South Africa Wine Technical Yearbook 2025

Under-vine cover crops – risks and rewards Even the space directly under the vines – usually kept weed-free – is getting a regenerative rethink. While under vine cover crops may increase humidity and raise disease concerns, studies have found they can also improve soil porosity, reduce compaction, and increase active carbon and microbial activity. Notably, cooler soil temperatures under cover crops may help protect vines during heat waves. There’s even potential to harness under-vine cover crops as hosts for beneficial arbuscular mycorrhizal fungi (AMF), which can support vine nutrient uptake and resilience. In low-rainfall regions, careful management of soil moisture is crucial, as cover crops can compete with vines for water and nutrients, potentially causing vine stress. Managing termination – when and how matters While RV aims to maintain living roots as long as possible, there are practical reasons for terminating cover crops, especially to control humidity and competition for water. How growers end a cover crop’s lifecycle – mechanically, chemically, or through grazing – has a big influence on soil outcomes, such as weed suppression achieved by a cover crop, as well as its impact on soil health parameters. Mechanical methods like tillage can boost short-term nitrogen levels by rapidly decomposing plant matter, but they also risk disrupting soil structure, releasing carbon, damaging microbial networks and increasing erosion. Tillage termination of cover crops often results in higher soil water content than roller methods, likely due to more effective crop kill and reduced transpiration. However, it is important to recognise that the effects of cover crop incorporation on soil health are influenced by factors such as soil type, climate, and the specific cover crop species used. On the other hand, methods like flail mowing are gentler on the soil and have been linked to higher microbial biomass and nutrient cycling. Roller-crimping, which flattens the crop into a mulch, conserves moisture, suppresses weeds, and gradually releases nutrients back into the soil. Mulching also helps cool surface temperatures and reduce evaporation – a bonus for water-scarce regions. Timing also matters. Letting cover crops grow longer before termination, such as until grapevine budburst, has been shown to significantly increase soil microbial biomass, thanks to the added root exudates and biomass. Ultimately, the success of cover crops in RV depends on careful selection of species, management of water competition, and thoughtful termination strategies suited to local climates and soil types. Vine performance, yield and wine quality – does greener soil mean better grapes? The big question on many growers’ minds when considering cover crops is: ‘Will it be detrimental to my yield or wine quality?’ The good news is research generally says no – at least when cover crops are well-managed.

In fact, cover crops can be a valuable tool in vineyards, especially in fertile or high-rainfall regions where excessive vine growth can be problematic. Too much vigour leads to dense canopies and shaded grape clusters, which reduces airflow and increases the risk of diseases like Botrytis cinerea . By soaking up some of the available water and nutrients, cover crops can help rein in this excess growth, creating a more balanced vine canopy and improved fruit exposure – key factors for ripening and grape quality. However, the relationship between cover crops and vines isn’t always straightforward. In dry or Mediterranean climates, water can become a limiting factor. One Italian study showed that increased competition from cover crops reduced vine water-use efficiency, meaning the vines had to work harder for each unit of carbon gained through photosynthesis. Without supplementary irrigation, under vine cover crops in such areas have sometimes led to smaller berries and lower pruning weights – although sugar levels and acidity remained stable. Interestingly, mild water stress can boost concentrations of anthocyanins and phenolic compounds, potentially enhancing wine colour and complexity. But it comes with a trade-off: lower yields. Some studies report neutral or even positive yield effects from cover crops. For example, in Uruguay, a permanent cover of Festuca arundinaceae didn’t negatively impact grape yield or composition. Similarly, Festuca rubra in another trial helped moderate vigour and resulted in berries with higher sugar (TSS) and anthocyanin levels – though these treatments did rely on extra irrigation. When additional irrigation was not supplied, under-vine cover crops have resulted in lower pruning and berry weights, but TSS and total acidity were unaffected. Cover crop selection in vineyards must account for local water availability and the crop’s water needs. In some studies in organic vineyards, spontaneous ground covers and sown cover crops outperformed bare, tilled alleyways in terms of yield, showing that the choice of control treatment can dramatically influence study outcomes. Boosting vineyard biodiversity – life beneath and beyond the vines Beyond grapes and yields, cover crops play a critical role in enhancing functional biodiversity – another cornerstone of RV. Flowering species like clover and phacelia attract a rich array of pollinators and beneficial insects. While grapes are self-pollinated, these insects support broader vineyard ecology and pollinate other nearby crops and wild plants. Cover crops also serve as homes and hunting grounds for natural enemies of common vineyard pests. Research has documented increased populations of predatory beetles, parasitic wasps, and other beneficial arthropods in cover cropped vineyards. For example, flowering summer covers like buckwheat and sunflower have been shown to reduce

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TECHNICAL YEARBOOK 2025