Exploring Natural Alternatives for Annual Bluegrass Control

: A ﬁeld experiment was conducted to identify and examine the effectiveness of potential non-traditional chemical products and alternatives for controlling annual bluegrass ( Poa annua L.), a plant with increasing herbicide resistant problems. In managed turf systems annual bluegrass can be a difﬁcult winter annual weed to control having negative impacts on turfgrass quality. This study included 12 different treatments [untreated check, baking soda, white vinegar + lemon juice, Suppress herbicide, superphosphate (0-20-0), clove oil, Weed Zap, Avenger Weed Killer, Fiesta Turf Weed Killer, Ecologic Weed & Grass Killer, Alcohol (43% ethanol), and Pool Time Algicide] applied to a TifEagle bermudagrass ( Cynodon dactylon × C. traansvalensis ) putting green where a natural infestation of annual bluegrass was present. Treatments were assessed visually for annual bluegrass control and turf phytotoxicity 1, 2, and 4 weeks after application. Overall, no treatment provided long-term control which was non-selective. Control was short-lived with annual bluegrass recovery beginning approximately two weeks after applying treatments. Greatest Poa burndown (~85%) was temporarily (1 to 2 weeks after application) with a combination of caprylic and capric acids (Suppress Herbicide) and a combination of clove oil and dishwashing detergent but plants fully recovered by three weeks after application. These products also produced similar temporary turf phytotoxicity. Bermudagrass turf phytotoxicity from selective treatments was most evident one week after applying treatments and turf had mostly fully recovered by four weeks after treatment. Suitable alternatives were not identiﬁed from products tested.


Introduction
Annual bluegrass (Poa annua L.) is the most troublesome winter annual weed in managed turf systems and has grown to epidemic proportions [1,2]. Having a light green color, it negatively impacts turfgrass quality through prolific seedhead production, clumped growth habit, and lack of stress tolerance [3,4]. Controlling annual bluegrass can be difficult as it exhibits high levels of genetic diversity, rapidly adapts to different climates as well as practices, and has developed widespread herbicide resistance.
Current management programs rely heavily on herbicides for annual bluegrass control even though few truly effective options are available [5]. Frequent herbicide use with the same mode-of-action without implementation of other non-chemical management practices can lead to resistant species with resistance to at least nine different herbicide modes of action [3,6]. Therefore, alternative options for control are needed to reduce dependency of synthetic herbicides and to combat herbicide resistance.
Alternative non-chemical products are available to annual bluegrass control, but the effectiveness of these products has not been adequately tested in different turf systems and environments. There have also been untested claims about household compounds or homemade products being used to control weeds. Typically, natural or biological control options are non-selective with all treated plants (desired or not) sustaining damage [7]. The objectives of this study were to further investigate different non-chemical commercial Horticulturae 2021, 7, 365 2 of 5 products and household compounds for their effectiveness to control annual bluegrass and turfgrass tolerance.

Materials and Methods
An experiment was conducted at Clemson University, Clemson, S.C., United States in the spring of 2020 (Year 1) and 2021 (Year 2) to identify and evaluate the effectiveness of potential non-chemical products and alternatives for controlling annual bluegrass. A TifEagle bermudagrass (Cynodon dactylon × C. traansvalensis) putting green (Figure 1) was chosen as the experiment site due to sufficient natural infestation of annual bluegrass present. This study included 12 different treatments consisting of common household compounds (Table 1). The objectives of this study were to further investigate different non-chemical commercial products and household compounds for their effectiveness to control annual bluegrass and turfgrass tolerance.

Materials and Methods
An experiment was conducted at Clemson University, Clemson, S.C., United States in the spring of 2020 (Year 1) and 2021 (Year 2) to identify and evaluate the effectiveness of potential non-chemical products and alternatives for controlling annual bluegrass. A TifEagle bermudagrass (Cynodon dactylon × C. traansvalensis) putting green (Figure 1) was chosen as the experiment site due to sufficient natural infestation of annual bluegrass present. This study included 12 different treatments consisting of common household compounds (Table 1).    Treatments were applied on 30 March 2020, and 2021 using a carbon dioxide (CO 2 )pressurized backpack sprayer delivering 60 gal ac −1 (561 L ha −1 ). Applications were made during spring greenup, typically when bermudagrass is most sensitive to herbicide applications. Plots measured 4.9 × 4.9 feet (1.5 × 1.5 m) and were arranged in a randomized complete block design with three replications in each trial. The site was maintained to normal putting green standards at a mowing height of 0.125 inch (3.2 mm) and irrigation provided as needed to prevent wilting. Plots were not mowed during the study to better display results from treatments.
Treatments were assessed visually for annual bluegrass control and turf phytotoxicity 1, 2, and 4 weeks after application. Visual control of annual bluegrass was rated on a scale of 0% to 100%, where 0% = no control and 100% = complete control. Phytotoxicity on TifEagle bermudagrass was also rated visually on a scale of 0% to 100%, where 0% = no phytotoxicity, 30% = maximum acceptable damage level, 100% = complete turfgrass death.
Statistical analyses were performed using analysis of variance (ANOVA) within JMP ® Pro system (Version 13.2.0, SAS Institute, Cary, NC 27513, USA). Means separation was analyzed using a Student's t test at α ≥ 0.05.

