Napa’s Nested AVAs: Oak Knoll

"The location of the Oak Knoll District at the mouth of the Napa Valley, with our warm sunny days moderated by the daily cool breezes off of the bay, means that the fruit can ripen slowly, maintaining freshness and aromatic lift, while developing a soft and elegant mouthfeel with balance and finesse." - Steve Matthiasson, Matthiasson Wines

Oak Knoll District is located in the southern end of central Napa Valley, California and was officially designated an American Viticultural Area in 2004. Known for its relatively cooler climate and long growing season, the Oak Knoll District is one of Napa Valley's more renowned sub-appellations. Oak Knoll is one of Napa Valley’s flatter appellations and it enjoys primarily deep loam soils. Its location to the north of Napa city places it right in the path of the daily fog that creeps up from the Bay, making it one of the cooler AVAs.

The center of the appellation is dominated by rich fluvial soils, which are deposited over time by the rivers meandering path and regular flooding. These deep fertile soils make for fruity and charming wines (no strict mountain tannins here!). Things get slightly more complicated in the west, where a small alluvial fan sits at the base of Mount Veeder; here the soil is more gravelly and the terrain slightly undulating. From a climatic perspective, the regular fog keeps the temperatures down, which means the wines tend to be among the more elegant and understated in Napa Valley.

The valley's largest alluvial fan formed by Dry Creek creates the defining feature of the district. The northwest area is composed of volcanically derived soils, with stony or gravelly consistency. Southern and eastern areas transition from gravel to silty clay loam near the river, making irrigation often essential. The Oak Knoll District AVA and neighboring Yountville AVA are two of the coolest of the Napa Valley Nested Appellations, featuring long, cool growing seasons averaging eight months. The climate in this area is relatively uniform due to the broad, flat nature of the valley floor.

In 1851 Joseph W. Osborne was one of the first people to introduce fine vinifera varieties to the area, including Zinfandel, after purchasing an 800 acre plot of land in Rancho Napa. He named it Oak Knoll Ranch and as early as 1857 was already winning awards in the Bay Area for his wine made from "Foreign Grapes." Despite this early recognition, viticulture was not the most important agricultural activity here during this time, with only around 5% of available land used for vineyards in the 1880's. Then, local wine production declined even further with the onset of Phylloxera in the late 1890's and Prohibition in 1920.

Since the end of Prohibition, numerous vineyards have been planted in the Oak Knoll District, now totalling over 4,150 acres of planted grapevines. The Oak Knoll District was officially designated an American Viticultural Area in 2004.

The District lies at the southern end of the renowned Napa Valley where the growing season is longer—cooler in summer than the warmer Upper Valley and drier in winter allowing soils to warm earlier in the spring. The marine influence from San Pablo Bay is strong here with foggy mornings and cool summer nights. Summer daytime temperatures can average 10°F cooler than St. Helena. This creates the longest growing season in the valley, providing early bud break and a long, leisurely growing season for optimal, balanced ripeness. The cooler nights and slowly rising daytime temperatures create a naturally long hang time for fruit to achieve bright acidity, great texture, fruit-forward aromas and elegant flavors.

This even warming and cooling creates the quintessential “Sweet Spot”. One of the rare places in the Napa Valley where it’s warm enough to perfectly ripen red varieties like Cabernet Sauvignon, Merlot, Malbec, Petit Verdot and Zinfandel while being cool enough for grapes like Chardonnay and Pinot Noir. There are more than 18 wine grape varieties cultivated in OKD, with Cabernet Sauvignon, Chardonnay and Merlot being the most heavily planted.