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© 2005 Plant Management Network.
Accepted for publication 1 September 2005. Published 6 October 2005.

Chemical Methods of Moss Control on Golf Course Putting Greens

Brian P. Boesch and Nathaniel A. Mitkowski, Department of Plant Sciences, The University of Rhode Island, Kingston 02881

Corresponding author: Nathaniel A. Mitkowski. mitkowski@uri.edu

Boesch, B. P., and Mitkowski, N. A. 2005. Chemical methods of moss control on golf course putting greens. Online. Applied Turfgrass Science doi:10.1094/ATS-2005-1006-01-RV.

Abstract

Many golf courses throughout the United States are perennially affected by moss encroachment. Unfortunately, moss control products must often be applied at regular intervals throughout the season and can be very phytotoxic. In addition, application timing impacts efficacy. Copper in the form of copper hydroxide + mancozeb (Junction) can control moss when applied on a biweekly basis at 0.2 lb/1000 ft² with minimal phytotoxicity. Season-long application may be required on heavily infested greens and fall applications are often most effective. Higher rates can severely damage turf. Silver nitrate is extremely effective at eliminating moss infestations in 1 to 2 applications at 0.275 lb/1000 ft² without phytotoxicity. However, silver is currently not labeled as a pesticide. Carfentazone (Quicksilver) has been labeled for use against moss and appears to be very effective in reducing up to 90% of moss when applied twice at a 14-day interval of 6.7 oz/acre [* see erratum]. Terracyte applied at 14-day intervals at no more than 8 lb/1000 ft² can also successfully combat a moss infestation but season long application may be necessary and higher rates can cause dramatic turf injury. Other products that have met with limited success include iron, lime, chlorothalonil and soap-based products, which often fail upon repeated trials.

Introduction

Moss is often difficult to manage on golf course putting greens. In severe infestations, it can completely infest an entire putting surface within 1 to 2 months (Figs. 1 to 4). It has been anecdotally reported that moss problems have increased on golf courses in the past few decades. Several researchers and turf professionals have suggested that the increased prevalence of moss is due to an increased demand for "fast" putting surfaces (those that allow for the farthest ball-rolling distances), leading to overstressed turf that is vulnerable to pest invasion and pathogen infection. It is also possible that moss may be appearing more frequently due to the elimination of mercury and other heavy metal-bearing pesticides. Mercury has been proven highly toxic to mosses, and its absence in the environment is likely a contributing factor to many moss epidemics. Whatever the reason for moss' resurgence, the turf industry is still anxiously awaiting solutions. Diligent cultural management techniques are essential to prevent moss problems but it is unlikely that turf managers will significantly raise heights of cut or change current management practices designed to increase ball rolling distance unless the current golfing culture changes dramatically.

Fig. 1. Moss can quickly overcome golf course putting greens when plants are established or maintained under stress. Very low heights of cut and high amounts of irrigation exacerbate moss problems, as seen in this experimental SR7200 velvet bentgrass green.		Fig. 2. The protonema stage of the moss plant (right side) appears early in the lifecycle and is often mistaken for black algae.

Fig. 3. Moss often appears as large clumps spreading radially from an initial point of infestation. Moss can also spread from the breakage and transport of vegetative parts, particularly when subject to constant mowing.		Fig. 4. Moss rapidly turns gray then brown within days, ultimately dying following the application of silver nitrate.

In the past 10 years, university researchers have aggressively started to combat moss problems but there is no readily apparent convenient solution. Many chemicals do elicit some toxicity in moss but the persistency of this plant makes long-term control difficult. Only a few studies have been able to demonstrate excellent long-term chemical control of moss but these solutions have drawbacks. This paper summarizes results of several moss control trials on turfgrass performed over the past few decades, highlighting moss biology, the various agents used for chemical control of mosses, and the drawbacks to each chemical. Of the limited research undertaken, almost all of it has been done on a small scale and very little of the research has been peer-reviewed. Despite this drawback, we feel that the practical importance of the work currently available warrants its inclusion in this review.

