Basics: Diverse Soils and Coastal Winds: The Ultimate Guide to Sonoma | Wine Enthusiast
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Diverse Soils and Coastal Winds: The Ultimate Guide to Sonoma

About an hour northeast of San Francisco lies picturesque Sonoma, California. Its western boundary meets the Pacific Coast, while the Mayacamas mountain range soars at its eastern borders.

Here, 18 American Viticultural Areas (AVAs) boast a diverse range of microclimates. More than 400 wineries produce everything from spicy Zinfandel to fruit-forward Pinot Noir.

This vast region accounts for many different soil compositions that make it a fascinating study in geography and topography. Grape growing can change significantly from one area or another in Sonoma, where you are can make an appreciable difference in what you grow and how you grow it.

Agoston Haraszthy, who was responsible for early plantings of European vines in Sonoma / Alamy
Agoston Haraszthy, who was responsible for early plantings of European vines in Sonoma / Alamy

A brief history

Sonoma’s wine history kicked off in the early 1800s, when Russian colonists began to plant grapes along the Pacific Coast. A decade later, quantitative grape growing took hold at the San Francisco Solano mission. Thousands of grapevines were planted to Mexican Mission grapes, used for religious purposes. The plantings grew, but it was not until the mid-1850s that the first non-Mission grapes were cultivated in Sonoma.

It was during this period that a Hungarian Count named Agoston Haraszthy, lured to California by the Gold Rush, purchased the Salvador Vallejo vineyard in Sonoma Valley. There, he planted cuttings from vines from France, Spain and Italy. That vineyard would later become Buena Vista Winery, and it would set the precedent for European-style wines throughout Sonoma County.

A vineyard on rolling hills in the Rockpile appellation of Sonoma Wine Country in the Spring near Healdsburg, CA.
A vineyard in the Rockpile appellation of Sonoma / Alamy

The major AVAs of Sonoma

Many geographical features define Sonoma winemaking. One is the region’s maritime influence from the neighboring Pacific Ocean. The region’s altitude, born of the Mayacamas mountain range, also makes a difference, as does fog that hangs over the valley floor. Each AVA contends with one or more of these growing conditions, some of which winemakers see as natural assets.

In the microscopic Fountaingrove District, a Central-Eastern AVA that’s also Sonoma’s second newest, elevation can reach 2,000 feet. Grapes also benefit from a maritime influence because of a mountain gap in nearby Santa Rosa. In Fort Ross-Seaview, also affected by the ocean, vineyards are planted at the top of rounded ridges that often eclipse 1,000 feet.

In Green Valley of the Russian River, which falls into the larger, centrally located Russian River Valley AVA, fog settles on the valley floor. It’s a boon for Chardonnay and Pinot Noir, two varieties that need the sun for ripening, but that also prefer cool nights.

“Fog is a defining element of our microclimate,” says Joy Sterling, partner/CEO of Green Valley’s Iron Horse Vineyards, which is dedicated to sparkling wine. “The big plus is cooler temperatures, so essential for growing Chardonnay and Pinot Noir for bubbly, and also prime for Pinot Noir, generally.”

Sonoma Coast is the larger AVA into which the Russian River Valley falls. Fog plays the same integral role there, which facilitates massive temperature swings. At night, it can cause drops of nearly 40°F, which prevents Chardonnay and Pinot Noir from becoming overripe and too high in alcohol.

“Climate here is driven by the Pacific Ocean,” says Craig McAllister, head winemaker at La Crema Winery, which has vineyards in the Russian River Valley, Carneros and the larger Sonoma Coast AVA. “Diurnal temperature variation—the difference between daytime high and nighttime low temperatures—plays a key role in sugar accumulation, color, flavor and aroma development, and in preserving balanced natural acidity.”

The same is true in Carneros to the south (also within the Sonoma Coast AVA), known for Chardonnay and Pinot Noir, and the smaller Petaluma Gap, Sonoma’s youngest AVA, established in 2017.

But all that fog can prove problematic.

“The downside is that too much fog can lead to problems in the vineyard, which we have to mitigate through our farming practices, intense hand work like pulling off leaves to let more air circulate around the grapes,” says Sterling.

Vines in Bennett Valley, Sonoma / Alamy
Vines in Bennett Valley, Sonoma / Alamy

Bennett Valley, in south-central Sonoma, is a bit of an outlier. A small AVA with a cool climate, it’s known largely for Merlot. That sets it apart from other cool-climate AVAs, like those in the valley that grow Pinot Noir, or mountain areas that cultivate Cabernet.

Sonoma Mountain and Moon Mountain are two appellations geographically close to one another, separated by the narrow Sonoma Valley AVA. Located in Sonoma’s southwest, both are known for Cabernet Sauvignon grown at higher elevations. So is Pine Mountain-Cloverdale Peak, although it falls in the northwest of the county, above both the Alexander Valley and Knights Valley AVAs.

These cooler microclimates allow for expressive Cabernets that show different characteristics from those at low elevations next door in Napa Valley.

Warmer AVAs, like Sonoma Valley, Knights Valley, Alexander Valley and Chalk Hill, generally yield ripe-fruited expressions. Producers take advantage of the ample sunshine and well drained soils of these regions.

Northern Sonoma AVA incorporates Knights Valley, Alexander Valley, Pine Mountain-Cloverdale Peak, Chalk Hill, the Russian River Valley as well as parts of Green Valley and Rockpile.

The other grape variety that benefits from the warmth of some of these AVAs is Zinfandel, which thrives in Rockpile and Dry Creek Valley, and to a lesser degree in Alexander Valley and Sonoma Valley.

Sonoma's Russian River / Getty
Sonoma’s Russian River / Getty

The terroir of Sonoma

Sonoma’s soil can range from sandy loam and volcanic ash to rock, and that diversity can change the character of the grapes from one appellation to the next.

Chalk Hill, which borders the Russian River, is named for the vein of chalk-like soil that runs through it. It’s actually volcanic ash that provides drainage to the area’s Chardonnays and Cabernet Sauvignons.

Green Valley of the Russian River possesses a soil type known as Gold Ridge, which is extremely hospitable for grape growing.

“Gold Ridge soil is a sandy clay loam,” says Sterling. “Five million years ago, Green Valley was an inland sea that slowly tilted into the ocean, leaving behind the sandy soil bed. So, great drainage.”

Diversity practically defines Sonoma. As such, winemakers are vocal about their soil.

“At La Crema, we seek those soils that are free-draining and which impart relatively low vigor to the vine,” says McAllister. “Most are sandy or silty loams, but we do see some with higher clay or gravel content. The combined influence of climate and soil gives us wines that are complex, multilayered and balanced.”

Because the terroir is so different from one AVA to the next, Sonoma is home to around 50 different grape varieties. Among the most commonly grown are Pinot Noir, Chardonnay, Cabernet and Zinfandel. Varieties like Sauvignon Blanc, Viognier, Pinot Blanc and Syrah have begun to establish stronger footholds.

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