There is already an enviable fund of scientific knowledge in this area, so it is possible to form a green cover with different plants, depending on the goal of the winegrower: protection from erosion, suppression of weeds, enrichment with nitrogen, attraction of beneficial insects, improvement of the soil microbiome, etc. Regardless of the chosen cultures all green covers additionally provide green manure and significantly contribute to humus building. However, there are few winegrowers who leave an uncultivated area under the plant cover, in order. With conventional ones, this most often involves spraying with herbicides. With organic, mechanical processing. The Australian Wine Research Institute (AWRI) in collaboration with the University of Adelaide conducted a multi-year study in order to determine the effect of complete row vegetation removal versus multiple types of green cover: grasses, nitrogen fixers, established vegetation, mixtures and mulches. Before this research, there were practically no comparative scientific experiments that could answer the question of what effect different regimes have on vines and wine with more certainty. The issues that were considered are primarily the yield and quality of grapes (and wine quality), economic effects, then the diversity of microbes in the soil, the amount of earthworms present, the amount of mineral nitrogen, the length of ripening, the temperature of the soil and canopy, and the carbon content of the soil. The research was conducted from 2014 to 2018 in four commercial vineyards with a variety of climates. The vineyards also had a variety of soils: from heavy clay to sand. The best results were achieved in the Barossa Valley with a moderately warm and moderately humid climate. The combination of legumes and grasses successfully suppressed weeds without losing the grape crop. The carbon content in the soil has significantly increased. There was a reduction of AUS$1,700/ha compared to the previous practice of using herbicides to control row vegetation. Oenologists blindly compared wines from all different treatments (including herbicide) and found no significant sensory differences. The worst results were in the Riverland vineyard, which has a very hot and dry climate. The combination of annual grasses and legumes failed to suppress weeds to a greater extent, while in the rows sown with exotic grasses, weed suppression was successful, but with yield loss.
Ten different combinations were planted, each in four places. The seeds were planted with a direct seeder, in one row on each side of the plant cover, 20 cm from the plant cover and at a depth of 10 mm. No fertilizer was applied. Triticale hay 55t/ha was used for the trial with mulch, and in the herbicide trial, total was used as needed and recommended by the manufacturer. All plants were allowed to grow and die naturally, without mowing or rolling. Some annual seeds were added in the second year, as the plants themselves did not drop enough to keep the mix balanced. The presence of plant species and dry mass was measured three times a year. Canopy and crop balance was assessed by measuring the weight of the pomace and grapes, while the canopy balance was measured by leaf area index using the VitiCanopy mobile phone application. The petioles were taken on the opposite side of the flowering leaf at the 80% pre-flowering stage and the presence of macro and micro elements was analyzed. The harvest was done by hand, and the AWRI protocol for processing red wines was followed. Sensory evaluation was done by a panel of 26 oenologists from the Barros Valley on a scale of 20 points. In order to evaluate life in the soil, three samples were taken by machine (cores up to a depth of 30 cm) in each trial, which were sent frozen to the laboratory where DNA analysis was done. Varieties and hybrids used Nuriootpa (N), Waikerie (W), Eden Valley (EV), Langhorne Creek (LC).
In the first year, the results were not measured because the green cover did not have time to work. Years 2015 and 2016 were significantly different in the vineyards of Nuriootpa, planted with Shiraz. 2015 was dry in spring, and 2016 was wet. This has had the greatest impact on the native prairie grasses wallaby grass and cocksfoot, which stop growing and dry out with the cessation of spring rainfall, thus creating a mulch cover. In 2016 and 2015, on plots with irrigation, they continued to grow and reduced the yield of grapes. A mixture of rye and beef clover (Medicago polymorpha) not only provided a higher yield compared to the herbicide treatment (28%), but the wines with that treatment also scored higher than the herbicide control. In Australian commercial terms this resulted in an increase in income of AUS$5,000/ha on an investment of AUS$200/ha.
The Eden Valley vineyard, planted with Cabernet Sauvignon, was chosen for producing high-quality grapes in a cooler climate with more rainfall. In the 2016/2017 rainy season, it was shown that the continuation of the growth of autochthonous grasses, which usually die at the end of spring, had already and impact on reducing yields. Mulch, which retained too much moisture, cooled the soil and delayed budding, also gave negative results. Here too, weed management with a mixture of grasses and legumes proved to be the best both in terms of yield and quality.
The results of the experiment are described in a very detailed report, which is impossible to convey here. The intention was to make our organic winemakers think and possibly make their own trials.
I recommend watching the webinar where this report is presented on YouTube: https://youtu.be/4Pks7-bCOpY
Since there was no mode of mechanical removal of vegetation under the plant cover in the trial, which is the only method in organic production, the results of the trial related to herbicides can also be applied to mechanical removal because both methods leave the soil bare.
The areas where the experiment was conducted have a climate very similar to the climate in Serbia, so the results can be directly applied to our conditions. However, the trial used plant species whose seeds are readily available in Australia. It is necessary to replicate the experiment here, using plant species that are easily available here.
In the conclusions, the authors warn that all experiments were conducted in conventional vineyards where herbicides have been used for years. In organic vineyards, the density of existing weeds and the amount of seed they drop each year can significantly reduce the power of planted green cover to crowd out weeds and may require replanting each year.
Scientific studies based on the experiments of planting a green cover under the bushes can also help in understanding this matter:
- Vukicevich, E., Lowery, T., Bowen, P., Úrbez-Torres, J.R., Hart, M. 2016. Cover crops to increase soil microbial diversity and mitigate decline in perennial agriculture. A review. Agronomy for Sustainable Development 36: 48.
- Bowles, T.M., Jackson, L.E., Loeher, M., Cavagnaro, T.R. 2017. Ecological intensification and arbuscular mycorrhizas: a meta-analysis of tillage and cover crop effects. Journal of Applied Ecology 54(6): 1785-1793.
- Pérez-Álvarez, E.P., García-Escudero, E., Peregrina, F. 2015. Soil Nutrient Availability under Cover Crops: Effects on Vines, Must, and Wine in a Tempranillo Vineyard. American Journal of Enology & Viticulture 66(3): 311-320.
- Karl, A., Merwin, I.A., Brown, M.G., Hervieux, R.A., Vanden Heuvel, J.E. 2016. Impact of Undervine Management on Vine Growth, Yield, Fruit Composition, and Wine Sensory Analyses in Cabernet franc. American Journal of Enology & Viticulture 67(3): 269-280.
- Penfold, C., Howie, J., Weckert, M., Nordblom, T. 2019. Under-vine cover cropping – a source of vine medication. Australian & New Zealand Grapegrower & Winemaker (671): 34-38.
Written by Branislav Anđelić