Technical Yearbook 2024

MARCH

Under-vine living mulches By Etienne Terblanche

Broadleaf annuals are gaining popularity and are frequently being used as under-vine living mulches. Cover cropping in the vineyard work row is an established practice in South African vineyards. The numerous advantages are well documented and stretch from increased-soil carbon and water infiltration to weed suppression and decreased surface temperatures and evapotranspiration. Thorough research by the late Dr Johan Fourie laid the groundwork for guidelines for cover crop cultivation, highlighting the importance of local soil and climatic conditions and desired crop service on cover crop choice and management strategy. Producers have since recognised the various benefits and are now turning to under-vine/living mulches to further reduce chemical inputs and leverage cover crop advantages. Try-outs on the producer level are yielding promising results, showing that in suitable conditions, the correct species of cover crop can effectively suppress the growth of weeds and contribute to soil improvement. Mode of action Actively growing cover crops compete with weeds for water and nutrients during winter and spring. Cover crops on the vine row suppress weed seed germination by blocking sunlight from reaching the soil surface. Cover crop residues remaining after natural senescence or termination suppress weeds during summer when retained as a mulch. This mulch acts to reduce evaporation from the soil surface. In addition, by using nitrogen-fixing species which deposit nitrogen in the grapevine rootzone the vineyard inorganic N-fertiliser requirement is reduced. As indirect benefits, flowering cover crops also provide habitat for beneficial insects and have been shown to increase the microbial biomass in the soil and, therefore, advantageous in terms of general soil health. Establishment Under-vine living mulches should ideally be established in March or early April to harness the last heat before autumn and winter set in. This also ensures that your cover crop gets a head start before the rainy season begins and competition from autumn weeds starts. Preferably, seeds should be planted directly under the irrigation emitter (drip or micro sprinkler) to improve the chances of germination. From experience, a surface free from actively growing weeds allows for better initial growth. Due to the creeping growth habit of recommended species, the vine berm is soon colonised by the

living mulch. The living mulch will actively suppress weeds, and in the case of ineffective suppression, selective herbicides could help eliminate the competition. Using grass-specific herbicides in broadleaf cover crops is very effective, while manual removal of large weeds is also effective. As cover crops naturally near the end of their life cycle, they form a dense mat that suppresses summer weeds and conserves soil moisture. In vineyards that are frequently irrigated, the life cycle is generally extended compared to supplementary irrigated vineyards. It is important to note that one could manage cover crop and vineyard water needs by differentiating between superficial (short) or deep (long) irrigation cycles. If the cover crop is allowed to complete its full cycle, a large number of seeds are produced, which will aid a self-seeding process for the seasons to come. Preferably, one would sow annually for the first two to three years to ensure complete coverage before relying solely on hardy dry material as well as re-seed effectively is very important. Viable options (especially in winter rainfall regions) include medics and subterranean clover. These broadleaf cover crops can be established during early autumn before the first rain arrives with the help of drip or micro irrigation. Medics work well in this regard by aggressively spreading via stolons. Therefore, even if only sown underneath the emitters, it can still easily cover the vine berm. Where sufficient moisture is present, perennial options like white clover perform well and provide year round cover. White clover has a low growing and creeping growth habit and spreads with stolons while also providing prominent flowers that attract beneficial insects. The existing trellis system, specifically the cordon height, will also dictate the choice of cover crop; in the case of higher trellising systems, aggressive climbers such as vetch can be considered. In contrast, crops with low growth habits, such as subterranean clovers, should be considered for low trellis systems where living mulches can easily climb into the fruiting zone. On a practical level, living mulches can be rolled, dragged mechanically or simply flattened through human trampling, e.g. when pruning or suckering. Special considerations • Cover crops, especially during the seedling phase, are susceptible to pests such as the Red-legged earth mite natural re-seeding. Choice of species In South Africa, the first and foremost consideration in determining the ideal species is soil moisture availability. This will dictate if an annual or perennial cover crop is required. In the case of annual, its ability to create sufficient/

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TECHNICAL YEARBOOK 2024