Moss Species, Biology, and Life Cycle

Few moss species can survive broad differences in climatic conditions, but collectively, moss plants inhabit many different environments on each continent. Oceans may be the only ecological habitat where mosses can’t survive, although some species are more salt-tolerant than others (16). Bryum, however, is one genus commonly seen on every continent. The USDA plants database lists 66 different species of Bryum (20). Bryum argenteum Hedw. is a true pioneer species; it grows at nearly all altitudes and commonly inhabits rocks or concrete, frequently growing on dry, compacted soils and sandy areas (8). It is commonly thought that mosses on golf courses will generally inhabit cool, wet, or shady locations. However, moss is most successful in the absence of competitive pressure, in addition to the other environmental parameters that favor it. Why then does moss grow on putting greens? Putting greens provide moisture, adequate (and even excessive) nutrient levels, and are frequently noncompetitive, especially if the resident turfgrass stand is stressed by intense management practices. Additionally, because of the low cutting heights of putting green turf, mosses have access to considerably more light than in many other locations. The biology and life-cycle of these plants are truly unique and they can provide some understanding as to why moss grows on putting greens.

Mosses are bryophytes, a group of non-vascular plants within the plant kingdom that also includes liverworts and hornworts. These plants do not have vascular systems; they are ectohydric organisms that absorb water and minerals throughout their entire body. Nevertheless, the physiology of a moss allows it to lay dormant and survive long periods of dehydration (18). When rehydrated, mosses will assume normal metabolic functions if conditions are favorable. Bryum argenteum is an excellent example, it does not need a stable environment to survive (16).

The moss life cycle begins with a small haploid spore. The cells of a germinating spore multiply to form a filamentous structure called the protonema (18). The protonema grows rapidly when moisture is available, and this stage of moss growth closely resembles algae. In fact, the protonema actually develops into a slimy black mat that can easily be mistaken for algae growth. However, the protonema forms buds on its thread-like filaments which grow leafy tissue called the gametophyte, consisting of small, green, leafy plant-like structures with rhizoids (18). Gametophytes form the small tufts of moss colonies. Rhizoids resemble roots, and function to anchor moss plants to the substrata. Some species have specialized rhizoids that aid in the conduction of water to the plants; however, their primary function allows the moss to stick to a surface. The gametophyte is a perennial structure capable of enduring extreme conditions through dormancy, but it can also grow sex organs (an antheridum and an archegonium) that produce sperm and egg cells. The union of sperm and egg forms a diploid zygote that grows from the gametophyte and is called a sporophyte. A sporangium is produced by the sporophyte that contains haploid spores which continue the life cycle when dispersed (18). Mosses spread rapidly via asexual reproduction. Foot traffic, mower traffic, and mechanical cultivation can fragment any piece of a moss plant, ultimately regenerating to form new plants. Because of their asexual method of reproduction, small moss colonies can rapidly spread, infesting an entire putting surface or multiple surfaces, much in the same way many fungal pathogens might spread.

Silver Thread Moss (Bryum argenteum) is the most commonly encountered moss species on putting greens (6,9,11). Amblystegium trichopodium (Schultz) C. J. Hartman, Bryum lisae De Notaris, and Brachythecium spp. are other moss species occasionally seen in turfgrasses (9). Bryum agrenteum can be distinguished by the shiny or silvery appearance of the moss colonies. Bryum lisae often grows in clumps and has a greenish-yellow color, while Amblystegium trichopodium and Brachythecium spp. can usually be discovered on wet and poorly drained soils (9). The latter two mosses are also typically seen in higher-cut turf, but can encroach onto putting greens (9).

