Brain on Winemaking: The Shade Cloth Conundrum

Also, doesn't it seem counterintuitive to remove leaves from the fruit zone in the spring (which is almost universally done in red varieties) only later to install shade cloth in the summer to return protection to the clusters?

Let's first look at the problem growers are trying to solve. High heat combined with direct sun exposure leads to sunburn on the grape skins, as the temperature inside the grapes can increase 20 degrees Fahrenheit (–6.6 Celsius) over the ambient air temperature. This results in rapid cellular death and deterioration as well as compromised skin integrity, which leads to accessible sugar diminution and microbial spoilage, but also dehydration and crop loss. When daily temperatures hit 100 degrees Fahrehheit or higher, color, flavor and tannin also begin to degrade, leading to thin and simple wines.

LEAF REMOVAL: THE NECESSARY EVIL

So farmers should just leave the leaves, right?

Wrong.

Removing leaves is critical to opening up the fruit zone to light and air, which helps push out moisture and prevent mildew growth. Another major benefit is that removing basal leaves allows for much more effective spray treatments. This is typically done early in the season when conditions are moist and mildew can first set in. More effective spraying decreases the amount of active agent used and may even allow for more organic or environmentally gentle active agents to be employed.

We also pull leaves in the fruit zone to hedge our bets against cool growing seasons (like 2023), with more light on the fruit needed to promote ripe character and degrade green aromas. The leaves must be removed early if the work is to be effective – always well before the extreme weather is forecast. So if we have established that leaves often have to be removed for good farming, is shade cloth the answer?

The idea is that instead of clusters receiving direct sun for some of the day and shade for the rest, with shade cloth the sun is more constant but diffused and deflected, so overall less impactful. Studies by David Parrish, a grower in Paso Robles, have shown a significant decrease in maximum cluster temperature when protected by cloth, and that different colors of material significantly influence the way light diffuses and hits the grapes.

And if the cloth is installed in a way that covers the basal leaves, sugar accumulation can be slowed significantly. Winemakers know that slow and steady ripening produces wines of character and concentration.

Typically made from woven or knitted polyethylene, shade cloth is designed to offer varying degrees of shading – from 20 percent to 90 percent or so. The choice of shading percentage depends on factors such as grape variety, the local climate, vineyard row orientation and the desired sensory characters of the resulting wine. When complex flavors and aromas with herbs and spices are desired, a higher degree of shading should be used.  If even and ripe fruit-based flavors are the goal, a more open weave might be the way to go.

One of the primary applications of shade cloth in viticulture is temperature moderation. Winemakers in Napa learned in 2022 that extreme temperatures of 115 degrees Fahrenheit lead to significant flavor and aroma loss, as well as a reduction of tannin concentration in wine grapes, which are all crucial components of  a great Napa cabernet sauvignon. A reduction in peak temperatures in the fruit zone leads to less fruit dehydration and allows phenolic ripening to better progress.

It works best in vineyards in hot locations, with lots of direct afternoon sun. Thinner soils, younger vines and naturally low-vigor vines also benefit most from this technique, as well as vines that don't typically get good basal leaf shade on the fruit.

The economic feasibility of shade cloth deployment varies depending on vineyard size, climate, and intended outcomes. Many of us don’t know how well these fabrics hold up because we haven’t been working with them long enough. Also, how well do they hold up to the punishment of installation and tear down? This will impact economic viability significantly. Studies that investigate crop yield and shade cloth are hard to trust because the shade cloth is also acting as a protective barrier against birds – another great benefit!

Savings may also be found by employing mechanical leafing instead of more precise hand leafing. Mechanical leafing is clumsier and rougher but can definitely be made more useful and quality-focused when later covered by shade cloth.

Cost of the material, installation and negative ecological impacts of production and disposal are definitely the major disadvantages of the technique. But there is also a risk of overshading the fruit if the wrong weave is used or if the vintage turns cool, leading to decreased ripening and bud fertility for possibly several subsequent vintages, especially when the cloth is installed too early in the season. Shade cloth may significantly inhibit sugar accumulation so this could be a problem in traditionally low-brix blocks.

JUSTIFIED EXPENSE

While initial investments in materials and installation can be significant, the potential benefits in terms of improved grape quality and increased yields often justify the expense over time, especially in high-cost fruit, which commands more than $15,000 per ton in top Napa Valley cabernet sauvignon sites. Advancements in technology and manufacturing have also led to more affordable and customizable shade cloth solutions, making them accessible to a broader range of vineyard operators.

It would obviously be better if we didn’t need to use shade cloth, which might be possible if we had overhead sprinklers and unlimited water supply. It would also be better if we had east-west row direction with a relaxed trellis system than north-south rows trained for vertical shoot positioning that allow the afternoon side clusters to bake; better if we could grow grapes where temperatures don’t go over 100 degrees Fahrenheit; better if we had decomposable or at least organic cloth material; and better if we had some leafing technique that creates a perfect speckling of light on clusters through the day.

But these are not options for many established vineyards, almost all of which are dealing with increased average temperatures as well as extreme heat waves.

This amount of attention and effort put into our wine grapes makes me think about what techniques will emerge next to save us from our environment. Non-permeable bags around each cluster to prevent smoke exposure?

Well, not yet … but we may be getting close.

–Matt Brain