Introduction
Japanese stiltgrass is characterized as an annual, C4 invasive grassy weed species that has colonized areas from New York to Puerto Rico (Barden Reference Barden1987; Fairbrothers and Gray Reference Fairbrothers and Gray1972). Japanese stiltgrass has the adaptability to invade disturbed and undisturbed areas, including riverbanks, wetlands, woodlands, roadside ditches, utility corridors, and landscape bedding and turfgrass (Derr Reference Derr1999; Fairbrothers and Gray Reference Fairbrothers and Gray1972; Redman Reference Redman1995; Swearingen and Adams Reference Swearingen and Adams2008). One of the main reasons that Japanese stiltgrass has gained such broad invasion success is derived from its ability to grow under full sun to almost full shade (Horton and Neufeld Reference Horton and Neufeld1998). The ability of Japanese stiltgrass to tolerate shade distinguishes it from most other C4 grasses such as smooth crabgrass [Digitaria ischaemum (Schreb.) ex Muhl.] (Brown Reference Brown1977; Winter et al. Reference Winter, Schmitt and Edwards1982).
Winter et al. (Reference Winter, Schmitt and Edwards1982) demonstrated that Japanese stiltgrass can maintain dry matter production at 18% of full sun and grow in ways that are equivalent to plants maintained under full sun. Japanese stiltgrass can grow and produce viable seeds even with just 2% to 8% of full sunlight (Cheplick Reference Cheplick2005). Japanese stiltgrass exhibits phenotypic plasticity, can grow without sufficient nutrients under challenging environmental conditions, and inhibits the growth of native species (Swearingen and Adams Reference Swearingen and Adams2008; Ziska et al. Reference Ziska, Tomecek, Valerio and Thompson2015). Japanese stiltgrass can also form large, sprawling mats that can grow from 0.6 to 1.0 m tall, which can shade out other native plants and produce up to 1,000 seeds per plant (Swearingen and Adams Reference Swearingen and Adams2008; Miller and Matlack Reference Miller and Matlack2010). These traits allow Japanese stiltgrass to outcompete most understory native species, making it a serious threat to native plant communities and ecosystem function (Culpepper et al. Reference Culpepper, Wang, Koralewski, Grant and Rogers2018; Miller Reference Miller2003).
Japanese stiltgrass management is challenging due to the plant’s ability to spread rapidly and produce seeds that can remain viable for up to 5 yr (Swearingen and Adams Reference Swearingen and Adams2008; Tu Reference Tu2000). Japanese stiltgrass can be controlled via manual or mechanical measures, but herbicides are recommended for controlling large infestations (Shelton Reference Shelton2012; Tu Reference Tu2000). Previous researchers have assessed several preemergence and postemergence herbicides for their ability to control Japanese stiltgrass in forest environments (Flory Reference Flory2010; Gover et al. Reference Gover, Johnson, Kuhns and Burton2003; Judge et al. Reference Judge, Neal and Derr2005a; Ward and Mervosh Reference Ward and Mervosh2012), but only a few extension publications (e.g., Nitzsche and Rector Reference Nitzsche and Rector2023) have reported selective Japanese stiltgrass control in cool-season turfgrass despite increasing infestations.
Judge et al. (Reference Judge, Neal and Derr2005b) demonstrated that preemergence herbicides labeled for control of large crabgrass [D. sanguinalis (L.) Scop.] in cool-season turfgrass may also be able to control Japanese stiltgrass. Those preemergence herbicides included dithiopyr, prodiamine, trifluralin, oxadiazon, isoxaben, and pendimethalin. Selective and nonselective herbicides applied postemergence have also been effective in controlling Japanese stiltgrass. They include sethoxydim, clethodim, imazapic, fenoxaprop, MSMA, fluazifop, glyphosate, and glufosinate (Gover et al. Reference Gover, Johnson, Kuhns and Burton2003, Judge et al. Reference Judge, Neal and Derr2005b, Weaver et al. Reference Weaver, Brown, McCarty and Gambrell2020), but only fenoxaprop and fluazifop are registered for selective weed control in cool-season turfgrass lawns (Anonymous 2022a). In order to expand the number of options available for Japanese stiltgrass control in managed cool-season turf, more information is needed on its response to traditional postemergence lawn herbicides such as topramezone, mesotrione, quinclorac, and combinations of topramezone + triclopyr and mesotrione + triclopyr. We hypothesized that herbicide combinations that contained topramezone, mesotrione, or quinclorac will control Japanese stiltgrass to an equal or greater extent than fenoxaprop and fluazifop. Thus the objectives of this study were to assess the effectiveness of various herbicides in controlling Japanese stiltgrass that grows in tall fescue lawns, and to assess the ability of selective herbicides to control Japanese stiltgrass in Kentucky bluegrass.
