J. Aquat. Plant Manage. 53: 165–170 Control of delta arrowhead (Sagittaria platyphylla) in Australian irrigation channels with long exposure to endothall dipotassium salt during winter DANIEL CLEMENTS, TONY M. DUGDALE, KYM L. BUTLER, AND TREVOR D. HUNT* ABSTRACT ma, South Africa, and the former Soviet Union (Adair et al. 2012). Delta arrowhead is a perennial, monocotyledonous Delta arrowhead [Sagittaria platyphylla (Engelm.) J.G. Sm.] herb, which reproduces by seed (achenes) and vegetatively is an emergent, aquatic plant, originating from North via stolons and tubers (Jacobs 2011). There are two main leaf America, which has invaded aquatic environments in forms; the emergent, upright, petiolate leaf form and the Australia. The plant is particularly problematic in southeast submersed, phyllodial leaf form (Haynes and Hellquist Australia, where it invades earthen irrigation channels and 2000). The emergent, petiolate leaf form bears flowers and drains. Hydraulic capacity is subsequently reduced, leading grows to 150 cm tall and tends to occur in slow-moving to a reduction in the efficiency of water delivery. Options water bodies. Leaf size and shape is highly variable and for controlling delta arrowhead in irrigation channels and dependent on environmental and management factors. The drains are currently underdeveloped. Previous trials have submersed, phyllodial leaf form produces linear, strap-like indicated that a potential control option is to treat leaves and is typically found in deeper water than the irrigation channels that hold standing water during tem- emergent, petiolate leaf form. However, phyllodial plants perate winter conditions with the contact herbicide can transform into petiolate plants or remain phyllodial endothall. This article reports on a field experiment to indefinitely, depending on environmental conditions. Delta determine the dose–response relationship for endothall arrowhead is frost sensitive, but regrowth occurs from dipotassium salt and delta arrowhead. A 3-wk exposure submersed or subterranean organs (Adair et al. 2012). period of endothall dipotassium salt to delta arrowhead À1 In southeast Australia, delta arrowhead has spread during winter at 5 mg ai L provided 69% biomass significantly since its introduction in about 1960 becoming reduction of the emergent petiolate growth form and 92% a major weed of irrigation and drainage systems (Adair et al. biomass reduction of the submersed phyllodial growth form 2012). In 2012, the Australian Government declared the 6 wk after treatment (WAT). Control remained evident at 15 species a Weed of National Significance because of its WAT; however, by 26 WAT differences between treatments invasiveness and potential impacts to the economy and were not detected. This reduction in biomass confirms environment. An increase in capacity to manage the weed is endothall dipotassium salt to be a useful tool to reduce delta required because current control methods are underdevel- arrowhead biomass during the spring irrigation period. oped (Australian Weeds Committee 2012). Key words: aquatic herbicide, aquatic vegetation, aquatic Excessive growth of aquatic plants in earthen irrigation weed control, chemical control, concentration exposure systems reduces their water-carrying capacity, thus com- time, Sagittaria platyphylla, water delivery. promising the reliability of water delivery to primary producers (Bill 1969, Bakry et al. 1992, Dugdale et al. INTRODUCTION 2013). Control of delta arrowhead in irrigation channels and drains in Australia relies on foliar applications of herbicide Delta arrowhead [Sagittaria platyphylla (Engelm.) J.G. Sm.] to emergent parts of the plant. The herbicides glyphosate (also called sagittaria; Alismataceae) is an emergent, aquatic and 2, 4-D are commonly used, and imazapyr, amitrole, and plant, originating from North America, which has subse- dichlobenil have also been used. High-dose and multiple quently become naturalized in Australia, Indonesia, Pana- herbicide applications each year are often required to achieve levels of control in which channel hydraulic *First,second,andfourthauthors:ResearchScientist,Senior capacity is not compromised (Adair et al. 2012). Research Scientist, and Senior Technical Officer, Department of Environment and Primary Industries, Biosciences Research Division, A potential alternative-control option for delta arrow- Centre for AgriBiosciences, 5 Ring Rd., La Trobe University, Bundoora, head in irrigation channels is to target the submersed parts Victoria 3083, Australia. Third author: Senior Research Scientist, of the plant. Submersed aquatic weed control with Department of Environment and Primary Industries, Biometrics Unit, herbicide is dependent on the relationship between Agriculture Research and Development, Mt Napier Rd., Hamilton, Victoria 3300, Australia. Corresponding author’s E-mail: Daniel. herbicide concentration (achieved by dosing the water to [email protected]. Received for publication October 20, 2014 a target concentration) and exposure time (Netherland and in revised form March 4, 2015. 