What is a "field blend"?

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Dear Dr. Vinny,
In the Douro, several varietals are often planted in the same vineyard and vinified together in a "field blend." Does that mean wine made from that vineyard has the same percentage of each varietal every year, or is the winemaker able to vary it by separating grapes by varietal at harvest?
—Fred, La Cañada, Calif.
Dear Fred,
You have the basics down of what makes a field blend: more than one grape variety planted together in the same vineyard. Field blends happen all over the world, though they're now rare. In the past, before folks fretted about varietals or clones, they just planted different grapes in their vineyard as an inexpensive (yet limiting) way to blend wines. All the grapes are harvested at the same time and fermented together—a true field blend doesn't separate by varietals at harvest; the "blend" is whatever Nature gives that vintage.
Modern winemakers generally prefer to plant and pick each variety separately, knowing that they don't always ripen evenly. Sometimes a winemaker will ferment multiple grapes together (a practice called co-fermentation), which invokes the spirit of field blends. But most winemakers blend different lots of wine together after the fermentation is finished to better control the process.
—Dr. Vinny