Efficacy of Preemergence and Postemergence Herbicides for Controlling Common Purslane

Common purslane (Portulaca oleracea L.) can be problematic in thin turf, along sidewalks and drives, and especially during turfgrass establishment. Little published research exists evaluating herbicides that will control purslane and are also labeled for turfgrass. Thus, our objective was to evaluate the efficacy of preemergence (PRE) or postemergence (POST) herbicides labeled for use in turf for controlling purslane. Experiments were conducted once in 2011 and twice in 2012 to evaluate nine PRE herbicides at one-half maximum and maximum label rates applied over immature perennial ryegrass (Lolium perenne L.). The PRE herbicides isoxaben and simazine consistently resulted in the best purslane control for all three PRE experiments. Experiments in 2011 and 2012 evaluated 25 POST herbicides at full label rates applied to mature purslane plants. The POST herbicides fluroxypyr, triclopyr, and metsulfuronmethyl were most effective in controlling purslane. Turfgrasses that establish quickly can resist weed colonization, and stands with high turf density may reduce weed competition (Busey, 2003). Conversely, stresses that thin turf and expose the soil to sunlight could allow annual weed infestation (Busey, 2003). Common purslane (Portulaca oleracea L.) is a warm-season summer annual weed often found in thin turf, bare soil areas (Matthews et al., 1993), or during turfgrass establishment. For spring establishment of turfgrass or in thin turf along sidewalks and drives, herbicide control of purslane may be necessary. Labels of many turfgrass herbicides list purslane as a weed species controlled, but limited published research is available on herbicide control of purslane in turf. Several studies in crops other than turfgrass have evaluated purslane control with herbicides, but often only report limited purslane data because it was not the primary target and/or because this weed appeared inconsistently during the studies. Of the research reported, few of the mentioned herbicides are labeled for use in turf, whereas others are not labeled for use in the United States. Among the products labeled for use in turf, Stacewicz-Sapuncakis et al. (1973) found that purslane was sensitive to the POST herbicide dicamba, but the lethal rate depended on plant age. Postemergence applications of clopyralid resulted in less than 45% control of purslane at harvest of leafy greens (Norsworthy and Smith, 2005), but clopyralid is no longer labeled for use on residential lawns. An evaluation of PRE herbicides for weed control in pumpkin (Cucurbita spp.) found sulfentrazone or dimethenamid resulted in 74% or greater control of purslane 21 d after treatment (Brown and Masiunas, 2002). Bensulide or pronamide applied PRE reduced purslane density in lettuce (Lactaca sativa) by 52% to 98% (Haar and Fennimore, 2003). Preemergence application to control purslane in leafy greens with pendimethalin resulted in greater than 84% control, whereas bensulide plus dimethenamid resulted in less than 78% control (Norsworthy and Smith, 2005). Among herbicides for controlling purslane that are not labeled in turfgrass or not for use in the United States, POST herbicides nitrofen and oxyfluorfen increase membrane permeability resulting in stomatal closure, membrane disruption, ethylene synthesis, and ultimately leaf abscission in purslane (Gorske and Hopen, 1978). Doohan and Felix (2012) report between 11% and 95% purslane control in green onion with oxyfluorfen applied at three labeled rates. Postemergence treatments with phenmedipham resulted in less than 45% control of purslane at harvest of leafy greens (Norsworthy and Smith, 2005). The PRE herbicides diethatyl, diphenamid, diethatyl plus diphenamid, or S-metolachlor reduced purslane in vegetable crops (Cavero et al., 1996; Norsworthy and Smith, 2005). Imazethapyr applied either PRE or POST in lettuce provided greater than 80% PRE control and greater than 85% POST control of purslane (Dusky and Stall, 1996). The soil fumigant methyl iodide was as effective as methyl bromide in reducing purslane seed germination rates in a laboratory study for the two highest rates tested (Ohr et al., 1996). Because many previously researched herbicides are not currently labeled for use in turf, the objective of our study was to conduct herbicide screens to determine the efficacy of PRE or POST herbicides labeled for turfgrass to control purslane. Materials and Methods Preemergence and POST herbicide studies were conducted in 2011 