications.

TifGrand bermudagrass

Seedhead emergence. Photoperiod triggered the initial seedhead emergence at all mowing heights more than temperature (growing degree-day) estimates (Figure 2). In 2013, the photoperiods measured 13.7 hours on the day initial seedhead emergence was observed for bermudagrass in mowed plots and at 13.3 hours for non-mowed plots. Seedheads emerged later in spring of 2014 and 2015 compared with 2013, which may have resulted from bermudagrass maturity after establishment. The initial seedhead emergence was consistent across mowing regimens in subsequent years. In 2014 and 2015, the non-mowed bermudagrass had initial seedhead emergence at 14.0 hours and 13.8 hours, respectively. All plots that received mowing programs had seedheads emerge at 14.2- to 14.3-hour photoperiods. These timings were approximately three to four weeks later than the plots that were not mowed.

Figure 2 TifGrand bermudagrass seedhead coverage in plots maintained at the various mowing regimens evaluated.

Seedhead production. Although photoperiod was more influential on the initial emergence of bermudagrass seedheads, calendar date (week number) and temperature (growing degree-day) had greater associations with seedhead production from January until August. Bermudagrass maintained at the lowest height evaluated (0.25 inch two days per week) averaged about 50 percent less seedhead coverage than the other mowing programs at peak emergence in August. Results suggest that TifGrand bermudagrass seedhead production can partially be suppressed by regular mowing at 0.25 inch compared to higher mowing heights.

Bermudagrass seedheads declined by late August from peak levels noted at all mowing heights. This decline in seedhead cover was consistent with the decline in turf quality, reductions in photoperiod and cooler temperatures in fall. Bermudagrass shoot growth is highly dependent on photoperiod, light intensity and temperatures. Researchers have noted that bermudagrass exhibits reductions in biomass, shoot height and tiller density as temperature and photoperiod are decreased. These physiological responses to changes in temperature and photoperiod in fall likely are associated with the decline of seedhead production of TifGrand bermudagrass.

Sea Isle 1 seashore paspalum

Seedhead emergence. Seashore paspalum quality and seedhead emergence had similar seasonal trends at all mowing heights. Seedheads emerged in spring, reached peak cover in late summer of all years, and declined in fall.

Photoperiod was more influential on the initial seedhead emergence of seashore paspalum at all mowing heights than temperature (growing degree-day) estimates. In 2013, the photoperiods measured 13.5 hours at all mowing heights on the day seedheads were initially observed. Seedheads emerged later in spring of 2014 and 2015 compared with 2013. In 2014, the initial seedhead emergence of non-mowed plots was observed at a 13.8-hour photoperiod, while mowed plots had initial emergence at 14.2 hours. The initial observation of seedhead emergence for seashore paspalum was at a 13.8-hour photoperiod in 2015 at all mowing regimens. The growing degree-day estimates over years were erratic when initial seedhead emergence was noted on seashore paspalum. These results suggest photoperiod has a greater value for predicting seashore paspalum seedhead emergence than temperatures.

Seedhead production. Calendar date (week number) and temperature (growing degree-day) were more influential than photoperiod on seedhead production from January until August for seashore paspalum maintained at 0.5 inch two days per week, 1.5 inches once per week, or not mowed during the experiment. Seashore paspalum maintained at the lowest mowing height averaged about 30 percent to 40 percent less seedhead coverage from other mowing programs at peak emergence. Turf that was mowed at 0.5 inch two days per week and 1.5 inches once per week had comparable seedhead cover to the non-mowed plots in 2013 and 2014. Peak seedhead coverage ranged about 20 percent to 30 percent less in 2015 for these mowing regimens than previous years. Seashore paspalum seedheads declined in September from peak levels consistent with trends in turf quality.

Diamond zoysiagrass

Seedhead emergence. Zoysiagrass had two flushes of seedhead emergence in spring and fall, while turf quality increased from spring to summer and declined in fall (Figure 3). Peak emergence was noted in May and October for the spring and fall seedhead flushes, respectively, and declined from May to August and October to December, respectively. Diamond zoysiagrass generally did not go completely dormant in the winter at any mowing height.

Figure 3 Diamond zoysiagrass seedhead emergence in spring.

The initial seedhead emergence was consistent over years from calendar dates, growing degree-day and photoperiod across all mowing regimens. The initial seedhead emergence occurred within approximately eight calendar days for the spring and fall timings over years. These dates had photoperiods ranging from 12.7 to 12.9 hour in both fall and spring. The growing degree-day on dates of initial seedhead emergence in spring and fall ranged 167 to 196 and 1949 to 2230, respectively, in all years across mowing regimens.

Seedhead production. Seedhead cover from January to peak emergence in spring had a stronger association with temperature compared with photoperiod. Reduction in photoperiod from July to October had a greater influence than calendar date and temperature (growing degree-day) for zoysiagrass seedhead cover during this period.

PGR application timing

Research was conducted over three years to evaluate the efficacy of PGRs applied at various timings based on cumulative temperature measurements (growing degree-days). Two PGR treatments, including ethephon + trinexapac-ethyl (TE) or mefluidide, were applied at five growing degree-day timings: 139, 278, 556, 833, or 1111 C (250, 500, 1000, 1500, 2000 F) with a base temperature of 10 degrees C (50 F) beginning Jan. 1.

