Field Evaluation of Herbicide and Mulch Combinations for Weed Control Efficacy

Weeds have been managed by manual weeding and /or herbicides for many years by the nursery and landscape industry. Increasing labor costs by hand weeding and environmental impact due to the use of chemicals have compelled scientists to look for alternate weed control strategies. Herbicide treated mulch is one alternative which can be a potential environmental friendly approach to control weeds for a longer period of time, while reducing the weed control costs and herbicides in the environment.  The objective of this study was to evaluate granular herbicides and mulch combinations at various depths of mulching compared to liquid formulations of herbicides combined with mulches.  An experiment was conducted at The Ohio State University, Columbus, Ohio with two types of mulches, hardwood and pine nuggets, at different depths (1, 2.5, and 5 inches) combined with two formulations of trifluralin +isoxaben [a liquid and a granular (Snapshot)] in a randomized complete block design.  Two separate experiments were replicated in time; one experiment was started in fall 2006 and repeated in fall 2007 and another experiment was started in spring 2007 and repeated in spring 2008. The Snapshot was applied directly on the top of the mulch; the liquid formulation was applied below or above the mulch or used to pretreat the mulch.  There were total of 35 treatments including untreated control, mulch alone without herbicide, and herbicide alone without mulch, and two commercially available mulches that came premixed with a herbicide.  Evaluations for weed control efficacy were done at 30, 60, 90, and 120 DAT for spring start experiments and 30, 180 and 210 DAT for fall start experiments using visual ratings. Visual ratings were based on a scale of 0 (no control) to 10 (complete control), with 7 and above commercially acceptable.  Herbicide residue analysis was also done using gas chromatography and high pressure liquid chromatography techniques.  One inch depth mulch either alone or in combination with herbicides in all the experiments did not show consistency in weed control. There were also no consistent differences between 2.5 inches depth mulch and 5.0 inch depth mulch alone or in combination with herbicide. Pine nuggets had better weed control efficacy compared to Hardwood mulch. Soil analysis for herbicide residues revealed that mulches act as slow release carriers for herbicides for longer period of time compared to the herbicides applied alone treatments.