Deep Dive: Shifting weeds with subsurface irrigation

That was one of the findings of a project conducted by Mark Krasnow of Thoughtful Viticulture, as part of herbicide reduction research in the Bragato Research Institute’s (BRI) Vineyard Ecosystems Programme.

“The real difference and the really exciting difference for me is we are just moving where those weeds are,” Mark told audiences at the three New Zealand Winegrowers and BRI Grape Days events around the country in June. “And we’re moving them from a really difficult to control area to a much easier to control area.”

He showed results of the trials, where six subsurface blocks – three in Hawke’s Bay and three in Marlborough - were allocated the same water as the above ground control blocks. When the irrigation season began, the drip line blocks sprouted vegetation in the “extremely difficult to control undervine area” while the subsurface, located 30cm below the surface and 30cm into the row, pulled that growth away from the vine. “Our undervine could look like our midrow looks in summer,” he said. “That would dramatically reduce the need for weed control full stop, and certainly for chemical weed control in the future, where it is going to become less available to use and have negative connotations with wine consumers.”

Mark noted that grasses can access water from the subsurface irrigation at 30cm depth, because it is wicked up through the soil. But the further into the row the watering, the easier those weeds are to deal with. While 30cm is a typical measurement, ensuring young plants can access water, some companies are running trials with subsurface closer to the midrow, and Mark is confident mature vines will send roots to the resource. He’s also keen to trial deeper lines, so weeds cannot access the water, but grapes can.

There was no significant impact on yields in the subsurface irrigation trial blocks, but there are “huge benefits” of underground infrastructure, including grazing sheep and running machinery without damaging lines and junctions, Mark told Grape Days audiences.

One of the first “fortuitous” findings of the research was that the subsurface irrigation can be installed in a much wider window than previously thought, he said. Typically, the lines are put in in winter, when the vines are dormant, but due to logistical issues, the trial sites were installed after budburst, he said, admitting to concerns that shallow roots would be cut and the vines put on the “back foot”. But while there were “certainly” some very large roots severed by the mole plough that installed the lines, there was no negative effect, he said.

Mark also spoke to the audiences about earlier work by Thoughtful Viticulture, through the Vineyard Ecosystems Programme, on reducing and eliminating herbicides in New Zealand vineyards. “The simplest way to reduce herbicide use is to reduce the number of sprays we do,” he said. The project looked at the impact of spraying once early in the season, mitigating the frost risk and reducing competition to get the canopy up and the crop on. After that there was no herbicide treatment, with mowing or cultivation used for weed control instead.

The team found that one spray instead of multiple sprays reduced herbicide use by anywhere from 50 percent to 75 percent. “The additional positive side is we didn’t see any significant effect of letting all of this green stuff grow under the vines - on the canopy growth or on the yield,” said Mark. Many vineyard managers and owners like to see “perfectly straight lines of herbicide strips under the vines” he said. “But they’re absolutely not necessary to get the yields that we expect from our vineyards.”

The one consistent difference they noted between the control and trial blocks was the reduction in yeast assimilable nitrogen (YAN) in the reduced herbicide blocks, especially in Marlborough, where it plays a part in “those big thiol bombs that we’re so famous for”, Mark said. That might mean some extra fertilisation in the vineyard or supplementation in the winery might be necessary, although he noted that the YANs were not so low as to risk the ferment.

The key takeaway was that vineyards don’t need to use as much herbicide, he concluded. “The undervine does not need to be clean for the entire season…getting that canopy up and getting past set is really the important stage. A little bit of competition from undervine vegetation after that doesn’t affect our bottom line; but it dramatically affects our chemical usage.”