A new study has shown that even a single season of the ‘mulch and compost’ weed control method can dramatically improve Shiraz berry chemistry and soil nutrition.
The study also found that mechanical weed control was a highly effective method for undervine management and led to very few differences from herbicide usage.
The study, undertaken in the Limestone Coast region by Thomas Lines as part of Wine Australia’s Incubator Initiative, offers growers and winemakers food for thought.
Herbicide has long been the ‘go to’ approach to undervine weed control, but in recent years the use of mechanical weed control or mulching, with or without the addition of compost, has become increasingly popular.
‘However, impacts on soil health and vine physiology associated with these new management practices are highly variable and are dependent on many factors’, Thomas said.
‘This trial was designed to take in data from many facets of the vineyard to better understand how mulching and mechanical weed control (finger weeding with light surface tillage) might affect vine performance, and ultimately help determine whether these practices might be feasible for growers in the Padthaway region to adopt’, said Thomas.
The study involved two sites in the Padthaway region – one growing Cabernet Sauvignon and the other growing Shiraz grapes. At the Cabernet Sauvignon site, a wide range of data was collected, including soil analysis, leaf analysis, soil moisture, nitrogen levels and harvest and berry chemistry; while at the Shiraz site, only harvest and berry chemistry data was collected.
No significant differences in yield were detected across either site, although Thomas said this wasn’t surprising given the lower yields across every South Australian region in 2020.
However, what did surprise him was the rapid response from the vines to the different treatments.
‘Despite a relatively late addition of mulch to the soil, canopy sizes were noticeably different towards the end of the season. It’s a shame it was a lower-yielding vintage, because it would have been interesting to see yields on a higher yield year.
‘Whether this is a result of improved surface soil moisture, reduced soil temperatures, or perhaps reduced salinity or hardness it’s hard to know’, Thomas said.
The team found undervine weed cover was most effectively reduced by the mulch treatments, with mechanical weeding proving the least effective, but these differences were not significant by the end of the season.
‘This means that if control of undervine competition is a viticulturist’s high priority, any of the options trialled here are feasible solutions, though they may have differing effects on the vine’s nutrition and berry quality.’
But the biggest surprise was in berry chemistry at the Shiraz site.
‘The inclusion of compost underneath the straw lead to surprisingly strong effects on the juice. With mulch alone, pH was higher than herbicide, though with compost, pH was lower than herbicide. Brix was also lower for the mulch and compost treatment’, Thomas said.
‘These effects suggest that the mulch and compost treatment is the best, but also perhaps that they needed either more time on the vine, or less water throughout the season, to ripen and bring up the sugars.’
Thomas said while mulch was a good option for growers wanting to reduce herbicide use, it can be a more costly option.
His advice to growers was to start testing alternatives early, so they could fine tune the most ideal option for their particular situation, and stay tuned for another year’s data, now with undervine cover crops, so confirm that these results can be consistently achieved.