California has zinfandel vines that are more than 140 years old. But when you buy a bottle of “old vine” zinfandel, don’t assume that the grapes are from such venerable vines.

There’s no legal definition for what constitutes an “old vine,” nor are there rules about putting the term on a wine label. The Alcohol and Tobacco Tax and Trade Bureau, the federal agency that approves wine labels, requested public comment on the matter in 2010, but nothing ever came of it. Other label terms with no legal definition include “ancient vines” and “heritage vines.”

Many vintners agree that a vine is old at 50. Zinfandel Advocates & Producers, an organization that celebrates zinfandel and its place in U.S. wine history, hasn’t taken an official position on “old vine.” But Rebecca Robinson, ZAP’s executive director, points out that only vineyards that were more than 60 years old were used for cuttings in the Heritage Vineyard Project, a project of ZAP and the UC Davis. The first phase was planted in 1995 with cuttings from pre-Prohibition vineyards around the state.

The issue of vine age is complicated, says Joel Peterson, founder and winemaker for Ravenswood Winery in Sonoma. Peterson knows a thing or two about old vines: He works with vineyards in Sonoma County that date back to the 1880s — “we would all agree those are old,” he says — and with other old vineyards in Lodi and the Napa Valley.

“Really old vineyards are a mix of vines,” Peterson says. In addition to the original vines on the property, a vineyard almost always has some newer vines that have replaced dead or diseased plants. “Are we talking about the average age? … It’s hard to make a rule.” That’s one reason Peterson thinks it’s fine that the government didn’t wade into the issue.

Old zinfandel vines are so prized for several reasons. These twisted, gnarled plants produce naturally low yields of grapes that are concentrated, intense and flavorful. Peterson says these vines are more integrated with their surroundings and exhibit consistent behavior, in addition to producing a naturally small crop. However, he says, “There are vines that are 60 and 70 years old that aren’t behaving like old vines,” because they’re growing on a more fertile or productive site. At the same time, he says, a dry-farmed zin vineyard on a marginal site might produce old-vine-type fruit when the vines are younger.

“Yes, you have to have age, but it’s about the behavior of the vine,” Peterson says. In general, though, he says that “around 50 years, plus or minus five” is a good rule of thumb.

A lot of California’s early zinfandel was planted during the Gold Rush, which is why there are a number of old vineyards in the Sierra foothills. The oldest documented vineyard in California is the property now called the Original Grandpere Vineyard or Vineyard 1869 near Plymouth in Amador County, which dates back to at least 1869. Zinfandel was so popular during the late 1800s that a lot of it was planted elsewhere in Northern California, and old vineyards also remain in Sonoma, Napa and Mendocino counties, Lodi and even a few pockets of Contra Costa County.

A lot of truly old-vine zins don’t list that fact on the label. Ridge Vineyards, known for its outstanding zinfandels, makes wine from a lot of old vineyards — Lytton Springs, Geyserville and Pagani Ranch in Sonoma County and Dusi Ranch in Paso Robles — but when old vines are mentioned at all, it’s on the back label. Ravenswood’s single-vineyard zinfandels all come from vineyards that are more than 90 years old, but you won’t find “old vine” on these labels, either. (Three of Ravenswood’s county series zins and its inexpensive Vintners Blend are labeled as old vine; the Vintners Blend comes from vineyards with an average age of 50, while the others use fruit from vineyards that are even older.)

Because of low yields and labor-intensive farming, old-vine zinfandels can be expensive, but they don’t have to be. Some of the Ravenswood wines cost less than $20. Bogle Vineyards’ old-vine zinfandel ($11), according to the winery, comes from vineyards in Lodi and Amador County that are 40 to 80 years old.

Contact Laurie Daniel at ladaniel@earthlink.net.