CONTROL OF DIFFICULT WEEDS FOR CALIFORNIA STRAWBERRY PRODUCTION

California leads U.S. strawberry fruit production, with annual value in Ventura County alone over $ 350 million. Weeding costs range from $ 750 to $ 1,750/ha even after methyl bromide fumigation. Some alternative fumigants provide limited control of weeds with hard seed coats such as little mallow (Malva parviflora) and clovers (Medicago spp., Melilotus spp.) or perennial species such as yellow nutsedge (Cyperus esculentus). In eight studies, oxyfluorfen at half-rate provided 89-100% control of little mallow and most other broadleaf weeds, but a full rate was needed for 45-95% control of yellow sweetclover (Melilotus officinalis); oxyfluorfen did not control yellow nutsedge. A paper layer composed of recycled newspaper and gypsum placed under black plastic completely inhibited yellow nutsedge germination in strawberry beds in fall 2006 that otherwise germinated through plastic alone at a density of 6 plants/m². The paper deteriorated and nutsedge germinated in spring, however, prompting further investigation of other mechanical barriers in 2007. Additionally, wind-dispersed seeds from horseweed (Conyza Canadensis) and annual sowthistle (Sonchus oleraceus) can germinate in planting holes and furrows after fumigation. Flumioxazin and oxyfluorfen completely inhibited horseweed and sowthistle germination when applied 30 days pre-plant to bed tops and did not injure strawberry. Flumioxazin and oxyfluorfen/napropamide reduced wind-dispersed weeds by 84-95% at 4 weeks after application in the furrows and by 68% during the following 8 weeks, reducing weeding time by 50% or more. A simulated drift study showed severe strawberry fruit injury for 2 weeks after flumioxazin contacted fruiting plants, suggesting that seed landing on bed tops after strawberry transplanting currently cannot be chemically controlled without crop injury; these weed species growing in areas adjacent to the production field should therefore be destroyed before flowering. These studies showed that an integrated approach utilizing chemical and mechanical methods can manage weeds that are not controlled by fumigation.