and 2012 at the University of Nebraska–Lincoln’s John Seaton Anderson research facility near Mead, NE. Soil was a Tomek silt loam (Fine, smectitic, mesic Pachic Argiudolls) with pH 6.8 and 3.1% organic matter. Experimental areas were tilled in July the year before herbicide treatment to encourage purslane establishment. Preemergence experiments were conducted in 2011 (PRE2011) and twice in 2012 (PRE2012a and PRE2012b). Plot areas were seeded with 195 kg·ha of perennial ryegrass (Lolium perenne L.) in late September preceding herbicide application the next spring. The perennial ryegrass was killed with glyphosate at 3.1 kg a.i./ha 7 (± 3) d after experimental treatments were applied to limit competition and encourage purslane. Herbicide treatments for PRE2011 were applied 20 Apr. 2011. As a result of an unusually warm spring, herbicide treatments for PRE2012a were applied on 29 Mar. at a similar growing degree-day accumulation (300 ± 50 GDD base 10 C) as PRE2011 and a third experiment, PRE2012b, was applied on 24 Apr. 2012 (Fig. 1). For each experiment, nine PRE herbicides were applied once at maximum and half maximum label rates (Table 1) and watered in immediately after treatment with 2.5 mm irrigation. Herbicides were applied to plots measuring 1.5 · 1.5 m using a CO2 pressurized sprayer with three flat fan nozzles (LF8002; TeeJet Spraying Systems, Wheaton, IL) at 817 L·ha and 207 kPA. Purslane emergence was first noted on 2 June for PRE2011, 2 May for PRE2012a, and 14 May for PRE2012b. Percent purslane cover was visually estimated at 6, 8, and 10 weeks after treatment (WAT). Before analysis, percent of the control was calculated as 1 – (treated percent cover/untreated percent cover) *100 Eq. [1] for each of the rating periods. Plot areas were watered with 7.6 mm irrigation once every 2 weeks. Chlorantraniliprole at 0.28 kg·ha was applied on 21 June 2011 and 22 May and 14 June 2012 to prevent white-line sphinx (Hyles lineata) infestation. Postemergence experiments were conducted in 2011 (POST2011) and 2012 (POST2012). Plot areas were tilled in late April to control winter annuals or earlygerminating summer annuals and purslane was encouraged to naturally invade plot areas with light watering and hand-weeding of nonpurslane weed species. On 15 June 2011 and 4 June 2012, 25 different herbicides were applied at the high label rate to purslane plants with 10-cm or greater main stem length (Tables 2 and 3). Eight of the herbicides were applied both with and without the labelrecommended surfactants, and the surfactants were also applied alone as standards. Herbicides were applied to plots measuring 1.5 · 1.5 m using a CO2 pressurized sprayer with three flat fan nozzles at 817 L·ha and 207 kPa. Received for publication 30 Nov. 2012. Accepted for publication 7 May 2013. To whom reprint requests should be addressed; e-mail [email protected]. 902 HORTSCIENCE VOL. 48(7) JULY 2013 The PRE herbicide isoxaben was applied at 1 kg·ha on 17 June 2011 and 5 June 2012 over all treatments to limit emergence of new purslane after POST application. Percent purslane cover was visually determined at 1, 2, 3, and 4 WAT. Percent of the control was calculated using Eq. [1] as described previously. Plot areas were watered with 7.6 mm irrigation once every 2 weeks. Chlorantraniliprole at 0.28 kg·ha was applied on 21 June 2011 and 1 June 2012 to prevent white-line sphinx infestation. Treatments in all research trials were arranged as a randomized complete block with three replications. The PRE study was a nine· three · two factorial with nine herbicide treatments, three experimental runs, and two herbicide rates. The POST study had two factors, herbicide treatment and experimental run. Data were analyzed as a general linear model, which assumes homogeneous variance; however, variance for some factors such as year or experiment tends to be heterogeneous. Therefore, within each study (PRE or POST), variance for each experiment was modeled separately using PROC GLIMMIX in SAS (SAS Institute, 2009) to account for heterogeneous Fig. 1. Cumulative growing degree-days (GDD) using base 10 C model and soil temperatures recorded at 10-cm depth from 1 Jan. to 31 Aug. in 2011 and 2012 at the University of Nebraska–Lincoln John Seaton Anderson turfgrass research facility near Mead, NE. Herbicide application dates for the PRE2011, PRE2012a, PRE2012b, POST2011, and POST2012 studies are indicated. Table 1. Percent purslane control by experiment following preemergence herbicide treatments. Herbicide Purslane control Common name Half maximum