What does it mean if a vineyard is "dry farmed"?

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Dear Dr. Vinny,
What does it mean if a vineyard is "dry farmed”?
—Paul, Lancaster, Pa.
Dear Paul,
Dry farming isn’t quite what it sounds like: It doesn’t mean that vineyards never get any water, but it typically means that the vines aren’t irrigated, relying only on rain. Not only is dry farming environmentally responsible, proponents suggest it yields more intensely-flavored grapes. In some regions, irrigation has been the norm for centuries; in others, it’s prohibited.
Water shortages have been a serious concern in California in recent years, and freshwater conservation is generally a global concern. So why isn’t dry farming more prevalent? It’s a little more complicated than just turning off the hose: Irrigated vines have shallow roots, and it can take years for deeper root systems to develop. Vineyards surrounded by trees or other crops competing for water would have more trouble without irrigation, and certain climates, soils, rootstocks, grape varieties and planting densities are more successfully dry farmed than others. Young grapevines also typically require more water than older vines.
Some vintners also take a looser approach to dry farming, making it their standard practice but resorting to irrigation if conditions imperil the vines or the crop.
—Dr. Vinny