Chemical Control

In recent years, golf courses struggling with moss have been armed with more effective solutions than in the past. Not long ago turf managers were experimenting with everything from dishwashing soap to fungicide cocktails for the control of moss, usually with little effect. The sections below review several chemicals tested for controlling moss on golf courses, focusing primarily on their efficacy. Unfortunately, the results found in many individual studies are difficult to compare to each other due to the unique conditions of each experiment. These conditions include factors such as climatological influences, application procedures, turfgrass management practices, and moss species. In cases in which dramatically unique experimental conditions are reported by researchers, they are reported here and their significance discussed.

Lime. Hummel (11) reported that hydrated lime could be used to kill moss on putting greens but indicated that the incidence of moss appeared to be unrelated to soil pH. Unfortunately, subsequent research determined the amount of lime required to injure moss (up to 10 lb/1000 ft²) could be very toxic to putting green turf (19). Lime applications to control moss are not recommended due to the detrimental effects that are associated with over-liming the turfgrass.

Iron. High concentrations of iron may be toxic to moss; however, consistent and long-term moss control is difficult to achieve with iron products. Hummel (11), reported that high soil iron levels correlate with lower incidence of moss on putting greens but no specific range of concentrations was mentioned. Since then, several researchers have tested iron products against moss in field trials, but the results vary between studies. In general, iron provides minimal moss control. A recent publication reported up to 6 weeks of good control in the summer, but moss would eventually return a few weeks later (22). Others reported 0% moss control or marginal control that was not statistically different from control treatments (5,15,19). The work reported by Cook et al. (6) includes the most successful treatment program to date using iron. Five to seven applications of a ferric or ferrous iron product applied at 0.15 to 0.2 lb of Fe per 1000 ft² biweekly provided up to 90% moss control on a ‘Providence’ creeping bentgrass putting greens. These studies were performed in the fall and winter months under cool and rainy conditions. Summertime applications may be slightly more toxic to the grass (6). This study was repeated by Landschoot et al. (13), who found similar results in summer trials, achieving 75% moss control, but fall-winter trials only achieved 45% moss control. In summer trials, slight blackening of the turf blades was observed, but did not lead to turf decline (13). At the rates applied, no turf discoloration was observed in fall-winter trials.

Copper. Copper sulfate and copper hydroxide are two commonly-applied copper pesticides. Both chemicals will kill moss; however, copper sulfate has been reported as being too toxic to putting green turf. Taylor and Danneberger (19) and Cook and Hsiang (5) conducted trials that indicated copper sulfate is not effective for moss control on putting greens. Cook et. al. (6) later indicated copper sulfate causes unacceptable turf injury at rates that will kill moss (0.1 to 0.15 lb of Cu per 1000 ft²).

Copper hydroxide is less harmful to the turfgrass than copper sulfate. Kocide (copper hydroxide) and Junction (copper hydroxide + mancozeb) are commonly used turfgrass fungicides, but also can be used for moss control. Junction is best applied in 2-gal spray volume with a pH of 6.5 or less (but typically above 5.5) in the fall (17). Solution pHs of 7.0 or above can drastically reduce control, while lower pHs may exacerbate turf chlorosis (iron deficiency) (17). In addition, copper becomes more available as soil pH decreases, potentially allowing for copper toxicity, although this phenomenon has not been reported in moss control trials. Spring applications of copper-based products are less effective than fall treatment (17). Cook et al. (6) at Oregon State University tested a very effective program, similar to their iron control program, that may completely eradicate moss from creeping bentgrass putting greens. Five to seven applications of Kocide or Junction applied at 0.1 to 0.15 lb of Cu per 1000 ft² at biweekly intervals provided excellent moss control for up to 2 years (6). This study was performed in the fall-winter season under cool and damp conditions. Unfortunately, copper accumulation in the soil can induce iron deficiency in plants (3). On turfgrass this appears as a chlorosis. Researchers recommended applying iron at a rate of 0.05 lb of Fe per 1000 ft² to eliminate or prevent turf chlorosis (6). In 2004, Landschoot et. al (13) achieved successful moss control using Junction with the method described by Cook et. al. (6). This study performed at Pennsylvania State University found that Junction was not effective for controlling moss during the summer, but they obtained 100% moss control in the fall-winter trials. Other moss trials using copper hydroxide report different results depending on the methods or location of the study. Summer moss trials in Arkansas found Junction provided about 25% moss control when applied biweekly at 0.2 lb/1000 ft² (15). A May-to-October trial at Cornell University showed 1 oz of Junction per 1000 ft² applied biweekly will prevent moss from encroaching into a green (17). The authors undertook a similar study to confirm the efficacy of various moss control products on moss-infested velvet bentgrass in the summer of 2004 on the variety ‘SR7200’ (Table 1). Applied in the middle of the summer, two 14-days applications at a rate of 0.2lb/1000 ft² were made and provided approximately 15% moss control.

