ASHS 2024 Conference

Plastic mulch films and geotextile landscape fabrics are not compatible with specialty crops grown at high densities with narrow in-row spacing. As a result, many growers resort to hand weeding these specialty crops, including leafy greens, carrots, and matted-row strawberries. Our objective was to develop and test value-added weed barrier mulches designed specifically for these crops to grow on and root through. We have conducted 13 trials across 3 crops (lettuce, carrot, and strawberry) and 3 years to evaluate 9 different biobased, compostable weed barrier mulches (BCMs) that include polylactic acid (PLA) resin. The BCMs tested varied in weight basis and material composition including: 1) PLA-only (80, 120, and 180 g/m2), 2) PLA embedded organic fertilizers (soybean meal and composted turkey litter; 150 and 210 g/m2), and 3) PLA paper (105 and 210 g/m2). The BCMs are installed in the field like a typical mulch film roll but are then covered with a 1 to 2.5 cm layer of compost (or other weed-free media) and crop seeds. Data was collected to evaluate effects of BCMs on weed density, soil nitrogen availability, moisture, and temperature, microbial abundance, and crop establishment, yield, and quality. Across all crops and site-years, BCMs reduced weed emergence by 80% to 97% compared to bare soil, and weed suppression was best in PLA paper. The BCMs immobilized some available soil nitrogen and embedded fertilizers in the BCM did not affect this outcome. In carrot, BCMs increased mycorrhizal fungi abundance by up to 59% compared to bare soil. In strawberry, BCMs increased crop canopy area by up to 51%. Yields were usually not different between BCMs and bare soil because weeds were removed weekly after counting. However, poor seedling establishment on BCMs due to heavy rainfall after planting in 2022 reduced carrot yield by 13% and lettuce yield by 20%. To address this issue, lettuce in 2023 was planted in two shallow seed furrows filled with compost on the BCM (instead of broadcast planting across the entire BCM bed top as in previous years). Lettuce establishment in the furrows was two times greater than in bare soil due to improved tilth and reduced erosion, and yield was not different from bare soil. Volumetric soil moisture in BCM compost seed furrows was 27.1% compared to 35.7% in bare soil, highlighting differences in texture and the importance of irrigation in the BCM system during establishment.