Mulch and Organic Herbicide Combinations for In-Row Orchard Weed Suppression

Trials were established at two locations to evaluate weed control provided by mulches and organic herbicides alone or in combination. Mulch treatments included barley straw, pine wood chips, paper, and no mulch (bare-ground control). Herbicide treatments included three organically certified herbicides: lemon grass oil (14% v/v), clove oil (8% v/v), and two rates of acetic acid (10 and 15% v/v). Other treatments included pelargonic acid (7% v/v), glyphosate (1.06 kg ai/ha), and no-herbicide (control). Individual herbicide treatments were applied according to weed pressure and were determined every 7 to 10 days. Herbicide applications were made two and three times in 2009 at Logan and Kaysville, respectively. Treatments were carried out for a second year at both locations with herbicide treatments applied a maximum of four times. A third trial was established in 2010 at Kaysville with four application times. Weed control evaluations were made once a month, and weed population densities were evaluated at the beginning and end of the season. Weed biomass and percent weed cover were collected at the end of the 2010 season. Mulches each provided significant weed suppression in the first year, but each mulch presented unique challenges over time. For example, paper mulch effectiveness decreased over time as cracks developed during wet-dry cycles allowing weed seedlings to emerge. Volunteer barley became problematic in straw mulch plots in the first year of each trial, and annual weed seed germination on top of the wood chip mulch became problematic in the second year. The straw mulch consistently provided among the highest weed control and the least end-of-season weed biomass. Glyphosate and pelargonic acid gave near 100% weed control among all mulch treatments, including the non-mulched plots. Lemon oil and clove oil displayed the same weed control ratings as the no-herbicide treatment in the three mulches (straw, wood chip, and paper), but displayed 41-95% weed control when applied without mulch. While organically certified herbicides generally did not provide sufficient weed control alone, some mulches and organic herbicide combinations provided weed suppression similar to conventional herbicide application.