Materials and Methods
Initial Screen of Postemergence Herbicides
Four field experiments were conducted from 2014 to 2019 to evaluate the tolerance of tall fescue to multiple postemergence herbicides and their efficacy in controlling Japanese stiltgrass in lawns (Table 1). The study consisted of a randomized complete block design with three replications and four temporal runs across two locations. The response of tall fescue to herbicide treatments was assessed in all four studies, whereas Japanese stiltgrass control was evaluated in two of the studies (Table 1). Each plot measured 1.8 m by 1.8 m. A comprehensive list of treatments, including common names, product names, manufacturer information, and application rates, is provided in Table 2. All herbicides were applied with a CO2-pressurized hooded boom sprayer equipped with two TTI11003 flat-fan nozzles (TeeJet Technologies, Glendale Heights, IL), spaced 36 cm apart, calibrated to deliver 281 L ha−1 of spray solution at 4.8 km h−1. Japanese stiltgrass was at the 4- to 6-tiller stage at the time of herbicide application. The experiment sites were mowed regularly at 1-wk intervals to a height of 6.4 cm throughout the study period.
Table 1. Field study information.
a Selected herbicides were applied sequentially at a 3-wk interval during the study.
Table 2. Herbicide common names, trade names, manufacturer, and rates used in field experiments to assess tall fescue tolerance and Japanese stiltgrass control.
a Nonionic surfactant at 2.5 mL L−1.
b Methylated seed oil at 5 mL L−1.
c Applied sequentially at a 3-wk intervals.
d Manufacturer locations: BASF Corp., Research Triangle Park, NC; Bayer Environmental Science, Cary, NC; Dow Agrosciences LLC, Indianapolis, IN; Syngenta Crop Protection, LLC, Greensboro, NC
Tall fescue and Japanese stiltgrass cover, control, and injury were visually assessed on a scale of 0% to 100%, with 0% being no control, no cover, or no injury and 100% being complete plant death or complete cover. Data were assessed at 0, 1, 2, 4, 6, and 8 wk after initial treatment (WAIT). Japanese stiltgrass counts were taken in each plot at 8 WAIT using a 1-m2 quadrant. Visual cover of tall fescue or Japanese stiltgrass was converted to relative cover based on the percent cover of a nontreated control plot within each replication. All response variables were subjected to ANOVA using the GLM procedure with SAS software (v. 9.3; SAS Institute, Cary, NC). Treatment was considered as a fixed effect, while experimental run and block were treated as random effects. The mean square of the treatment effect was tested for all assessed parameters using the mean square associated with the experimental run (McIntosh Reference McIntosh1983). Means were separated using Fisher’s protected LSD (α = 0.05). Means were presented separately by experimental run if the treatment-by-experimental run interactions were significant; otherwise, means were pooled over experimental runs.
Assessing Fenoxaprop Combinations and Fluazifop
Four field experiments were conducted in 2015 and 2017 to assess Japanese stiltgrass control and Kentucky bluegrass tolerance after selective herbicide treatments (Table 1). The experiment was a randomized, complete block design with three replications and two temporal runs for each species. Treatments included a nontreated control, a premix of fenoxaprop + fluroxypyr + dicamba at 421 g ha−1 (with or without a nonionic surfactant), fluazifop at 53 g ha−1 with a nonionic surfactant, fluazifop at 105 g ha−1 with a nonionic surfactant, and quinclorac at 840 g ha−1 with methylated seed oil. A detailed list of herbicide common names, trade names, manufacturer names and locations, and rates evaluated in the study is provided in Table 3. Herbicide application method, site maintenance, and data collection were the same as the previous study except that digital photographic images were also taken to quantify turf cover using TurfAnalyzer (Green Research Services, Fayetteville, AR) to detect the green pixels in each image. Turf cover was converted to relative cover based on the percent green cover in the nontreated plot within each replication. Data were analyzed as described above because the experimental design and response variables are similar for both studies. Means were separated using Fisher’s protected LSD (α = 0.05).
Table 3. Herbicide common names, trade names, manufacturer, and rates used in field experiments to assess the response of Kentucky bluegrass and Japanese stiltgrass.
a Nonionic surfactant at 2.5 mL L−1.
b Methylated seed oil at at 5 mL L−1.
