2019 ASHS Annual Conference

Mulch, preemergence herbicides, or a combination of these two materials may be used in landscape planting beds for weed management. Several types of mulch materials and crop residues have been shown to intercept and bind with preemergence herbicides and reduce control in cropping systems. This has led to some debate as to whether or not preemergence herbicides can or should be used in combination with common organic mulch materials, and which mulch material is most compatible with preemergence herbicides. Experiments were conducted in 2018 to assess herbicide movement through three mulch types including pinebark, pinestraw, and hardwood. Weed species evaluated were large crabgrass [Digitaria sanguinalis (L.) Scop.], garden spurge (Euphorbia hirta L.), and eclipta [Eclipta prostrata (L.) L.]. Liquid formulations of prodiamine, dimethenamid-P + pendimethalin, and indaziflam were applied to soils that were either mulched with the above listed materials at a depth of 5.1 cm or left non-mulched. After herbicide treatments were applied, 3.8 cm of irrigation was applied over 3 days and then all mulch was removed and weed seeds were sown to determine herbicide movement through each mulch type. Data collection included weed counts and biomass determination at 4 weeks after seeding. Control of each weed species in pots that were originally mulched was then compared to control of each weed species that contained no mulch at the time of application. Results from seeding studies showed only 67% eclipta control was observed in pots originally mulched with hardwood, while pinebark and pinestraw had no effect on efficacy. Large crabgrass data showed that pinebark (65% control) was the only mulch type that caused a significant reduction in prodiamine efficacy. Dimethenamid-P + pendimethalin efficacy on garden spurge was reduced in pots originally mulched with hardwood or pinebark, but all treatments provided ≥94% control. While differences were observed with all preemergence herbicides evaluated, pinestraw was the only mulch material that did not decrease the control of any herbicide. Based on the results of this study, pinestraw may be a more compatible mulch material for use in combination with preemergence herbicides.