Peter Luthi is into his 21st crush as winemaker at Trefethen Vineyards. Not many people who make wine in the Napa Valley have had as long a tenure at one place as has the Swiss-born Luthi. The 550 acres of Trefethen’s vineyards just may be the largest contiguous vineyard in the valley under single, private ownership. And Luthi knows the terroir there with as much precision as a Swiss watch. And having worked for 23 years with Trefethen’s owner Janet Trefethen, Luthi is equally intimate with the soils and nuances of the Oak Knoll District.

That’s because, it was Janet Trefethen who led the campaign to get the region, due north of the city of Napa, named an American Viticuture Area (AVA). Two years ago, the Oak Knoll District of Napa Valley was granted AVA status, thus becoming the 14th and latest so-named sub-region of the Napa Valley.

Oak Knoll is one of the rare places in the valley that is warm enough to ripen Cabernet Sauvignon and Merlot and yet is not too warm for Chardonnay and Pinot Noir. The region is generally cool, but soil variation within the vineyards produces some sites where the earth warms more quickly in the spring. These areas begin growing earlier and thus have a greater capacity to ripen, which is important for Cabernet. Combining these warmest sites with limited crop yield allows Cabernet and Merlot to achieve maturity.

The marine influence gives Oak Knoll cooler than average temperatures and a higher humidity than most of the rest of the valley. These characteristics are essential for preserving aromatics.

Under Luthi’s guidance, Trefethen produces 70,000 cases a year of Viognier, Chardonnay, Pinot Noir, Cabernet Franc, Cabernet Sauvignon, Merlot, and Riesling. It is the Riesling, (along with the winery’s Cab and Chardonnay) that has brought the most attention to Trefethen since it was founded in 1973.

In fact, Trefethen's 20 acres of Riesling -- from which is produced a dry as well as a late harvest style made from botrytis-affected grapes -- is considered to be the largest such Riesling vineyard on the North Coast.

Luthi spoke to Appellation America’s Napa Valley correspondent Alan Goldfarb recently about these subjects and others.