Results
Significant differences occurred between evaluation dates and years; therefore, results are presented separately by evaluation date and year. Annual bluegrass control from treatments were mostly non-selective and visually appeared to burndown everything, including the desirable turf (Table 2). However, results were relatively short-lived with most products providing little to no long-term control. During Year 1, only plots treated with Suppress Herbicide (caprylic acid + capric acid) and Avenger (citrus oil) had visual control ratings statistically higher than the untreated check. Annual bluegrass control was most noticeable one week after treatment (WAT) with Suppress Herbicide provided best control at~50% followed by Avenger at 20% (Figure 2). Results began to wane the second WAT with significant control only observed with Suppress Herbicide (23%). Annual bluegrass plants continued to recover and by the fourth WAT, no statistical control differences were evident between treatments and untreated check.
During Year 1, only plots treated with Suppress Herbicide (caprylic acid + capric acid) and Avenger (citrus oil) had visual control ratings statistically higher than the untreated check. Annual bluegrass control was most noticeable one week after treatment (WAT) with Suppress Herbicide provided best control at ~50% followed by Avenger at 20% (Figure 2). Results began to wane the second WAT with significant control only observed with Suppress Herbicide (23%). Annual bluegrass plants continued to recover and by the fourth WAT, no statistical control differences were evident between treatments and untreated check. In year 2, seven treatments had visual control ratings statistically different than the untreated check. Similar to year 1, annual bluegrass control was most noticeable 1WAT. Suppress Herbicide, Clove Oil + dishwashing soap, Weed Zap (clove oil + cinnamon oil) and Avenger Weed Killer providing best control at between ~57 and 87% ( Table 2). By the second WAT, results began to wane with significant control only observed with Suppress Herbicide (~67%), Clove Oil + Dawn (53%), Weed Zap (37%), and Avenger Weed Killer (27%). Recovery from all treatments was observed by 4WAT at which time, no statistical differences occurred between treatments and untreated check.
Bermudagrass turf phytotoxicity was observed following the application of selective treatments during both years (Table 3). Phytotoxicity was most evident 1WAT in plots treated with Suppress Herbicide and baking soda plus dishwashing soap but, phytotoxicity was only observed in plots treated with Suppress Herbicide 2WAT during Year 1 (Table 3). No other treatment caused phytotoxicity above the acceptable level at this point in Year 1. The turf had mostly fully recovered by 4WAT.  In year 2, seven treatments had visual control ratings statistically different than the untreated check. Similar to year 1, annual bluegrass control was most noticeable 1WAT. Suppress Herbicide, Clove Oil + dishwashing soap, Weed Zap (clove oil + cinnamon oil) and Avenger Weed Killer providing best control at between~57 and 87% (Table 2). By the second WAT, results began to wane with significant control only observed with Suppress Herbicide (~67%), Clove Oil + Dawn (53%), Weed Zap (37%), and Avenger Weed Killer (27%). Recovery from all treatments was observed by 4WAT at which time, no statistical differences occurred between treatments and untreated check.
Bermudagrass turf phytotoxicity was observed following the application of selective treatments during both years (Table 3). Phytotoxicity was most evident 1WAT in plots treated with Suppress Herbicide and baking soda plus dishwashing soap but, phytotoxicity was only observed in plots treated with Suppress Herbicide 2WAT during Year 1 (Table 3). No other treatment caused phytotoxicity above the acceptable level at this point in Year 1. The turf had mostly fully recovered by 4WAT.  (Table 3). By 2WAT, phytotoxicity above the 30% maximum level was only observed with Suppress Herbicide (73%) and Clove Oil + Dawn (57%). Turf fully recovered by the fourth WAT with no phytotoxicity from treatments observed.

Discussion and Conclusions
From this study, several non-synthetic chemical products tested could potentially provide some short-term annual bluegrass burndown. However, results were intermediate at best with little to no long-term control from most products evaluated, similar to the low weed control efficacy observed after applying non-chemical products in other turfgrass systems [8]. None of the products tested appear to provide acceptable commercial control with only a single application for this winter annual weed. Control from these products could possibly be improved through multiple applications as some tested product labels mention repeat applications may be necessary for effective control [9,10]. Additional research is needed to determine whether multiple applications of these products can effectively control annual bluegrass without injuring the desired turf.
Certain products also caused short-term (1 to 2 weeks) undesirable turf phytotoxicity. Other research has found similar, non-chemical products have caused injury to desired turf as well [8,11]. Similar visual burndown is often observed after applying non-selective herbicides such as glyphosate, diquat, or glufosinate [3]. It is not recommended to apply the products utilized in this research for annual bluegrass control as none are registered herbicides and turf safety has not yet been adequately tested.