Research: Too much California sun may require new wine grape vineyard systems

Switching away from one of the most common and widely used trellis systems to carry grapevines as they grow could alleviate the effects of extreme temperatures on cabernet sauvignon and other grapes common in the North Bay, a University of California at Davis study concludes.

As anyone who has driven through the North Bay can see, vines grow on structures. Most of what people see in the rows of vines are those using vertical shoot position, or VSP trellises. Vine shoots are trained to grow up in vertical, narrow rows with the fruit growing lower to the ground, allowing for greater exposure to sunlight.

But in a six-year study, published in the journal Frontiers in Plant Science, researchers suggest there’s another way. A single high-wire trellis systems, on the other hand, “allows the vine leaves to shade the grapes.”

These trellises are about 5½ feet tall and not only do they do a better job of protecting grapes from sun damage, they can produce better yields, researchers say.

“We found that with the single high-wire trellis systems, growers get a more marketable yield for the amount of water they would have to apply because the system has the most ideal leaf area to fruit ratio,” said lead author Kaan Kurtural, professor of viticulture and enology and an extension specialist at UC Davis.

Kurtural said growers could easily switch to these high-wire trellis systems without having to replant an entire vineyard. The conversion might take about 18 months.

“This seems like a great way to move into the next century of grape growing,” Kurtural said.

To get more information, the North Bay Business Journal asked another study author, Runze “Cliff" Yu, assistant professor of Viticulture California State University, Fresno. Yu’s responses have been edited for length and clarity.

What has been the reason that Vertical Shoot Position (VSP) trellises have been used for so long in the industry. Is it done for a particular advantage?

The VSPs are tidy and can be used in narrower row spacing without inter row shading issues, this is very beneficial to the coastal areas because land prices are high.

Also, they have relatively more openness inside the canopy, which can be good to keep a good air flow when the disease pressure is high especially, this is especially prevalent in the coastal regions.

Has it not become apparent that this system is detrimental to grapes, or that is the case now, with changes in climate?

Here in California, the sunlight is our best blessing, and it can turn out to be our worst enemy as well.

The openness offered by VSPs is just not beneficial anymore. With the constant and frequent heat waves and average air temperature, the clusters/berries on VSPs are very easy to get sunburned, which will cause losses in yield and quality.

This is extremely common in the Napa area. From our other studies, one single heatwave with a duration of three to four days, anthocyanins, flavonols, and tannins can be significantly deduced (30-40%) with VSPs. You can imagine how bad it can be if there are multiple heatwaves in one summer, VSPs are just not providing enough protection.

Has it always been the case that sunlight is not good for grapes? Why has no one taken the time to find a better way to shade them?

Sunlight is usually good for grapes, it is just we are having too much here in California, compounded with the increasing heat. The North Coast regions are becoming hotter and hotter. The growers have realized this and are trying to use preventative methods, but compared to establishing a vineyard with the correct trellis setup, those approaches will be costly in time and money.

How much better yield from this system versus the old?

From this study, we have seen a yield increase from 16%–20% yield increase — or more, if trying to maximize yield — with single high wire with better quality.

The differences were constant during the two years of this study. The growers in the North Coast might not be thrilled about the yield increase, but they cannot say no to the efficiency and better quality.

The biggest obstacle will be if the Napa growers want to adapt, since they are trying to keep the image of this traditional look to attract tourists and change their common practices. They are concerned if the vineyards will look like what's in the Central Valley, which might make them look not as "premium." But in the end, the changing environment is just not allowing them to do the same things as 20-30 years ago, so they have to be adaptable accordingly.

As traditional as the wine industry is, what do the authors have to say about how they think their idea will be received?

I think this trellis might receive some resistance, certainly from the growers in the North Coast. But like I mentioned, with the changing climate and also labor shortage, the growers just cannot do the same practices and hope the outcomes will be the same. If they do, they might have to compromise their wine quality, aging potential, and the overall profitability.