Table 1. Results of the moss control study conducted at the University of Rhode Island in Summer 2004.

 ^x Applications were made on 7/26/04 and 8/09/04.

 ^y Numbers followed by the same number in either column are not statistically difference according to the Waller-Duncan k-ratio test at P ≥ 0.05.

 ^z Negative values indicate a decrease in moss within percentage columns.

Mancozeb is a component in the fungicide Junction and chief active ingredient of the fungicide product Fore. Cook and Hsiang (5) found that mancozeb gave some moss control in a summer trial; however, these results were not always statistically different from control plots. McCalla et al. (15) reported that when the fungicide Fore was applied biweekly in the summer at 0.2lb/1000 ft², a ~17% reduction in moss was observed.

Peroxide. There are few reports of successful moss control using hydrogen peroxide; however, there are a number of peroxide-based products available for moss control on turf, in addition to household hydrogen peroxide. In a July-through-October experiment, Landschoot et al. (13) tried a mixture of Ivory dishwashing detergent (8.0 oz/1000 ft²) and household hydrogen peroxide (8.0 oz/1000 ft²) applied biweekly and found the combination to be ineffective at controlling moss. Commercial formulations of peroxide-type products do exist, primarily TerraCyte and Zerotol (BioSafe Systems Inc., Glastonbury, CT). These products contain hydrogen dioxide, an oxidizing agent much like a peroxide. While Zerotol is not labeled for moss control, TerraCyte is specifically intended for that purpose. Zerotol has been tested for moss control and found ineffective when used alone in summer trials at 12 oz/1000 ft² (15). When applied in the spring and fall, Rossi (17) reported good moss control using Zerotol as a follow-up treatment to TerraCyte applications at multiple application rates. Results of research trials using TerraCyte have been consistently positive. Eight pounds of TerraCyte per 1000 ft² have been recommended as the optimal application rate to control moss with minimal damage to the turfgrass (13,14,17) . TerraCyte will burn turf and elevate soil pH if not used carefully. Rossi (17), and Mahady (14) showed that rates higher than 8 lb/1000 ft² cause unacceptable damage to the turfgrass. Landschoot et al. (13) further noted that consecutive day treatments of TerraCyte may be too injurious to the turf, contrary to indications in the product labeling. Good moss control can be achieved if multiple applications (8 lb/1000 ft²) are separated by 2 weeks (13). Fall TerraCyte treatments are preferred over summertime application because the turfgrass is more susceptible to injury while under heat stress. In addition, Rossi (17) notes that moss has an acclimation period following the winter during which chemical treatments are less effective. Rossi (17) reported 70 to 80% moss control in fall trials (using 4, 6, and 8 lb/1000 ft²) while Landschoot et al. (13) reported 52-78% control in summer trials and 80% control in fall trials at 8 lb/1000 ft². Mahady (14) reported, at best, 66% moss control when applied 3 times in January in California at 8 lb/1000 ft² on predominately Poa annua greens. When the authors repeated these experiments on ‘SR7200’ velvet bentgrass in the summer of 2004 (Table 1), moss infestation was slowed but not stopped at either the 6 or 8 lb/1000 ft² rate in 8 weeks. At the 12 lb/1000 ft², however, almost a 60% most reduction was observed but with a significant amount of turf burn injury.

