Why don't more California wineries dry farm?

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Dear Dr. Vinny,
On a recent tour of wineries in Sonoma, I stumbled upon one that uses dry farming, a technique they say improves the quality of their wine. Given the current severe drought in California, dry farming seems to make good sense. Why don’t more California wineries dry farm?
—Karen O., Stevenson Ranch, Calif.
Dear Karen,
I’ve explained the principles of dry farming before, which is what it sounds like: relying only on natural rainfall for growing grapes. It’s popular not only with water conservationists, but as you point out, many vintners believe the practice leads to more intense grapes and wines.
With all the talk of the severe drought in California, the topic of dry farming is coming up as an alternative—in some cases. The problem is that transitioning from an irrigated system to a dry system isn’t as simple as turning off the faucet. It would need to be more gradual, allowing time for the grapevines to develop root systems that seek water deeper in the soil. Dry-farmed vines can have root systems as deep as 30 feet, while irrigated vines typically have much shorter roots, only a couple of feet down. It takes time for those roots to grow. It’s further complicated because young vines require more water to establish their roots, and not every type of soil is conducive to deep root systems. That’s why winemakers are always talking about dirt.
Dry farming is most successful when it starts out as dry farming, and honestly, it’s very site-specific. It starts with figuring out what kind of crop size you can get from what amount of rainfall you can expect, as well as assessing climate, soil type, rootstock and grape variety. Dry farming isn’t suited to large, densely planted vineyards, or vineyards that are surrounded by trees or other crops that would compete for the groundwater. Without irrigation, winemakers lose some ability to react to extremes in weather, which makes grapegrowing a much riskier (and thus, more expensive) proposition. Considering how much of the winemaking process is left up to the whims of Mother Nature, I can’t blame growers for wanting more control.
—Dr. Vinny