Cover crops are an important component of integrated weed management programs in annual and perennial cropping systems because of their weed suppressive abilities. They influence weed populations using different mechanisms of plant interaction which can be facilitative or suppressive. However, the question often arises if cover crops can be solely relied upon for weed management or not. In this review we have tried to provide examples to answer this question. The most common methods of weed suppression by an actively growing cover crop include competition for limited plant growth resources that result in reduced weed biomass, seed production, and hence reductions in the addition of seeds to the soil seedbank. Cover crop mulches suppress weeds by reducing weed seedling emergence through allelopathic effects or physical effects of shading. However, there is a great degree of variability in the success or failure of cover crops in suppressing weeds that are influenced by the cover crop species, time of planting, cover crop densities and biomass, time of cover crop termination, the cash crop following in the rotation, and the season associated with several climatic variables. Several studies demonstrated that planting date was important to achieve maximum cover crop biomass, and a mixture of cover crop species was better than single cover crop species to achieve good weed suppression. Most of the studies that have demonstrated success in weed suppression have only shown partial success and not total success in weed suppression. Therefore, cover crops as a sole tool may not be sufficient to reduce weeds and need to be supplemented with other weed management tools. Nevertheless, cover crops are an important component of the toolbox for integrated weed management.

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.