2009). In Australia, the herbicide acrolein is used for J. Aquat. Plant Manage. 53: 2015 165 controlling submersed aquatic weeds in irrigation channels dipotassium salt (0.3, 0.6, 1.2, 2.4, 4.8 mg ai LÀ1) and an (Bowmer and Smith 1984) but has been found ineffective on untreated reference, randomized in three replicate blocks delta arrowhead (M. Finlay, pers. comm.). Diquat is also of six plots. registered for aquatic use, but it is inactivated rapidly in Herbicide treatments were applied in mid July 2012 by turbid water (Simsiman et al. 1976), which is common in the mixing the required volume of endothall stock solution,1 irrigation districts of southeast Australia. This results in a based on measured plot volume, in 20 L of water and short exposure time and, consequently, ineffective control broadcasting it over each plot. After 3 wk, the herbicide- (Bowmer 1982, Hofstra et al. 2001, Clements et al. 2013). treated water in each plot was pumped out to a holding The contact herbicide endothall is widely used to control channel, and then the bund walls were removed, allowing submersed, aquatic vegetation in the United States (Sprech- plots to be refilled with fresh water from the adjacent area er et al. 2002). Endothall is effective against a wide range of of the channel. Plots that contained no herbicide were also submersed species (Skogerboe and Getsinger 2001, Skoger- pumped out and refilled in the same manner. boe and Getsinger 2002, Dugdale et al. 2012, Dugdale et al. 2013) and has a low-sorption coefficient (Reinert et al. Water quality and herbicide concentration 1996); thus, it maintains efficacy in turbid water (Hofstra et al. 2001). In the United States, it is used effectively in flowing Water samples were taken from each plot before and at irrigation channels (Sisneros et al. 1998). Although endo- intervals after herbicide application to determine endothall thall is usually used on submersed aquatic weeds, the concentration and turbidity. An enzyme-linked immuno- emergent genera burreed (Sparganium spp.) is listed on the sorbent assay (ELISA)2 was used to determine endothall product label, and recently, endothall has provided control concentrations. Turbidity was measured using an instanta- of flowering rush (Butomus umbellatus L.), which is a member neous turbidimeter.3 Water depth and temperature was of the Alismatales order (Poovey et al. 2013). logged continuously over the trial period using data The primary mode of endothall decay is microbial loggers.4 activity (Reinert et al. 1986). Therefore, we would expect to achieve a longer exposure period during cold water Efficacy of delta arrowhead control conditions compared with when water temperatures are higher and microbial action is greater. Further, in Victoria To determine efficacy of delta arrowhead control, a (35 to 368S), Australia, delivery of irrigation water ceases for range of abundance metrics were used before treatment (0 about six weeks each winter (June to August) and irrigation wk after treatment [WAT]) and at 6, 15, and 26 WAT (all channels hold standing water (static water conditions). This posttreatment intervals are measured from the end of the 3- provides an opportunity for herbicide treatment with wk exposure period). At 0 and 6 WAT, all above ground longer exposure times to be achieved compared with delta arrowhead biomass from five preselected quadrats undertaking control during the irrigation season (Clements (0.09 m2) was harvested in each of the 18 plots by pulling out et al. 2013). A confounding factor to winter treatments is entire plants from within quadrats. Dry-weight biomass of reduced herbicide efficacy because plants are not actively viable aboveground plant material was determined for each growing in cold water (Netherland et al. 2000), and plants plot. For each quadrat in each plot, plants were classified are large with fibrous stems and crowns. into two growth forms—petiolate or phyllodial—and the Effective control (. 95% biomass reduction) of delta five quadrats from each plot were combined. Excess water arrowhead has been achieved in field trials in which the slow from each combined sample was removed by spinning in a decay of endothall in static water irrigation channels during commercial salad spinner until no droplets were produced, winter was exploited. However, only a single concentration before sample weighing (wet weight), and then subsampling and exposure time were used (7 mg ae LÀ1 for . 32 d; and drying