The under-vine zone of the vineyard contains the greatest concentration of vine roots, with management directly impacting vine yield, quality and profitability. The removal of weeds involves regular use of herbicides or cultivation, neither of which is recognised as best practice soil management. This project investigated the growth of cover crops below the vine which are both beneficial to the soil and the vine while competing with or suppressing the growth of weeds. At four South Australian sites, annual pasture legume and grass cover crops generated yields and financial gross margins equivalent to or greater than the herbicide controls.
The typical view of a vineyard shows the area between the vines to be covered with vegetation that is mown to prevent excessive growth, which keeps it looking respectable while reducing the loss of soil moisture. The area under the vines is maintained in a bare state with herbicides, as weeds can otherwise grow into the vine canopy while utilising soil moisture that should be available to the vines. The maintenance of this soil in a bare state is however not in line with best practice soil management, prompting a research program to develop an environmentally benign alternative system which does not result in a financial impost through reduced productivity.
Using lessons from the broadacre farming sector, field trials using plant species from arable pasture systems were tested in the vineyard context. The species chosen were intended to either suppress weed germination or growth by providing a mulch on the soil surface. They were also expected to improve soil quality via active root systems and their associated microbiology, while the legumes would supply biologically fixed nitrogen. The reason why this work had not been undertaken before in Australia was the expected yield reduction that would be associated with competition for water and nutrients. It was suspected though that the benefits of improving soil quality by providing active plant growth over the winter and spring period, would then generate improved vine root function, accessing previously unavailable soil moisture. It was essential however that the plants chosen to be sown under-vine either died off or reduced their growth in spring to minimise competition with the vine.
Many pasture species have been bred in Australia with adaption to our Mediterranean environment. The annual legumes and grasses germinate with rains in autumn, grow vigorously into spring then set large quantities of seed before senescing. Perennial native grasses have adapted over thousands of years of evolution, while the exotic species have been bred for summer dormancy to ensure their long-term survival.
In replicated trials, a range of eight annual and perennial native and exotic species were mechanically sown under-vine in commercial vineyards throughout South Australia in 2014. They were compared to herbicide and straw mulch controls. They were closely monitored over three seasons for their growth patterns and impact on the weed populations, vine productivity, grape quality, soil health and ultimately financial returns.
Overall, the results have been very encouraging, to the extent that current practices of maintaining a bare under-vine zone can now be questioned with some authority, as an alternative management strategy can now be tested on a larger scale. At all sites, the growth of annual pasture legumes and grasses as a mixture, or legumes alone, did not reduce productivity, and at 3 out of 4 sites, it was improved. While they did not prevent the growth of weeds, their growth was suppressed significantly. Soil carbon, an important indicator of soil quality, improved significantly, particularly where legumes and grasses were sown as a mixture. This was achieved at approximately the cost of one year’s herbicide application, which should be a one-off cost for purchase and sowing of the pasture seed. With such a low establishment cost and reduced maintenance requirements, plus improved yields, the gross margins increased by up to $1,700/ha, which was achieved at the Barossa site. An initial tasting by Barossa winemakers of wine made from fruit grown and influenced by each of the treatments suggested there is no detrimental effect on quality.
The benefits were not universal across all sites however. The warm, dry Riverland environment provided a considerable challenge. While there was no yield reduction from the legume and annual grasses, they were unable to suppress the growth of some very aggressive summer weeds. Weed control was achieved by an exotic perennial grass species, but at the expense of grape yield.
It is intended to continue this research for another two years. This will enable the delivery of results to grapegrowers with greater certainty and assuredness that the results are real and growers can adopt the practices without penalty. In the meantime, greater awareness of the project through extension efforts and a blog site will occur, and several wine tastings from the 2017 vintage will seek to confirm the benefits or otherwise of under-vine cover crops to the end product.