Mulch Type and Depth, Herbicide Formulation, and Postapplication Irrigation Volume Influence on Control of Common Landscape Weed Species

Abstract

<jats:p>Mulch is often applied in landscape planting beds for weed control, but little research has focused specifically on mulch and preemergence (PRE) herbicide combinations. The objectives of this research were to determine the efficacy of herbicide + mulch combinations and which factors significantly affected weed control, including herbicide formulation and posttreatment irrigation volumes. Additional objectives were to determine efficacy derived from mulch or herbicides used alone under herbicide + mulch combinations and to identify differences in the additive (herbicide + mulch combinations) or singular (herbicide or mulch) effects compared with the use of herbicides or mulch only. Large crabgrass (<jats:italic>Digitaria sanguinalis</jats:italic>), garden spurge (<jats:italic>Euphorbia hirta</jats:italic>), and eclipta (<jats:italic>Eclipta prostrata</jats:italic>) were used as bioassay species for prodiamine, dimethenamid-P + pendimethalin, and indaziflam efficacy, respectively. The experiment consisted of a factorial treatment arrangement of two herbicide formulations (granular or spray applied), three mulch types [hardwood chips (HWs), pine bark (PB), and pine straw (PS)], two mulch depths (1 and 2 inches), and three levels of one-time, posttreatment irrigation volumes (0.5, 1, and 2 inches). Three sets of controls were used: the first set included three mulch types applied at two depths receiving only 0.5-inch irrigation volume, the second set included only two herbicide formulations and three one-time irrigation volumes, whereas the last set received no treatment (no herbicide or mulch) and only 0.5-inch irrigation volume. High levels of large crabgrass and garden spurge control (88% to 100%) were observed with all herbicide + mulch combinations evaluated at mulch depths of 1 inch or greater. When comparing mulch types, the best eclipta control was achieved with hardwood at 2 inches depth. The spray formulation of indaziflam outperformed the granular formulation in most cases when used alone or in combination with mulch. Overall, the results showed that spray formulations of prodiamine and dimethenamid-P + pendimethalin were more effective than granular formulations when applied alone, whereas indaziflam was more effective as a spray formulation when used both alone and in combination with mulch. Increasing irrigation volume was not a significant factor for any of the herbicide + mulch combinations when evaluating overall weed control.</jats:p>

Short-term Preemergence Herbicide Adsorption by Mulch Materials and Impacts on Weed Control

“…Herbicides were chosen because they are commonly used in both landscape beds and in nurseries and represent three commonly used herbicide mechanisms of action used in and around ornamental plants in Florida (Neal et al, 2017). These herbicides and mulch types also have been evaluated previously for weed efficacy, both when in combination and when used individually over a longer-term study (Saha et al, 2019). Indaziflam, prodiamine, and dimethenamid-P + pendimethalin were applied at labeled rates (0.04, 1.5, and 1.5 + 2.0 lb/acre, respectively) to mulched containers using a carbon dioxide backpack sprayer (Bellspray; R&D Sprayers, Opelousas, LA) calibrated to deliver 20 gal/acre using a flat-fan nozzle tip (8004; TeeJet Technologies, Wheaton, IL) at 30 psi on 6 June 2018 (run 1) and 12 July 2018 (run 2).…”

“…Eclipta was used for indaziflam, large crabgrass for prodiamine, and garden spurge for dimethenamid-P + pendimethalin. These weed species were chosen because they are problematic in nurseries and landscapes in Florida and have previously shown a high degree of sensitivity to the aforementioned herbicides (Neal et al, 2017;Saha et al, 2019). The experiment was a completely randomized design with six single pot replicates per herbicide and mulch combination and the nontreated, nonmulched control group.…”

“…straw mulch and reported that although soil concentrations of metolachlor were significantly lower when applied in the presence mulch, weed control increased with increasing mulch level due to the weed suppression ability of the mulch. In ornamental production, synergistic herbicide and mulch interactions have been reported, as mulch may act as a slow-release herbicide carrier, extending the longevity of weed control, or provide control of weeds when mulch alone or mulch depth is insufficient (Case et al, 2002;Mathers and Case, 2010;Saha et al, 2019). However, observed synergism with herbicides and mulch typically results when mulch is applied at an adequate depth, usually more than 2 inches (Marble, 2015).…”

“…This commonly occurs in nurseries, as containers are completely filled with potting media, and depths of 2 to 3 inches may be cost prohibitive or difficult in large landscapes. As many common landscape and container weed species can emerge through mulch depths of 2 inches or more, herbicide adsorption could decrease control if depths of 2 inches or less are used (Richardson et al, 2008;Saha et al, 2019;Teasdale and Mohler, 2000). Second, mulch is often lost in container plants from blow over during weather events or moving containers during normal operations.…”

“…Consequently, significant adsorption could lead to poor weed control if mulch were removed from containers. Most research on mulch and herbicide use in ornamentals has focused on long-term efficacy of various mulchherbicide combinations (Bartley et al, 2017;Case et al, 2005;Mathers and Case, 2010;Saha et al, 2019). For landscapes and nursery scenarios, it is unknown how much herbicide is immediately adsorbed or intercepted by mulch or how efficacy would be affected if mulch levels were not sufficient for weed control or if mulch was lost soon after application.…”