The application rates and products used for treatments included ethephon (Proxy 2L, Bayer Environmental Science) plus trinexapac-ethyl (Primo Maxx 1ME, Syngenta Crop Protection,) at 5 oz./1,000 sq. ft. + 5.5 oz./acre, respectively, or mefluidide (Embark 2S, PBI Gordon) at 0.5 lb. a.i./acre. Seedhead cover was rated weekly for four weeks after treatments on a percent scale. Turfgrass injury was visually rated weekly on a percent scale (Figure 4).

Figure 4 Seashore paspalum plots used for PGR application timing research for seedhead control in 2014.

The ethephon + trinexapac-ethyl (TE) treatment provided approximately three times greater seedhead control of bermudagrass than mefluidide at all application timings. However, seedhead control never exceeded 69 percent on bermudagrass from one to four weeks after treatment. The ethephon + TE treatments also were more effective than mefluidide for controlling seashore paspalum seedheads on every date. Control ranged from 75 percent to 82 percent from two to four weeks after treatment with ethephon + TE. However, this treatment caused excessive injury (30 percent) on several dates and is not recommended for superintendents (Figure 5).

Figure 5 Seashore paspalum injury from ethephon plus trinexapac-ethyl treatments.

Mefluidide provided poor control on every date (greater than 70 percent) but caused acceptable injury. The application timing in late spring (approximately 550 to 1100 growing degree-day) generally was more effective for controlling seedheads with the PGRs at rates tested compared to later timings in summer. The application timing also was more important on bermudagrass than seashore paspalum, which may have resulted from differential tolerance levels of these grasses to the PGR treatments.

Injury was excessive (greater than 20 percent) from ethephon + trinexapac-ethyl treatments on bermudagrass and seashore paspalum, and would be unacceptable for golf course turf. Further research is needed to refine PGR rates and regimens for TifGrand bermudagrass seedhead suppression. It’s also recommended that further research focus on application timings based on photoperiod rather than temperatures for bermudagrass and seashore paspalum.

Diamond zoysiagrass was relatively less responsive to PGR applications than the other species. Single applications of ethephon + trinexapac-ethyl or mefluidide provided less than 40 percent control of zoysiagrass seedheads. These results suggest that multiple applications of these PGRs and modifications of application timings in late winter are necessary for successful seedhead suppression. Screening PGRs and herbicides when photoperiods reach approximately 12.5 to 12.6 hours or growing degree-day ranges of approximately 150 to 160 (base 10 C at Jan. 1) will enhance the potential for treatments to be effective.

Managing warm-season grass seedhead suppression

The initial flush of seedheads on TifGrand bermudagrass and Sea Isle 1 seashore paspalum is triggered by photoperiod in late spring. However, seedhead production is associated with temperatures until peak emergence rather than with photoperiod. The genetic variability among cultivars could contribute to differences in seedhead production under various photoperiods. Further research is needed to compare the influence of photoperiod on the initial seedhead emergence of bermudagrass and seashore paspalum cultivars under various maintenance regimens.

Turf managers can use photoperiod or growing degree-day models for predicting seedhead emergence of Diamond zoysiagrass in spring. However, photoperiod was more indicative of initial seedhead emergence in fall compared with growing degree-day. The initial emergence of Diamond zoysiagrass seedheads occurred at approximately the same photoperiod (12.7 to 12.9 hour) in fall and spring. Numerous zoysiagrass cultivars used for golf course turf may exhibit differing seedhead production patterns, and further research is needed to determine the influence of photoperiod and growing degree-day on the seasonal seedhead production of other zoysiagrass cultivars.

A significant limitation to warm-season turfgrass management is the selection of PGRs available for seedhead suppression. Trinexapac-ethyl (TE) is the most popular PGR used for enhancing turfgrass color, quality and stress tolerances. However, trinexapac-ethyl doesn’t provide acceptable seedhead suppression of bermudagrass, seashore paspalum and other warm-season grasses. Tifway bermudagrass seedheads have been reported suppressed 67 percent to 70 percent by sequential trinexapac-ethyl applications at 0.18 followed by 0.09 lbs. a.i. per acre. It also is noted that lower rates of TE are less effective for controlling seedheads. Mefluidide suppresses warm-season turfgrass seedheads with minimal injury potential. However, mefluidide is no longer being manufactured, and its supply for the turf industry will be limited in the future.

Mowing is an important cultural practice for seedhead suppression. All three grasses mowed at 0.25 inch two days per week had about half the amount of seedhead coverage of plots mowed at higher heights and the non-mowed turf. Mowing frequency also has been shown to influence turfgrass seedhead production during active growth, and this warrants further research with these cultivars. Further research will be needed to refine recommendations for PGR use and cultural management of warm-season turfgrass seedheads.

Patrick McCullough, Ph.D., is a turfgrass scientist at the University of Georgia-Griffin Campus. You may reach him at pmccull@uga.edu for more information.

Photos by: Patrick Mccullough

Acknowledgements

The author would like to thank Seth Williams, Jialin Yu and Bob Perry at the University of Georgia Griffin campus for technical assistance, Jerry Davis at the University of Georgia for assistance with statistical analysis and results presentation, and the United States Golf Association for financial support for this research.

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