Enhancing late-fall nitrogen on greens

Complement late-fall nitrogen with a plant growth regulator to improve winter hardiness and nutrient sufficiency of greens.

Chase M. Rogan and Max Schlossberg, Ph.D.

Read this story in GCM's digital edition

When turfgrass consumes nitrogen under optimal growing conditions, the cells in leaf and shoot tissue become hydrated and nonstructural carbohydrates are depleted (3). This highly typical response to nitrogen was the probable impetus for early discouragement of substantial fall nitrogen applications to turfgrass (1). However, several more-recent field studies in cool-season turfgrass have shown that fall nitrogen fertilization has not increased the turf’s susceptibility to biotic and abiotic stresses in winter (11,12,13).

October 2013 research: Enhancing nitrogen photo 1

The study site was Penn State’s Valentine Turfgrass Research Center
in University Park, Pa., where Penn G-2 creeping bentgrass
and annual bluegrass was maintained as a putting green.
Photo by Brad Bartlett

In the early 1990s in Wisconsin, the timings of fall nitrogen fertilizer applications were compared within a standard annual program of 4 pounds nitrogen/1,000 square feet (19.5 grams/ square meter) (5). Season-end nitrogen delivery (1.5 pounds urea-nitrogen/1,000 square feet [7.3 grams/square meter]) was applied to a Penncross creeping bentgrass (Agrostis stolonifera L.) putting green in mid-September, mid-October or mid-November. Although root-growth was unaffected, response to the mid-October and mid-November applications included enhanced winter color and spring green-up, and delayed (to late May) spring nitrogen fertilizer requirements. Mid-October nitrogen application fostered significantly greater spring growth than the mid-September timing, but less growth than the mid-November timing (5). Excessive growth in early spring in response to fall nitrogen application(s) remains a significant concern for superintendents. 

Adding a plant growth regulator 
Trinexapac-ethyl is a foliarly absorbed plant growth regulator commonly used in turfgrass management. Greenhouse results indicate trinexapac-ethyl improves canopy color and reduces vertical shoot growth of both cool- and warm-season turfgrass species for weeks (14). Likewise, trinexapac-ethyl has been shown to positively influence freezing tolerance of cool-season turfgrasses (8,10). 

Repeated trinexapac-ethyl applications to hybrid bermudagrass [Cynodon dactylon (L.) Pers. × C. transvaalensis Burtt-Davy] have also decreased nutrient concentration in leaves while increasing nutrient concentrations in rhizomes by 8% to 36%, resulting in a net increase in bermudagrass nutrient retention (7). Similar trinexapac-ethyl-induced reduction in nitrogen requirements has been reported for creeping bentgrass putting greens (4).

The objective of this study was to determine how timing of fall applications of trinexapac-ethyl and/or nitrogen influence subsequent growth, nitrogen status and spring canopy density of creeping bentgrass/annual bluegrass (Poa annua L.) putting greens.

Materials and methods

Studies were initiated in September 2009 and 2010 at the Pennsylvania State University Valentine Turfgrass Research Center (University Park, Pa.). The site was a mature, push-up putting green with a 3-inch (7.6-centimeter) sand cap overlying a Hagerstown silt loam.

October 2013 research: Enhancing nitrogen photo 2

Grass clippings were collected in fall 2009 and 2010, and spring 2010 and 2011 to measure clipping yield, an indicator of turfgrass growth and vigor.
Photo by Derek Pruyne

Year 1 
Maintenance and fertilization. The Penn G-2 creeping bentgrass and annual bluegrass putting green was irrigated to prevent wilt. Throughout the 2009 season, the green was mowed six days/ week at a height of 0.126 inch (3.2 millimeters) and clippings were removed.

In early September, granular urea, potassium sulfate and magnesium sulfate fertilizers were applied to deliver nitrogen, potassium, sulfur and magnesium at rates of 0.57, 2.56, 1.35 and 0.23 pounds/1,000 square feet (2.78, 12.49, 6.59 or 1.12 grams/square meter), respectively. Leaf clippings were collected Sept. 28 and analyzed. Following clipping collection, soil was randomly sampled to a depth of 4 inches (10 centimeters) and analyzed.

