Do native and introduced cover crops differ in their ability to suppress weeds and reduce seedbanks? A Case study in a Table Grape Vineyard.

Cover cropping is considered a valuable tool for integrated weed management. However, weed suppression by cover crops can be inconsistent. We hypothesized that a native cover crop species may have greater capacity to suppress weeds than an introduced species owing to adaptive advantages. A study was conducted from 2020 to 2022 in a newly-established Autumn King tablegrape vineyard in Parlier, CA. Two cover crop treatments, a native plant species, phacelia (Phacelia tanacetifolia), or an introduced species, rye (Secale cereale L. Merced), were compared to a no cover crop, herbicide-managed control for impacts on weed control. Cover crops were sown in 2019 in the center 1.8 m of the 3.6 m space between the grapevine rows. The experimental design was a randomized complete block with four replications. Percent cover, weed density, and weed biomass in different seasons were assessed in the inter-row spaces and the vine rows. Soil samples were collected in 2021 from the grape row and interrow spaces to assess the weed seedbank. After the cover crops were terminated, percent weed cover was lower in the interrow of the phacelia compared to the no cover crop plots at two sampling times. When cover crops were actively growing (winter/spring), phacelia plots had a 7-fold higher cover crop to weed biomass ratio compared to that of the rye plots, thus indicating greater weed suppression. However, weed seedling emergence from the seedbank samples of the cover crop plots were 2- to 4-fold greater (in the phacelia and rye, respectively) than from the no cover crop plots. In summary, phacelia suppressed weeds more than rye; however, they both resulted in a bigger weed seedbank size compared to the herbicide-managed no cover crop plots, so additional management practices will be needed for long term weed management with cover crops in vineyards.