Organic pesticides. Daconil (chlorothalonil) is one of the most tested organic pesticides against moss. Some studies claim Daconil is an excellent moss control agent while others disagree. A North Carolina State University moss-control study tested Daconil Zn (6 and 11 oz/1000 ft²), and Daconil Weatherstik (4 and 8 oz/1000 ft²). Two applications were made 2 weeks apart and researchers greater than 90% moss control for as long as 6 weeks after initial application. No significant differences were observed between any of the Daconil formulations or spray rates in this study (22). McCalla et al. (15) also obtained good results with Daconil, reporting 99% moss control up to 8 weeks after the initial treatment at 10 oz/1000 ft² applied biweekly. In contrast, an earlier study testing Daconil for moss control reported that it was completely ineffective but did not report the details of the experiment (19). Cook et al. (6) also found Daconil Zn and Daconil Ultrex had no activity on moss in fall-winter trials in Oregon but did not report details of the experiment. One possible explanation for the contrasting results is that Daconil may require high temperatures to elicit a toxic effect on moss. The studies by McCalla et al. (15) and Yelverton (22) were both performed in warm and humid conditions. The difference in results could have been due to early spring versus summer application timing. Additionally, it is unclear exactly why chlorothalonil should be toxic to mosses and what biochemical mechanism it is interfering with in the moss plant.

The chemicals oxyfluorfen (Goal) and flumioxazin (SureGuard) reportedly give excellent moss control in greenhouse plantings (7). These chemicals are Protox inhibitor broadleaf herbicides but are not labeled for turfgrass use. A new chemical called QuickSilver (a.i.= carfentazone) is also a Protox inhibitor and is labeled for use against moss on golf course greens in 45 states and Washington D.C. at two 6.7-oz/acre applications 14 days apart  [* see Erratum]. The authors are unaware if carfentazone has been reported in published moss control literature; however, it has been demonstrated to be approximately 90% effective with 2 applications in informal University trials including Clemson and North Carolina State University. Phytotoxicty appears to be low and thus this product may provide an excellent solution for moss control, but further study is warranted to determine the effects temperature and application timing on this product.

Soaps and fatty acids. Anecdotal stories of dishwashing detergents or mouthwash used for moss control have been passed around for many years. Many superintendents will attest that Dawn Ultra dish soap cured their greens of moss, but research trials have failed to substantiate these claims (12). Some recent studies have mentioned that cryptocidal (spore-killing) fatty-acid soap products work quite well against moss on putting greens.

Happ (9) reported that Dawn Ultra has showed some promise as a cryptocidal moss control agent from between 2 to 8 oz/1000 ft². A summer trial with Dawn Ultra provided 74% control 6 weeks after treatment but fell to 30% control after 10 weeks; turf quality was also reduced by the treatments (2). Later research indicated Dawn Ultra may not be as effective as originally thought. McCalla et al (15) obtained 0% moss control using Dawn Ultra (4 oz/1000 ft²) or DeMoss (3 oz/1000 ft²), a fatty acid soap product labeled for moss control. In 2004, Landschoot et al. (13) reported Dawn Ultra was ineffective at controlling moss in their trials at either 4 or 8 oz/1000 ft². Application methodology seems to influence the efficacy of these types of products, primarily Dawn. Spot treatments tend to be much more effective than broadcast sprays, probably because of the extremely high rate of application and optimal rate of water (4,12). Taylor and Danneberger (19) incorporated potassium salts of fatty acids with ammonium sulfate and iron sulfate applications and found this was the most effective moss treatment in a 2 year study. Both iron and sulfur were also used in this experiment and these treatments were phytotoxic in high temperatures. Cook et al. (6) screened many fatty acid soap products and found them to be effective moss killers. According to researchers, a product called No-Mas achieved excellent control after 2 applications (0.63 gal/1000 ft²) applied 2 weeks apart (6) but no specific value indicating the reduction in moss levels was reported. Broadcast applications of fatty acid soaps are most effective when used in extremely high volumes of water (6 to 12 gal of water per 1000 ft²), unfortunately, these water volumes are rarely achievable on a golf course (6).