c Manufacturer locations: BASF Corp., Research Triangle Park, NC; Nufarm Americas, Inc., Alsip, IL; PBI Gordan Corp., Kansas City, MO;
Results and Discussion
Initial Screen of Postemergence Herbicides
The main effect of treatment was visual injury to tall fescue, and it was highly significant (P < 0.0001) and not dependent on trial (P > 0.05), leading to the pooling of data across all four experimental runs (data not shown). Only treatments that contained mesotrione caused injury to tall fescue, with injury levels between 15% and 25% at 2 WAIT. Those treatments, however, did not affect the relative turf cover (data not shown). Transient injury to tall fescue from mesotrione has been noted in other studies (Goddard et al. Reference Goddard, Willis and Askew2010; Willis et al. Reference Willis, Beam, Barker and Askew2006), whereas the plant exhibited no injury from triclopyr (Dernoeden et al. Reference Dernoeden, Kaminski and Fu2008). Fenoxaprop did not alter tall fescue color or density beyond commercially acceptable levels (McCarty et al. Reference McCarty, Higgins, Whitwell and Miller1989) and was generally safe to turf even with frequent applications (Johnson and Carrow Reference Johnson and Carrow1995). Finally, in other previous reports, topramezone and quinclorac treatments resulted in less than 7% injury to tall fescue without affecting turf quality (Brewer et al. Reference Brewer, Willis, Rana and Askew2017; Patton et al. Reference Patton, Braun, Bearss and Schortgen2021).
The herbicide treatment-by-experimental run interaction was significant for Japanese stiltgrass control (P < 0.0001), relative weed cover (P = 0.0032), and weed shoot density (P < 0.0001) at 8 WAIT (Table 4). In nontreated control plots, Japanese stiltgrass density was 224 and 314 shoots m−2 at the Newport and Blacksburg locations, respectively (Table 4). Japanese stiltgrass was controlled by ≥90% with fenoxaprop, regardless of rate, cover was reduced to <15% and shoots to ≤6 m−2 at both study sites (Table 4). These results align with those reported in previous research in forest ecosystems, where even reduced rates of fenoxaprop were found to be as effective as the labeled rate for controlling Japanese stiltgrass (Peskin et al. Reference Peskin, Mortensen, Jones and Booher2005; Ward and Mervosh Reference Ward and Mervosh2012). Topramezone, applied at 27 or 54 g ha−1 (alone or with triclopyr), provided ≥80% control of Japanese stiltgrass, and relative weed cover and shoot density were reduced to ≤22% and 35 shoots m−2, respectively (Table 4). Although triclopyr did not improve the performance of topramezone in its ability to control Japanese stiltgrass (Table 4), applications of both triclopyr and metribuzin led to sustained or enhanced topramezone performance in goosegrass [Eleusine indica (L.) Gaertn.] control (Brewer et al. Reference Brewer, Craft and Askew2022; Cox et al. Reference Cox, Rana, Brewer and Askew2017) Topramezone is specifically labeled for Japanese stiltgrass control in cool-season turf and recommended for selective management of troublesome weeds (Anonymous 2022b; Cox et al. Reference Cox, Rana, Brewer and Askew2017; Landschoot et al. Reference Landschoot, Abbey and Delvalle2023; Peppers et al. Reference Peppers, Elmore and Askew2023). Treatments with mesotrione, quinclorac, and triclopyr applied individually resulted in decreased Japanese stiltgrass relative cover and shoot density, but weed control was <68% (Table 4). Other studies have similarly reported poor control of Japanese stiltgrass following applications of quinclorac on golf course naturalized areas and triclopyr on cool-season grass forages (Flessner et al. Reference Flessner, Lassiter and Bamber2019; Weaver et al. Reference Weaver, Brown, McCarty and Gambrell2020). Overall, these data suggest that fenoxaprop and treatments with topramezone were able to effectively and selectively control Japanese stiltgrass in tall fescue lawns without causing significant turfgrass injury.
Table 4. Effect of herbicide treatments on Japanese stiltgrass control, relative cover, and shoot density at 8 wk after initial treatment. a
a Means followed by a different letter within the same column are different based on Fisher’s protected LSD (α = 0.05).
b Nonionic surfactant at 2.5 mL L−1.
c Methylated seed oil at 5 mL L−1.
d Applied sequentially at 3-wk intervals.
Assessing Fenoxaprop Combinations and Fluazifop
The main effect of treatment was significant for both Kentucky bluegrass injury (P = 0.0004) and relative turfgrass cover (P = 0.039) at 4 wk after treatment (WAT), but these response variables were not dependent (P > 0.05) on the experimental run (Table 5). Treatments containing fluazifop resulted in ≥25% injury to Kentucky bluegrass and a reduction of ≥20% in digitally assessed relative turf cover at 4 WAT (Table 5). Fluazifop resulted in 25% to 46% Kentucky bluegrass injury, similar to previous reports (Warren et al. Reference Warren, Skroch, Monaco and Shribbs1989).
Table 5. Effect of herbicide treatments on Kentucky bluegrass injury and relative cover compared to nontreated control at 4 WAT and Japanese stiltgrass control, relative weed cover, and shoot density at 8 WAT.a,b
a Abbreviation: WAT, weeks after treatment.
b Means followed by the same letter within each column are not different based on Fisher’s protected LSD (α = 0.05).
c Nonionic surfactant at 2.5 mL L−1.
d Fluazifop was applied at 53 g ha−1.
e Fluazifop was applied at 105 g ha−1.
f Methylated seed oil at 5 mL L−1.
