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Nitrogen fertilization and silvery-thread moss

By |  October 15, 2015 0 Comments

Figuring out N’s contributions to STM success
By Zane Raudenbush, Ph.D., and Steve Keeley, Ph.D.

If you’re a superintendent managing cool-season grass putting greens, you’ve most likely dealt with silvery-thread moss (STM). Its biological and ecological traits are well-adapted to life in a putting green, and that creates challenges when attempting to develop control strategies.

Typically, a STM infestation begins as small, 1- to 2-inch diameter colonies that can rapidly increase in size, especially in cool weather. Researchers are interested in determining factors that contribute to STM’s success in order to determine best management practices. Nitrogen fertilization has been shown to dramatically impact STM growth. In particular, research has proven that spraying urea enhances STM competitiveness compared to applying insoluble nitrogen.

Unfortunately, superintendents prefer not to apply insoluble nutrients, specifically N, because some insoluble granular products emit a foul odor after application, granular fertilizers may stick to the rollers on reel mowers and create a marcelling effect, and mowing may remove granular fertilizers, reducing their effectiveness. By contrast, superintendents can add soluble N to a spray tank and apply in conjunction with pesticide and plant growth regulators.

Urea and ammonium sulfate are two soluble N sources that superintendents commonly use. Ultimately, both N sources are converted from NH4+ to NO3– in the soil via nitrification. However, agronomists often recommend ammonium sulfate as the primary N source for high pH soils. Ammonium sulfate may be the ideal nitrogen source when attempting to reduce root zone pH, but it has a higher potential than urea to burn turfgrass leaves because of its higher partial salt index. A high partial salt index can create an osmotic gradient that draws water directly from the cells of turfgrass leaves, causing discoloration. With STM, this effect on osmotic potential is of interest because bryophytes (moss) are especially susceptible to osmotic shock.

Because of its higher partial salt index, we initially thought that substituting ammonium sulfate for urea would be detrimental to STM, so we set out to test our hypothesis.


We first did a greenhouse study to determine the effects of N source on STM growth in a controlled environment. We established silvery-thread moss collected from a local putting green in PVC containers (4 inches in diameter by 10 inches deep) filled with sand conforming to USGA guidelines for a putting green root zone. We applied ammonium sulfate or urea weekly at 0.1 lbs. N/1,000 square feet for seven weeks. As always in this type of research, some pots were left untreated for comparison. We applied fertilizers with a single-nozzle (TeeJet XR8004EVS) sprayer, with a spray volume of 1 gallon/1,000 square feet. We determined the percent of moss cover weekly using digital analysis. After trial completion, we harvested the moss shoot material, dried it and recorded the dry weights.


We conducted field studies the next year from May to October at Rocky Ford Turfgrass Research Center and Colbert Hills Golf Club in Manhattan, Kan. The Rocky Ford putting green had a sand-based root zone and contained Declaration creeping bentgrass (Agrostis stolonifera L.), which we mowed six days a week with a flex-cutting unit set to a bench cutting height of 0.115 inch. The Colbert Hills green was of California-style construction and contained Penn G-2 creeping bentgrass. We mowed it six days a week also, but with a triplex mower set to a bench cutting height of 0.115 inch. We irrigated both sites every two to three days at 100 percent ET replacement, estimated from on-site weather stations.

We applied treatments at the same rates and intervals as in the greenhouse experiments but for a longer time period, specifically, from May 15 to Oct. 15. Initial STM cover at Rocky Ford ranged from 5- to 15-percent cover with an average cover of 9 percent, and the STM cover at Colbert Hills ranged from 5 percent to 21 percent, with an average cover of 13 percent. We determined silvery-thread moss cover of the 3-foot-by-3 foot plots by using a rating grid containing 961 intersections on 1-inch centers (Figure 1). We registered a count if moss was present under an intersection.


To our surprise, while both ammonium sulfate and urea generally increased STM cover, ammonium sulfate caused the greatest increases by far (Figure 2). Originally, we hypothesized ammonium sulfate’s higher partial salt index would injure STM. Perhaps the “spoonfeeding” N rates we used (chosen because they represent the approach many superintendents use) did not result in a sufficiently high spray-solution salt content to cause injury to STM. Higher rates of ammonium sulfate possibly could cause foliar burn to STM (as sometimes happens with turfgrass), but substantially higher rates of soluble N are not typically applied to greens.

Dry weight of STM shoot growth also was highly affected by N source. Silvery-thread moss treated with ammonium sulfate had twice as much shoot mass compared to moss fertilized with urea, and three times that of the untreated moss. The increased dry weight caused by ammonium sulfate was the result of longer shoot filaments, which ultimately reflects increased leaf production. Longer shoots enable STM to better compete for sunlight in the turfgrass canopy, while crowding out desirable turfgrass species. Additionally, longer shoots are more likely to be sheared off and spread around during mowing, which could increase the infestation in a green.