Silver nitrate/mercury. Metal elements such as iron and copper provide fair to excellent moss control, as previously noted. Another metal, mercury, has been used to effectively eradicate moss populations (11,21). It has been informally suggested that moss has become such a problem on golf courses over the past 20 years partly because mercury-based pesticides are no longer used. Metal-contaminated soils are toxic to mosses. Weber and McAvoy (21) proposed the toxicity sequence of metal contaminants (Hg > Cu > Pb > Ni > Cd > Zn > Mg) based on the electropotential series (E_v) of the ions. Metal ions with larger E_v values are stronger oxidizing agents, allowing them to disrupt photosynthesis in mosses. Silver ions (E_v ≈ 0.8v) are almost as effective as mercury ions (E_v ≈ 0.86v) given this mode of action (21). A 0.22% (w/v) solution of silver nitrate applied at a rate of 2 fl oz/1000 ft² eradicated moss on a putting green and no turf damage or moss reappearance was observed (21). Although silver nitrate may show promising results, it is not labeled for moss control on turfgrasses and its high cost would certainly limit its use as a pesticide. Treatment of a single 5000-ft² green could easily cost over $300 per application (the approximate cost of 1 lb of silver nitrate is between $230 and $360). Additionally, the fate of silver in soils is not well understood. Silver strongly adsorbs to clay and organic matter, suggesting minimal potential for leaching into groundwater, but silver leaching studies have yet to be undertaken (1). The authors found that a rate of 0.275 lb/1000 ft² of silver nitrate completely eliminated moss in two applications on ‘SR7200’ velvet bentgrass with no injury to the turf (Table 1). While silver appears to be an excellent solution for moss control, further investigation is necessary to validate silver as an environmentally safe product.

Conclusion

Of all the products recently tested for control of moss on golf course putting greens, copper-, silver-, and peroxide-based products have demonstrated the best results and have the highest level of reliability. It also appears that carfentazone may provide an excellent control option but data is still relatively scarce concerning some of the details of its use. Lime, iron, chlorothalonil, and soap-based products seem to provide some control but published positive results are sporadic. Factors such as application timing, chemical rate, and location have a major influence on these chemicals, whereas copper, silver, and peroxides are more universally effective. In none of the published studies was there much attempt to examine the spectrum of chemical activity against the many species and biotypes of mosses. While there are a few dominant moss species on golf course putting greens, there is substantial diversity that may contribute to pesticidal efficacy. No single chemical can be used to control every fungal turfgrass disease. Hence it is improbable that any one chemical will be always be universally effective against every species of moss. In the case of almost every chemical, the most significant drawback to the pesticide used against moss is the occurrence of phytotoxicity. At rates high enough to kill moss quickly, turf is almost always significantly damaged. Silver appears to be one of the few products that does kill moss in as little as a single application with no or little apparent phytotoxicty. Silver, however, is not labeled for turf use and there are significant concerns over its environmental fate and safety. While numerous chemical options do exist to eliminate moss, appropriate cultural methods are generally more effective at keeping it at bay. Using clean topdressing materials, providing adequate plant nutrition, avoiding excessive moisture, and increasing mowing heights can all prevent the establishment of moss and reduce its impact when it does begin to encroach in turfgrasses. In short, the most effective way to prevent moss from spreading is to maintain an environment that is not suitable to its successful establishment or propagation.

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