Design and treatments. The experimental design was a randomized complete block with 72 plots (3 feet × 6 feet [0.9 meter × 1.8 meters]). On Sept. 30, 2009, three treatments were applied to each of the six blocks. A CO2-pressurized, single-nozzle (Tee-Jet TP11008E) wand-sprayer was used to apply soluble nitrogen (1:1 urea– N:NH4NO3–N at 0.61 pound/1,000 square feet (2.97 grams/square meter) in a tank-combination with Primo Maxx (trinexapac-ethyl, Syngenta) plant growth regulator at 0, 0.1, or 0.2 fluid ounce/1,000 square feet (0, 0.31, or 0.63 liters/ hectare). Plots initially treated with Primo Maxx at the 0.1 fluid ounce/1,000 square feet rate were re-treated with an equal application of Primo Maxx eight days later. Thus, all plots treated with Primo Maxx received a total of 0.2 fluid ounce of Primo Maxx/1,000 square feet in either a full- or split-application regimen.

October 2013 Research: Enhancing nitrogen figure 1

This procedure was repeated on randomly selected plots remaining within each block on Oct. 10, Oct. 21 and Nov. 1, 2009. These four initiation dates, each 10±1 days apart and centered on Oct. 15 (the 30-year average date of first frost in University Park, Pa.), are the four experimental timing levels.

Fall clipping yield. Measures of turfgrass growth and vigor were first collected from treated plots on Oct. 9, and repeated every 10±1 days through Nov. 12, 2009. Thus, fall clipping yield data represent a single collection from plots treated Nov. 1 and cumulative collections from plots treated earlier. Clippings were immediately oven-dried, cooled in a desiccator and weighed. Mowing was discontinued in late November 2009, and plots were left uncovered over the winter. 

Spring canopy reflectance and clipping yield. Multiple canopy reflectance measures were collected from all plots on March 25 and April 1, 2010, using a Crop Circle ACS-210 (Holland Scientific). The reflectance values were used to calculate the green normalized differential vegetation index (GNDVI).

The GNDVI nondestructively estimates the quantity of chlorophyll between the sensor and soil and thus quantifies turfgrass canopy density in place (9). A strong correlation of GNDVI and chlorophyll content has been reported in bermudagrass and creeping bentgrass systems (2). GNDVI values, describing spring turfgrass canopy density of each plot, were averaged over both collection dates for statistical analysis. 

On April 6, 2010, clippings were collected from all plots at a 0.126-inch mowing height. Subsamples of repeated (fall) or single (spring) clipping yields were analyzed, for leaf tissue nitrogen concentration.

 October 2013 Research: Enhancing nitrogen figure 2

Figure 2. Mean spring canopy density (GNDVI) by fall treatment and year of study
(pooled fall application dates). Treatment means with overlapping error bars
are not significantly different.

Year 2 
Maintenance and fertilization. The putting green was irrigated to prevent wilt, fertilized at typical maintenance rates and mowed six days per week, with clippings continually removed, in 2010. Soil and clipping samples were collected and analyzed for baseline nutrient concentration and soil fertility. On Sept. 8, 2010, the putting green was fertilized with granular potassium sulfate to deliver 1.04 pounds potassium (5.07 grams/square meter) and 0.43 pound sulfur/1,000 square feet (2.09 grams/square meter).

Design and treatments. A second study was initiated in fall 2010 using a 48-plot design within a separate section of the same putting green. Treatments were applied as described for the first year on Oct. 1, and the plots remaining in each of four blocks were treated on Oct. 10, 20 and 30, 2010. These four initiation dates represent the four experimental timing levels.

Fall clipping yield. As in 2009, fall clippings were collected every 10 days and processed and analyzed as described. Mowing was discontinued Nov. 11, 2010, and plots were left uncovered over the winter.

Spring canopy reflectance and clipping yield. On March 30 and April 7, 2011, canopy reflectance was measured for all plots as described. On April 14, clippings were collected, and clipping yield samples were processed and analyzed as described.

Results and discussion

Conditions in the fall were typical of a cool, humid region in both years of the study. Mean September, October and November monthly temperatures were 62 F (16.6 C), 50 F (10 C) and 45 F (7.2 C) in 2009 and 65 F (18.3 C), 53 F (11.6 C) and 41 F (5 C) in 2010. Rainfall over this period totaled 7.7 inches (19.55 centimeters) in 2009 and 9.5 inches (24.13 centimeters) in 2010. First frost was later in 2010 (Nov. 2) than in 2009 (Oct. 14), but November 2010 was colder and had more rainfall. Likewise, December mean air temperature was lower in 2010, and January, February and March were colder in 2011 than in 2010.

