Long-term under-vine coverage by spontaneous vegetation changed plant community and soil dynamics without impacting yield at two South Australian vineyards

Under-vine management practices have traditionally consisted of herbicide applications or cultivation to manage weeds, thus limiting competition for resources with grapevines. However, increasing awareness of detrimental impacts of herbicides and cultivation on agricultural landscapes are shifting practices towards alternative options, including an unmanaged under-vine area where spontaneous vegetation proliferates. How this practice may affect the suite of soil, water, and nutrient relations and corresponding grapevine performance in a Mediterranean climate in the long-term is relatively unknown. Thus, this study utilised a unique opportunity to explore how undisturbed under-vine spontaneous vegetation compared to traditional herbicide (Eden Valley, loamy sand soil) or cultivation (McLaren Vale, silty loam soil) practices at two commercial vineyards. After five years of treatments, assessments were made on under-vine plant community and soil dynamics, grapevine responses, and juice composition. At Eden Valley, the spontaneous vegetation changed the plant community by increasing total plant coverage and biomass, while at McLaren Vale there was a lesser effect; however, the plant communities at both sites were similarly dominated by perennial grasses compared to higher proportions of broad-leaf, fast-growing species in the traditional herbicide and cultivation treatments. Soil water and nutrient availability were decreased by spontaneous vegetation in Eden Valley, while in McLaren Vale, the spontaneous vegetation enhanced soil phosphorus, water infiltration, and increased organic carbon by 40%. In Eden Valley, spontaneous vegetation decreased juice yeast available nitrogen; yet phenolics, anthocyanins, and total soluble solids were higher. At both sites, there was no significant impact of maintaining under-vine spontaneous vegetation on yields, however it decreased pruning weights at Eden Valley, while no effects were quantified on pruning weights or juice composition at McLaren Vale. We demonstrate that in the long-term, maintaining undisturbed spontaneous vegetation as an alternative to using herbicides or cultivation in the under-vine row can benefit the vineyard agroecosystem, while not adversely impacting grapevine yields depending on vineyard soil type and target winemaking outcomes.