Marin County AVAs

Marin County does not have any AVAs that are fully unique to itself. The northern half of the county is covered by the North Coast AVA, and the County has a portion of the Petaluma Gap AVA that also falls within it.

The county had a lot of vineyard acreage under vine in the 19th Century, but the San Francisco Earthquake of 1906, the Prohibition era (1920–33) and World War II (1939–45) all but eradicated the wine industry here.

Nowadays, there are only around 200 acres of vines within the appellation, belonging mainly to boutique wineries. George Lucas's Skywalker Vineyards can be found in Marin County, along with vineyards and wine cellars owned by wine producers in Napa and Sonoma counties.

Strong winds, persistent fog and temperature swings mean that growers have to select their sites carefully to protect their vines from the weather. The majority of vineyards are planted on slopes that are angled so as to avoid the most devastating effects of the winds, while taking advantage of the sun.

Influences from the Pacific Ocean ensure that the winters in Marin County are milder than in Sonoma and Napa valleys, meaning that bud break can often come quite early. However, the cooler summers mean that Marin County enjoys a long growing season where ripening can happen slowly, allowing the grapes to retain acidity while they develop delicate aromatics.

There is a range of soil types in the area, but viticulture usually takes place in hilly areas where sandstone, loam and shale predominate. These thin mountain soils offer excellent drainage, meaning that vines have limited access to water that would dilute the flavors in their grapes. The wines produced from these grapes offer excellent concentration of flavor and good varietal character.

Petaluma Gap AVA

Established as an AVA on 6 December, 2017.  The Petaluma Gap AVA is an area which spans 202,476 acres stretching through an 30 miles inland valley from the Pacific coast at Bodega Bay southeast to Highway 37 at Sears Point on San Pablo Bay straddling the border of northern Marin and southern Sonoma counties. The AVA lies entirely within the North Coast AVA and partially in the Sonoma Coast AVA.

The Marin County portion of the Petaluma Gap remained outside of the Sonoma Coast AVA, while the Sonoma County portion remained within the Sonoma Coast AVA. TTB allowed the partial overlap to remain, primarily because the name “Sonoma Coast” is associated only with the coastal region of Sonoma County and does not extend into Marin County.

The distinguishing features of the Petaluma Gap are topography and climate. Its topography sets the AVA uniquely from Sonoma Coast AVA. The ‘Gap’ literally describes the geological valley that is a 15 miles wide by 30 miles corridor between the coastal hills allowing the salty maritime winds and a cycle of morning fog, clearing sunny spells, more fog in the afternoon and clearer evenings to flow eastward through the region. A typical day in the valley can experience a temperature change of 40 to 50 °F.

These cycles contribute to a longer growing season benefiting cool-climate grape varieties. The lower elevations and rolling hills in Petaluma Gap allows the marine air to enter at a higher velocity than the surrounding regions where higher and steeper slopes disrupt the air flow. Although marine breezes are present during most of the day, the wind increases significantly in the afternoon hours because the rising inland hot air pulls the cooler, heavier marine layer from the coast causing a steady airflow.

The effect of these prolonged high winds on the grapes is a reduction in photosynthesis to the extent that the grapes have to remain on the vine longer (hang time) in order to reach a given sugar level, compared to the same grape varietal grown in a less windy locations. Grapes grown in windy regions are typically smaller and have thicker skins than the same varietal grown elsewhere. The smaller grape size, thicker skins, and longer hang time concentrate the flavor compounds in the fruit, allowing grapes that are harvested at lower sugar levels to still have the typical flavor characteristics of the grape varietal. Also with the wind, there is no botrytis or mildew issues and the grapes have a higher skin-to-juice ratio. 

Around 75 percent of plantings are Pinot Noir, with Chardonnay at 13 percent, and Syrah 12 percent.