Routine soil analysis showed a neutral soil pH and suggested minor recommendations (which were followed) to optimize nutrient availability. Analyses of putting green leaf tissue collected immediately before the experiments were started indicated sufficient nutritional status (data not shown).

October 2013 research: Enhancing nitrogen photo 3

Canopy reflectance was measured to calculate the green normalized
differential vegetation index (GNDVI). A radiometer was held at waist height
and carried the length of the center of each plot. The device measured canopy reflectance
of 590- and 880-nanometer wavelength radiation.
Photo by Max Schlossberg

Fall tissue nitrogen 
Fall tissue-nitrogen concentrations were significantly affected by application timing and the year. In 2009, plots treated before the first hard frost (Oct. 14) maintained 4.0%-4.2% tissue-nitrogen levels (Figure 1). Plots treated on the last two application dates of 2009 showed significantly lower tissue-nitrogen concentrations (3.2%-3.3%). The drastic drop in tissue nitrogen for the latter two application dates is attributed to lower temperatures and a reduced rate of physiological assimilation by plant leaf and root tissue following the first hard frost.

The same effect was not observed in fall 2010, when the first hard frost occurred after all treatments were applied (Nov. 2). Thus, the plants remained more physiologically active throughout the fall and showed a slight linear decline in nitrogen assimilation over the application timings (Figure 1). Regardless, leaf tissue-nitrogen concentration was greater in fall 2009 than in 2010; this is likely the residual effect of yearly variation in summer fertilization practices.

October 2013 Research: Enhancing nitrogen figure 3

Spring canopy density and growth 
Green normalized differential vegetative index (GNDVI) is a measurement of chlorophyll content and thus quantifies turfgrass canopy density. Figure 2 shows the mean spring canopy density by treatment and year (Figure 2). Over all fall 2009 application timings, Primo Maxx treatments yielded greater spring GNDVI relative to nitrogen alone (Figure 2). In spring 2011, differences in canopy density by treatment were less pronounced, perhaps due to lower winter temperatures and a comparatively delayed spring green-up.

Early spring vigor is analogous to shoot growth and is a sure sign that mowing is about to resume.

Although early spring vigor is sometimes perceived as beneficial, and welcomed by the maintenance staff, it may increase turfgrass susceptibility to crown hydration injury (1). Continued regulation of growth, particularly when putting greens break dormancy with sufficient canopy density and nitrogen status, may reduce the likelihood of deacclimation injury caused by rapid onsets of freezing temperatures in early spring. 

Early spring clipping yield data were pooled over both seasons to best illustrate the interaction of treatment and timing (Figure 3). Application timing had little effect on spring growth response to the fall nitrogen-alone treatment. However, applications of Primo Maxx made later in fall (after the first hard frost) reduced putting green spring growth by as much as 20%. In contrast, early spring growth of plots receiving early-October Primo Maxx treatments (before the first frost) exceeded that observed in plots treated at the same time with nitrogen alone (Figure 3). Spring growth in plots receiving early Primo Maxx treatments was as much as 13% greater than spring growth in plots treated with only nitrogen. This difference is likely a manifestation of post-regulation growth enhancement (6), also called the rebound effect. It is important superintendents recognize the sensitive nature of late-fall Primo Maxx application timing when considering such putting green treatments.

 October 2013 Research: Enhancing nitrogen figure 4

Figure 4. Mean spring leaf tissue nitrogen by fall treatment and year of study
(pooled fall application dates). Treatment means with overlapping error bars
are not significantly different.

Spring tissue nitrogen 
Nitrogen levels in spring putting green leaf tissue were unaffected by application date, which came as a surprise considering the effect on leaf nitrogen levels in fall (Figure 1). Spring leaf-tissue- nitrogen level was significantly enhanced by fall Primo Maxx treatment, regardless of timing or application regimen (split or full) (Figure 4). Thus, the late-fall Primo Maxx treatment resulted in greater nitrogen status compared to plots treated with nitrogen alone, yet also reduced early spring growth (Figure 3). This observation is in agreement with reports of nitrogen preservation within turfgrasses recently treated by Primo Maxx (4,7).


