Brain on Winemaking: The Winkler Index is an Elegant Relic

The Winkler Index is a system developed by University of California professors and wine aficionados A.J. Winkler and Maynard Amerine in the 1940s, to group California’s potential grape-growing climates into five categories based on heat accumulation. Region I (e.g., coastal Monterey County) was the coolest and Region V (southern San Joaquin Valley) the warmest. The index evaluates a region’s potential to ripen certain grape varieties during a growing season so that the wine industry historically could rely on this system to help plan viticulture investment and development.

Although originally designed with California regions in mind, the Winkler Index has been used very successfully in areas from Chile and Argentina to Oregon and Washington. It is an extremely well-thought-out guide that relies on both experimentation results (we are talking about thousands of fermentations analyzed in the lab!) as well as decades of anecdotal data for its construction.

However, the index was conceived at a time when vineyard development was possible at a fraction of current costs and the industry was more able to explore and take chances. Now, more accurate forecasting is needed, and proper site analyses will need to account for protection against climate extremes along with general heat values.

AIMING BEYOND SIMPLE

To this day, the index remains an elegant and simple tool that can translate basic temperature data into general varietal planting guidance. But the key word is “simple.” Only broad generalizations are able to be drawn from Winkler, as it was developed with large geographic areas in mind, not microclimates, and data collection sites were few and far between. Also, the index incorporates average daily temperatures, with no regard for how long through the day the high temperature persisted, which in the case of heat is truly important with regards to both ripening and vine stress.

In a different vein, the index only considers days with average temperatures over 50 degrees Fahrenheit (10 degrees Celsius), since the researchers observed that grape vines don’t grow below that temperature. However, lower temperature limits should really not be 50 degrees, as we know now that vines metabolize (sometimes quite efficiently) at temperatures in the 40s.

Also, upper limits should be established, as we also know fruit ripening does not take place above 95 degrees, so higher temps should not contribute ripening value. Since its development in the 1940s, several additions have made the reference more relevant, i.e., adding upper and lower limits to Regions I and V, and subdividing Region I into Ia and Ib. But to inform sound planting decisions, a number of other issues need to be taken into consideration.

Winemakers across Napa Valley are currently preparing to blend and bottle the 2022 vintage, and like me are probably remembering the extreme heat we experienced that summer. While I am remarkably satisfied with the quality of wines I was able to produce, the unprecedented heat spike definitely impacted aroma, texture and color concentration, as high temperatures above 115 degrees persisted over multiple days in some areas.

Not only are our growing regions becoming generally warmer over time, but we are also experiencing more extreme variations in temperatures from season to season. Varieties of grape vines respond quite differently to extreme conditions, and certain microclimates and growing sites are more favorable to ripening during these extreme temperatures. Extreme heat, like extended drought, is a serious threat to the quality and reputations of our beloved growing regions.

In 2022 I saw dry-farmed (non-irrigated) vineyards with deep roots on deep and easily penetrated soils survive the unprecedented heat wave exponentially better than frantically irrigated vineyards on rocky, poorly cultivated and prepared soils that were actually in cooler locations.

On Atlas Peak, where west-facing rocky soils reached extreme temperatures, grape varieties like petit verdot and malbec retained most of their color, tannin and depth of flavor, while certain cabernet/rootstock combinations in adjacent blocks produced weak and thin, simple wines. Temperature tables, of course, can never give insight into these microclimate and soil-specific lessons.

CONSIDERING THE ALTERNATIVES

At some point in vineyard development research, the focus must be pulled away from the average temperature data, and ripening value must come from more specific temperature and duration standards. The Huglin Index –a bioclimatic heat index for vineyards developed by French scientist Pierre Huglin in the 1970s – incorporates some of this critical data. The Biologically Effective Degree Day Index also considers more factors and data, but sometimes makes criteria for using the index more difficult to obtain.

After temperature has been investigated, thought must now be centered on the microclimate-varietal combinations that will allow for heartiness and acceptable ripening in both typical years as well as through more extreme climate swings.

These site factors may, in fact, have a lot to do with the soil. Less nutrient-deficient soils will help growers achieve a canopy with enough vigor to shade and protect fruit during scorching sun. Deeply rooted vines stay much cooler in general than shallow rooted vines, and cool roots help slow dehydration, which helps with vine ripening during high heat. East-facing slopes that are spared some of the afternoon direct sun and areas that cool rapidly in the evening (decreasing the time vines are subject to high temps during heat events) also do well during high-heat events.

Satellites and aerial photography that illustrate soil temperatures, fertility and ground cover uniformity can now be utilized to inform our planning. Vineyard soil preparation can also be informed from this imaging.

At some point though, the local (or most proximate) growing community should be engaged, because that is where the true expertise can be found. Experts, consultants and neighbors can lend their experience with varying growing years to give insights to the best bets for specific site development recommendations.

Once a brilliant innovation, the Winkler Index served the industry well for decades. But modern vineyard development requires using contemporary indices that are more informative. The Internet and other online spaces that connect us all provide professional networks and information only dreamed of by Winkler and Amerine. And though now a relic, the index should be cherished as an elegant and ingenious product of scientific minds whom luckily, also loved both vines and wines.

– Matt Brain