Undervine cover cropping for healthy and productive soils

Abstract

This project aimed to better understand the mechanisms behind the undervine cover crop and grapevine interactions in the vineyard. Experiments were established at three vineyard sites across South Australia: Nuriootpa, Langhorne Creek and at Monash in the Riverland. All cover crops reduced soil salinity at the Nuriootpa site, and did not induce water stress during hot conditions relative to the herbicide control. Cover crops that included Medicago sp. and annual grasses added nitrogen to the soil and increased carbon concentration and did not incur yield reductions relative to the herbicide or straw mulch treatments. Overall, this project has shown that in the Barossa and Langhorne Creek, undervine cover crops (with appropriate species selection) will perform as well or better than herbicide-based weed management.

This report only relates to the first two years of the Riverland experiment, which included mechanical weed control. The study demonstrated the effectiveness of Medic and annual Ryegrass to generate significant undervine biomass without use of additional water. After two years of data collection, most differences among treatments were not statistically significant, and further work is necessary to understand the full, long-term consequences of the tested practices.

Summary

Within vineyards, knowledge of undervine management practices remains largely under-developed owing to the relative paucity of data. It generally requires considerable resources to remove weeds that can generate competition with the vine for soil moisture and nutrients. Current standard practice sees growers use regular herbicide applications to maintain a bare undervine area, while very few growers allow plant growth in this space. Over time, this has been shown to deplete the soil of many important resources and can lead to soil compaction and hydrophobicity. An ideal agricultural system should be able to generate its desired product, while utilising the soil resources through the growing season, while replenishing them in the dormant season.

Across the 2019 and 2020 vintages, to test the effects of selectively growing a variety of grasses and pasture crops in the undervine space, an initial trial was designed and implemented in two South Australian wine regions; the Barossa Valley (Nuriootpa) and Langhorne Creek. Simultaneously, another trial in the Barossa Valley (Tanunda) explored the potential benefits of biofumigant crops on nematode abundance. Lastly, across the 2022 and 2023 vintages a fourth trial investigated cover-cropping and other weed management options in a Riverland vineyard. The goal of this extension was to test the effect of undervine cover cropping and mechanical weeding in a semi-arid, extreme environment.

At the Barossa Valley and Langhorne Creek trial sites, a variety of industry-standard vineyard-floor management practices were selected for testing. These included an exclusive legume treatment: Medic (Medicago); one containing a combination of Medic and annual Ryegrass (Lolium rigidum); a combination of the vigorous grass Fescue (Festuca) and the legume Strawberry Clover (Trifolium); the vigorous grass Kasbah (cv. Cocksfoot, Dactylis); and the native species, Wallaby Grass (Rytidosperma geniculata). These undervine cover crops were compared to both a straw mulch (Triticale) application and a herbicide-sprayed control (bare earth). Over two growing seasons, and building on the establishment of these trials four years prior, data was collected on the soil nutrients, soil moisture, soil hardness and soil microbiomes (including mycorrhizal fungi), vine nutrient status, canopy size, vine yield and berry chemistry.

By providing an active mulch layer on the soil surface which would act as a weed suppressant, the treatment species selected were expected to have a number of broad beneficial effects:

• reduced water evaporation
• improved soil carbon
• increased soil nitrogen (in leguminous cover crops); and
• increased soil biodiversity.

In addition to these benefits, the ideal cover crop should also provide:

• no reduction in yield
• no reduction in grape quality;
• no increase in watering; and
• no increased cost of management.

The third trial site in Tanunda, tested the effectiveness of biofumigant undervine species to control nematode populations. After two seasons of failed attempts to establish biofumigant cover crops, greenhouse generated 'Nemcon' biomass was incorporated into soils at the trial site. No effects on nematode populations were observed, though due to the limitations in establishing the cover crops these results are inconclusive and may yield different results at sites with differing soils.

Finally, using soil from the Langhorne Creek trial site, impacts of the cover crops on soil arbuscular mycorrhizal fungi (AMF) was also investigated. Findings from this aspect of our project suggest that soil inoculum potential is unchanged by presence of cover crops vs herbicide control.

Overall, the outcomes generated from 2018 to 2020 at Nuriootpa and Langhorne Creek show that selection of species is critical to achieving the requirements of an undervine cover crop. While the perennial grasses were included in the project because they were genetically programmed for summer dormancy, they continued to remain actively growing and competing with the vines throughout the season, and so were determined unsuitable for a role in the undervine space. By contrast, the annual Medic and Ryegrass species, which have been bred as pasture plants to thrive in the Mediterranean environment, fitted into a functional role within the vineyard environment by providing all the benefits noted above. The contribution of the legumes to supply soil nitrogen, and the grasses to ameliorate soil structure, appears critical to enabling the balance to swing in favour of the provision of soil cover and its associated additional water use, compared to the maintenance of a bare undervine strip. The Medic + Ryegrass species mix increased soil carbon concentration by 64% without compromising grape yields or using more water.

At the Riverland (Monash) site in 2022 and 2023, the limited timespan of the trial yielded few significant differences among treatments, though trends similar to those in the earlier trials may be present. Extension of this trial beyond 2023 is highly recommended as treatment effects on soil continues to accumulate.

Despite these positive findings, and the low cost of implementation, there remains considerable grower resistance to adopting these practices. It therefore requires a paradigm shift in growers' perceptions of the vineyard aesthetic. The 'untidy' nature of an undervine cover crop, as it cycles through maturity and senescence with minimal intervention, may not fit with traditionally-held perceptions of the archetypal vineyard. Just as it has taken, in some regions, many years for growers to stop cultivation of the mid-row, it will require a concerted effort to encourage them to move on from herbicide use and enable the ecosystem services that cover crops provide to perform the same function in vineyard floor management.