Applying nitrogen and Primo Maxx in late autumn can enhance the spring density and tissue nitrogen concentration of greens-height turfgrass systems. Application of Primo Maxx in late fall suppresses growth in early spring, even when tissue nitrogen is high. In fact, Primo Maxx significantly preserves tissue nitrogen, which can be beneficial for spring growth after Primo Maxx is no longer active and weather patterns stabilize. Conversely, Primo Maxx can be used to stimulate growth in early spring when applied up to two weeks before the first hard frost, but this is not recommended because early growth may diminish hardiness.

Spring density was significantly improved by the combination of Primo Maxx and fall nitrogen in the first year of the study. Results show tissue nitrogen levels >4.0% mass at the onset of dormancy did not have detrimental effects in early spring. When striving for such a high concentration late in the season (September or later), nitrogen should be applied with a plant growth regulator such as Primo Maxx. This will restrict top growth and improve density going into winter. In theory, a plant that is photosynthetically active but not growing will synthesize carbohydrates, contributing to energy reserves and winter hardiness. Where winter injury threatens the health of a turfgrass system, maintaining a dense turfgrass canopy may ultimately reduce turf loss and aid in accelerated spring recovery.


The authors thank The Pennsylvania Turfgrass Council for their financial support of this research, and Brad Bartlett, Sarah Fishel and Derek Pruyne for their technical assistance.

Literature cited

  1. Beard, J.B. 1973. Turfgrass: Science and culture. Prentice-Hall, Englewood Cliffs, N.J.
  2. Bell, G.E., B.M. Howell, G.V. Johnson et al. 2004. Optical sensing of turfgrass chlorophyll content and tissue nitrogen. HortScience 39:1130-1132.
  3. Joern, A., and S. Mole. 2005. The plant stress hypothesis and variable responses by blue grama grass (Bouteloua gracilis) to water, mineral nutrition, and insect herbivory. Journal of Chemical Ecology 31:2069-2090.
  4. Kreuser, W.C., and D.J. Soldat. 2012. Frequent trinexapac-ethyl applications reduce nitrogen requirements of creeping bentgrass golf putting greens. Crop Science 52:1348-1357.
  5. Kussow, W.R. 1992. Late season nitrogen fertilization. Pages 135-156. In: Proceedings 62nd Annual Michigan Turfgrass Conference, Lansing, Mich. Jan. 20-22, 1992. Michigan State University, East Lansing, Mich.
  6. Lickfeldt, D.W., D.S. Gardner, B.E. Branham and T.B. Voigt. 2001. Implications of repeated trinexapac-ethyl applications on Kentucky bluegrass. Agronomy Journal 93:1164-1168.
  7. McCullough, P.E., H. Liu, L.B. McCarty et al. 2006. Bermudagrass putting green growth, color, and nutrient partitioning influenced by nitrogen and trinexapac-ethyl. Crop Science 46:1515-1525.
  8. Rossi, F.S., and E.J. Buelow. 1997. Exploring the use of plant growth regulators to reduce winter injury on annual bluegrass (Poa annua L.). USGA Green Section Record 35(6):12-15.
  9. 9. Schmidt, J.P., A.E. Dellinger and D.B. Beegle. 2009. Nitrogen recommendations for corn: An on-the-go sensor compared with current recommendation methods. Agronomy Journal 101:916-924.
  10. Steinke, K., and J.C. Stier. 2004. Influence of trinexapac-ethyl on cold tolerance and nonstructural carbohydrates of shaded supina bluegrass. Acta Horticulturae 661:207-215.
  11. Walker, K.S., C.A. Bigelow, D. Smith et al. 2007. Aboveground responses of cool-season lawn species to nitrogen rates and application timings. Crop Science 47:1225-1236.
  12. Webster, D.E., and J.S. Ebdon. 2005. Effects of nitrogen and potassium fertilization on perennial ryegrass cold tolerance during deacclimation in late winter and early spring. HortScience 40:842-849.
  13. Wehner, D.J., J.E. Haley and D.L. Martin. 1988. Late fall fertilization of Kentucky bluegrass. Agronomy Journal 80:466-471.
  14. Wherley, B., and T.R. Sinclair. 2009. Growth and evapotransporation response of two turfgrass species to nitrogen and trinexapac-ethyl. HortScience 44:2053-2057.

Chase Rogan was a graduate student conducting this research in fulfillment of his M.S. degree requirements at Penn State University. He is currently GCSAA’s field staff representative for the Mid-Atlantic region. Maxim J. Schlossberg is an associate professor of turfgrass nutrition and soil fertility in the Center for Turfgrass Science at Penn State University, University Park, Pa.