Watering dormant vines during winter in dry seasons is far more effective at maintaining yields and quality than waiting until spring, a three-year study in South Australia has found.
The researchers say the benefits of the winter watering in dry years far outweighed the cost of the additional water and associated expenses, particularly in regions such as Barossa SA where Shiraz grapes fetched about $2250 a tonne.
In South eastern Australia, where the bulk of Australia’s premium wine grapes are grown, rainfall in autumn and early winter has only reached the long-term average a handful of times in the past 25 years.
South Australia is consistently responsible for about 50 per cent of Australia’s annual production and 75 per cent of premium wine production. If current weather patterns continue it means the vines will require a regular program of year-round irrigation.
During the research, covers were placed over 120 Shiraz vines in the Barossa between May and early September to mimic drought conditions while another
32 uncovered vines served as the control crop.
The vines under the covers were split into four groups:
Those irrigated with long term average amounts of winter ‘rainfall’ in Nuriootpa SA through under canopy sprinklers;
Vines given the same 230mm of water through drippers across the winter;
Then a reduced rain group that received only a third of the water of the other two; Finally, a spring rain set of vines that were subjected to a dry winter followed by 150mm of irrigation to refill the soil profile as the covers were removed just before budburst in early September.
The uncovered control crop performed best overall, averaging 10.86kg per vine over the three years. The sprinkler-irrigated vines at 10.06kg were next followed closely by the dripper-fed vines with a yield of 8.95kg.
The reduced rain vines had two poor years and one excellent season following strong rains either side of winter in 2016, bumping the average fruit yield up to 9.02kg per vine over the three years.
But the surprise result was the consistently poor performance of the vines left dry in the winter but then pumped full of water in September, which averaged 8.48kg per vine.
Australian vineyards in traditional regions rely on starting spring with a full soil water profile and growers expect that this will sustain the growth of those vines for a good part of the summer.
Growers’ are already aware of the benefits gained from winter rainfall and the importance of having a full profile, but nobody has really said what do we do about the lessening of winter rain, because in the past 95 per cent of the time they received a full profile, so it wasn’t that big a deal. But now with climate change there is less rain, especially during autumn and winter.
One of the biggest findings in the trial was the detrimental effect of holding back winter irrigation in dry years until spring. When the vines were irrigated in the spring, which is typically what a grower would do following a dry winter, there were decreases in yield and a detrimental effect on the wine sensory profile.
That was the worst outcome because growers were refilling the soil water profile in spring as opposed to during winter, but you were always getting a reduction in yield and always grew a much larger canopy, so the quality of the grapes wasn’t as good – it tended to grow a whole lot of leaves instead of fruit bunches.
There is now a real take home message from the three-year trials – if you’re having a dry winter you need to be putting the water on during the winter despite the vines being dormant. Don’t wait until spring because then you’re just going to grow a big canopy and not have as much fruit.