2019 ASHS Annual Conference

Perennial and annual broadleaf and grass weeds can greatly impact yield and labor costs of Hawaii green onion (Allium fistulosum) farms. Misuse of post-emergent herbicides can result in developed resistance by weed species as well as crop injury. Thus, local farmers expressed a need for pre-emergent herbicide recommendations. Three pre-emergent herbicides were evaluated against hand-weeding and an untreated control in a randomized complete block design. Herbicides included pendimethalin (Prowl H₂O, max rate=2 pints/acre), dimethyl tetrachloroterephthalate (Dacthal Flowable, max rate=14 pints/acre), and oxyfluorfen (GoalTender, max rate= 1 pint/acre). Green onion seedlings (Koba variety, 2 weeks old) were transplanted into prepared beds on September 13, 2017. Each treatment plot was replicated four times, separated by 1 ft between plots, and consisted of four rows, 14 seedlings per row, and 8 inch spacing between rows and seedlings. Pre-emergent herbicide treatments were applied immediately after transplanting at the maximum allowable rates. Hand-weeding of all weeds was conducted in specified plots 3 weeks after planting. Above-ground yield was measured and weed severity visually assessed within a 9 sq ft area for each plot at 6 weeks after planting. A five-point scale of severity consisted of 0=no weeds, 1=1-25% weed cover, 2=26-50%, 3=51-75%, and 4=76-100%. Weed species were also identified and tallied per plot. Data analyses included One-Way ANOVA with Tukey Comparison using Minitab 18.0. Oxyfluorfen was the superior pre-emergent weed control product. Oxyfluorfen provided the statistically greatest weed control (average weed score= 0.25; P<0.0005), followed by pendimethalin and hand-weeding (score= 2, 2.5). Untreated and dimethyl tetrachloroterephthalate (DPCA) treatments were completely overgrown with weeds at harvest (score= 4, 3.5). All four weed treatments produced greater yields than the untreated plots (P<0.0005), but oxyfluorfen (average yield= 1.43 lbs) exhibited significantly greater yields than DPCA and untreated plots (average yields= 0.98, 0.54 lbs). Effective treatments controlled weed species such as Amaranthus viridis, Hypochaeris radicata, Lepidium didymum, Portulaca spp., Sida spinosa, and Panicum spp. of grasses. Only oxyfluorfen was able to completely control Pyllanthus niuri, with pendimethalin and hand-weeding controlling to lesser extent. Oxyfluorfen and pendimethalin were also the most cost-effective treatments at $23 and $13 per acre, respectively, while DPCA and hand-weeding were ten- or hundred-times more expensive at $354 and $2823 per acre. Utilization and even rotation of oxyfluorfen and pendimenthalin pre-emergent herbicides can provide Hawaii green onion farmers with an effective and inexpensive means to control perennial and annual weeds.