In contrast, Kentucky bluegrass was highly tolerant to applications of dicamba + fenoxaprop + fluroxypyr (regardless of surfactant) or quinclorac; injury measured ≤3% (Table 5). Quinclorac, when applied at 840 g ha−1, did not cause injury to newly established Kentucky bluegrass (Reicher et al. Reference Reicher, Weisenberger and Throssell1999). Previous research also confirmed that tank mixing fenoxaprop with fluroxypyr does not compromise the efficacy of fenoxaprop against smooth crabgrass, nor does it cause injury to cool-season turfgrass (McCullough et al. Reference McCullough, Brosnan and Breeden2009).
The main effect of treatment was significant for Japanese stiltgrass control (P = 0.0002), relative weed cover (P = 0.0003), and weed shoot density (P < 0.0001) at 8 WAT, with these response variables showing no dependence on the experimental run; therefore, data were pooled over runs (Table 5). At 8 WAT, Japanese stiltgrass was controlled by ≥92%, relative cover was reduced to ≤7%, and shoot density was <5 shoots m−2 when the combination of dicamba + fenoxaprop + fluroxypyr was applied, both with and without a nonionic surfactant (Table 5). In other studies, when fenoxaprop was applied alone, Japanese stiltgrass was controlled by 93% and weed cover was reduced by 89% (Judge et al. Reference Judge, Neal and Derr2005a, Reference Judge, Neal and Derr2005b). A commercial premix of dicamba, fenoxaprop, and fluroxypyr is marketed for controlling Japanese stiltgrass and other problematic weeds in Kentucky bluegrass turf (Anonymous 2017). Although the current study shows that dicamba and fluroxypyr do not reduce the performance of fenoxaprop in its ability to control Japanese stiltgrass, in other studies (Cox and Askew Reference Cox and Askew2014), mixtures with other herbicides such as 2,4-D and mecoprop have antagonized graminicides for annual grass control.
Fluazifop treatments were also highly effective, controlling Japanese stiltgrass by ≥85% across various application rates, while reducing relative cover and weed shoot density to ≤17% and <20 shoots m−2, respectively, at 8 WAT (Table 5). Similar results were observed by Judge et al. (Reference Judge, Neal and Derr2005b), who reported 97% control of Japanese stiltgrass 8 wk after treatment with fluazifop. In the studies reported here, treatments were applied in July or August when Japanese stiltgrass was relatively mature and chances of subsequent germination were relatively low. Herbicides applied earlier in the season may not perform as well due to subsequent seedling emergence as has been demonstrated with Japanese stiltgrass (Judge et al. Reference Judge, Neal and Derr2005b) and other grassy weeds (Askew et al. Reference Askew, Shaw and Street2000). Quinclorac reduced Japanese stiltgrass relative cover by only 35% and shoot density to only 108 shoots m−2, indicating it is not effective in controlling this weed (Table 5). Fluazifop at 53 g ha−1 injured Kentucky bluegrass by 25% at 4 WAT, but even a low injury would not be considered completely safe by turfgrass managers. Additionally, this response is very different from the minimal injury to Kentucky bluegrass observed after dicamba + fenoxaprop + fluroxypyr treatments.
Practical Implications
This research provides information that will aid turf managers to selectively control Japanese stiltgrass in cool-season turfgrasses. Fenoxaprop selectively controls Japanese stiltgrass even at 35 g ha−1 and reduces the overall cost of managing this problematic weed in tall fescue. Topramezone-based herbicides also effectively control Japanese stiltgrass without compromising tall fescue safety. Fluazifop at 53 g ha−1 resulted in transient injury to Kentucky bluegrass but it effectively controlled Japanese stiltgrass. Reduced rates of fluazifop could be applied for selectively managing Japanese stiltgrass in Kentucky bluegrass if land and turf managers are willing to tolerate transient turf injury up to 4 wk after herbicide application. Dicamba, fenoxaprop, and fluroxypyr premix controlled Japanese stiltgrass by >90% without compromising Kentucky bluegrass safety. Although our study did not evaluate the tolerance of tall fescue to the dicamba, fenoxaprop, and fluroxypyr premix, this product is labeled for use on tall fescue and other cool-season turfgrass species.
Acknowledgments
We thank Sandeep Rana and Jordan Craft, former graduate students of the Turfgrass Weed Science Lab, for their technical support during research implementation and data collection.
Funding
Partial funding for this research was provided by Nufarm Americas Inc., Alsip, IL.
Competing Interests
The authors declare they have no competing interests.
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