Unsurprisingly, grapevines need water to survive. They’re good at seeking out water via their roots, often going deep to eke out moisture in the subsoil or parent rock.
This is because vines are structural parasites. Rather than grow their own trunks, if left to their own devices, vines will climb up trees and other plants to reach the light at the top of the canopy. Similarly, vine root systems establish themselves in challenging soils, often already populated by other thirsty plants. Given the depths that they will reach for water access, grapevines are also fairly drought-tolerant.
Historically, many venerated European vineyards in places like Bordeaux and Barolo have dry-farmed grapevines and forbidden irrigation. In regions with what’s called a Mediterranean climate, summers tend to be warm, dry and with very little rainfall.
These areas have many old vineyards with vines that are spaced widely and unsupported by trellising. This is called gobelet, or bush vine training, and it’s ideal for dry, sunny climates. It limits the vigor of the vine because a larger canopy would require more water. It allows in enough light and air, while still providing dappled shade to prevent the grapes from getting sunburned. The wider spacing allows each vine to develop an extensive root network to find available moisture.
In recent years, where permitted, many vineyards that previously used this type of training system have turned to irrigation. And, in regions where there simply isn’t enough rainfall to sustain vine growth, irrigation has always been the norm.
How much water does a vine need? As a rule of thumb, if the annual rainfall dips below 20 inches, growers will need a bit of supplementary water. However, a lot depends on whether it rains during winter or the growing season and the ability of the soil to retain moisture. Clay, limestone and organic material help.
In certain wine circles, a debate exists about whether irrigation leads to a loss of quality or terroir expression. For some, “dry grown” indicates quality.
One of the most striking methods of irrigation is in Mendoza, Argentina, a high desert with less than eight inches of rain annually. Farming here relies on a beautifully engineered series of irrigation canals that dates hundreds of years and repurpose meltwater from the Andes. The method mimics rainfall, with bursts of lots of water followed by dry periods, but requires a great deal of water.
Another way that winegrowers irrigate is by strategically positioning overhead sprayers. These aren’t a very efficient use of water, either, but they have the potential to mimic rainfall. One potential problem is that this method wets the leaves, which can increase the chances of disease on the plant.
The most widely used type of irrigation is a drip line, which aims a targeted amount of water at the root of each vine. It’s an efficient use of water, but it can encourage root growth only where the drips accumulate. A little-and-often watering pattern results in a diminished active root zone, which stops the vine roots from fully exploiting the soil. For this reason, some advocate irrigation only rarely, but in big bursts, to wet a larger soil profile.
Another factor to consider is the evapotranspirative rate, or how much water the vine stands to lose while it is transpiring. Plants face a dilemma. They open pores in their leaves, called stomata, to gather carbon dioxide for photosynthesis. In doing so, they lose water. So, they do a calculation. If they lose too much water due to heat, wind or dry conditions, they might shut their stomata. A vine on free-draining soils in a warm, windy spot will need more water to survive.
Irrigation has become so common that, in regions that don’t employ it, the term “dry grown” is sometimes used as a differentiator. In certain wine circles, a debate exists about whether irrigation leads to a loss of quality or terroir expression. For some, “dry grown” indicates quality.
Why is irrigation controversial? In some regions, it wouldn’t be possible to grow vines without it. Unfortunately, greedy farmers with access to water can irrigate to produce larger, lower-quality crops. That is why some classic regions ban the practice, even though some extra water could help in very dry years.
But there are ways that irrigation can be done smartly to produce quality grapes. Red varieties can benefit from a reduced water supply after veraison, when the berries change color and begin the final stage of ripening. The drying roots signal to the rest of the plant using the hormone abscisic acid, and the vine concentrates its resources on ripening the grapes.
Plus, regulated deficit irrigation, which restricts the vines’ access to irrigation water, has the dual benefit of conserving water and enhancing grape quality, especially for red varieties.
There’s evidence that the best vineyard sites enact this sort of mild water deficit at the right time. This is why “dry grown” is sometimes used as a badge of honor. It’s likely, in some situations, to create lower yields of better-quality grapes. But it would be unfair to think that irrigation is always inferior. It’s a tool and, like any tool, it can be used well or badly.