Preemergence Herbicides Applied in February Are Less Effective Than Herbicides Applied in April for Smooth Crabgrass Control in New Jersey

Previous research indicates November-applied dithiopyr, pendimethalin, or prodiamine provide adequate crabgrass control in Indiana, Illinois, Maryland, Pennsylvania, and Rhode Island (Bhowmik & Bingham, 1990; Dernoeden, 1993; Reicher & Throssell, 1993; Reicher et al., 2011). The few studies that directly compared spring and autumn applications in the northern U.S. were conducted in USDA Hardiness Zone 5 (research presented herein was in Zone 7). These studies determined pendimethalin, dithiopyr, and prodiamine were sometimes less effective when applied in November than in the spring (Fermanian & Haley, 1994; Reicher & Throssell, 1993; Reicher et al., 2011). More conclusively, Reicher, Sousek, and Gaussion (2014) found single applications of dithiopyr, prodiamine, and pendimethalin were less effective when applied in fall or late winter compared with the spring in Nebraska. Lawn care operators often begin applying preemergence herbicides in late winter after soils thaw. Other than the work of Reicher, Sousek, and Gaussoin (2014), late winter applications (February) have received limited investigation, especially in split application programs. The objective of this research was to evaluate smooth crabgrass control from common preemergence herbicides in single and split application programs initiated during late February and April. Four replicate experiments were initiated in February 2017 and 2018 in North Brunswick and approximately 25 miles southeast in Freehold, NJ on simulated perennial ryegrass lawns mowed weekly at 1.5 to 2 inches. Crabgrass cover was >95% in non-treated plots by 1 August at both locations. The North Brunswick site was a Nixon sandy loam with a pH of 5.5 and 2.3% organic matter at Rutgers Horticulture Farm No. 2, and the Freehold site was a Holmdel sandy loam with a pH of 6.3 and 3.0% organic matter at the Rutgers Adelphia Research and Extension Center. Sites were irrigated to prevent moisture stress. No fertilizers or fungicides were applied. Four application programs and three herbicides were evaluated in a two-way factorial treatment design with a non-treated control. Herbicides consisted of dithiopyr (0.5 lb acre−1, Dimension 2EW), prodiamine (1.0 lb acre−1, Barricade 65WG), and pendimethalin (3.0 lb acre−1, Pendulum AquaCap). Each herbicide was evaluated in four different application programs: (1) a single application in February, (2) a single application in April at full bloom of Forsythia, (3) a split application program of February followed by June, and (4) a standard split application program of April followed by June. Split programs applied half of the total herbicide rate at each application. Application dates and environmental conditions are presented in Tables 1 and 2 and Figures 1 and 2. Herbicides were applied with standard CO2–powered small-plot spray equipment using water carrier at 44 gal acre−1 through a 9504EVS nozzle and overhead-irrigated with 0.25 inches of water within 6 h. Plots measured 3 × 7 ft and were separated with a 1-ft non-treated border. Treatments were replicated four times and arranged in a randomized complete block design. Smooth crabgrass control was evaluated visually on a 0% (no control) to 100% (complete control) scale relative to non-treated areas. North Brunswick observations were arcsine square root transformed for the analysis of variance (ANOVA) and post-ANOVA mean separation; non-transformed means are presented. Data were subjected to ANOVA in SAS (v9.4) using the GLIMMIX procedure. Means were separated using Fisher's Protected LSD when F tests were significant at α = 0.05. Location-by-treatment interactions were significant and year-by-treatment interactions were not; thus, data from each location are presented separately and combined across years. Treatments generally provided less control in Freehold than in North Brunswick, which we partially attribute to higher soil organic matter in Freehold (2.3 and 3.0%, respectively). In North Brunswick, only application timing was significant. A single application in February provided less control than a single application in April (Table 3). Split applications initiated in February provided more control than single February applications but less control than split applications initiated in April. In Freehold, the herbicide-by-timing interaction was significant. Among single applications in February and April, prodiamine provided more control than dithiopyr and pendimethalin (Table 4). A single application of prodiamine provided less control than split applications of prodiamine or dithiopyr. All three herbicides provided similar crabgrass control (84 to 95%) in split application programs. Similar to our research, Reicher et al. (2011) determined split applications provided more control than November only applications. These experiments indicate preemergence herbicides are less effective if applied in February than in April during Forsythia full bloom. If logistics necessitate application several weeks prior to smooth crabgrass emergence, a split-application program (making the second application 6 wk after Forsythia full bloom) will substantially improve control. If crabgrass pressure is high or poor control was observed in the past, split applications may provide more control than well-timed single applications. If only a single application is practical, application timing should be at Forsythia bloom. The authors would like to thank Stephanie Alea, Alex Coward, and Benny Tran for assistance with this research. The Rutgers Center for Turfgrass Science provided financial support.