Because ammonium sulfate had such a dramatic effect on STM growth in the greenhouse, we applied the treatments on actual putting greens to see if we could duplicate the results. While N source effects took longer to observe compared with the greenhouse studies, the effects were similar in the end. At Colbert Hills, ammonium sulfate increased STM cover on the last two (out of five) rating dates compared with the untreated, and on the next-to-last rating date compared to urea (Figure 3). By the study’s end, ammonium sulfate increased STM cover by nearly 200 percent. Urea increased STM cover by 55 percent, but was not different from the untreated at any rating date.

We observed a similar trend at Rocky Ford. Specifically, ammonium sulfate appeared to increase STM cover more than urea and the untreated, but this effect was not statistically significant (Figure 4). However, it was obvious that STM was benefitting from the addition of soluble N, regardless of the source (Figure 5). Silvery-thread moss in plots receiving N appeared to be more robust and grew higher in the bentgrass canopy. Additionally, the distinctive silvery sheen of STM was more pronounced when fertilized with soluble N, making it easier to spot from a distance.

Because we observed the greatest increases in STM cover in September and October (Figures 3 and 4), superintendents should consider applying a selective herbicide, such as carfentrazone-ethyl, in the fall to reduce the competiveness of STM as temperatures decrease. This is especially important considering that STM is capable of relatively vigorous growth at temperatures as low as 41 degrees F, when the growth rate of turfgrass is low.


This research supports previous findings that spraying soluble nitrogen throughout the growing season is likely to increase the competitiveness of STM. Additionally, based on our research, superintendents struggling with STM should limit or avoid use of ammonium sulfate as an N source. Although granular fertilizers have drawbacks for putting green fertilization, previous research demonstrated that granular N sources did not increase STM cover in putting greens. Therefore, superintendents should consider including them in their N fertilization program, especially in the spring and fall when STM is highly competitive.

Additionally, many fertigation programs are designed to inject a small amount of soluble N with every irrigation cycle. While this practice was not directly studied in our research, it seems plausible that fertigating with soluble N will increase the competitiveness of STM.

Superintendents who struggle with STM and choose to spray soluble N should implement an STM control program. Such a program may include selective herbicide use along with cultural practices such as reduced irrigation frequency, cultivation and topdressing.

Zane Raudenbush, Ph.D., recently completed his Ph.D. at Kansas State University, and Steve Keeley, Ph.D., is a turfgrass scientist at Kansas State University. Zane can be reached at for more information.


Borst, S.M., J.S. McElroy, and G.K. Breeden. 2010. Silvery-thread moss control in creeping bentgrass putting greens with mancozeb plus copper hydroxide and carfentrazone applied in conjunction with cultural practices. Hort. Tech. 20(3):574-578.
Glime, J.M. 2007. Bryophyte Ecology. Volume 1. Physiology Ecology. Online. Ebook sponsored by Michigan Technical University and the International Association of Bryologists.
Kennelly, M. M., T.C. Todd, D.M. Settle, and J.D. Fry. 2010. Moss control on creeping bentgrass greens with standard and alternative approaches. Hort Sci. 45(4): 654-659.
Lyons, E.M., K.S. Jordan, I.T. James, D.M. Hudner, and D. McGowan. 2012. Irrigation frequency influences establishment of silvery thread moss (Bryum argenteum Hedw.) and rooting of creeping bentgrass (Agrostis stolonifera L.) on simulated golf greens. Acta Agri. Scand. Section B-Soil & Plant Sci. 62(1):79-85.
Raudenbush, Z. and S.J. Keeley. 2014. Combined effects of topdressing, cultivation, and carfentrazone-ethyl on silvery-thread moss infestation. Abstracts, 2014 CSSA International Annual Meetings. CSSA, Madison, WI.
Raudenbush, Z., S.J. Keeley, and L.R. Stark. 2015. A Review: establishment, dispersal, and management of silvery-thread moss (Bryum argenteum Hedw.) in putting greens. Crop Forage & Turfgrass Mgmt. doi:10.2134/cftm2014.0094.
Smith, L. 1999. Biological and environmental characteristics of three cosmopolitan mosses dominant in continental Antarctica. J. Vegetation Sci. 10:231-242.
Thompson, C., M. Kennelly, J. Fry. 2011. Effect of nitrogen source on silvery-thread moss on a creeping bentgrass putting green. Online. Appl. Turf. Sci. doi:10.1094/ATS-2011-1018-02-RS.

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