Few can claim to have had as profound an impact on modern-day viticulture as Albert J. Winkler. After a career that officially spanned 40 years and then continued long into retirement, Winkler (or "Wink" as he was known by those close to him) was described by some as the “world’s greatest horticulturist” and “the father of modern day Californian viticulture” (along with Maynard Amerine).

Albert J. Winkler & His Contributions to Viticulture

His work with wine, which was briefly interrupted by the period of Prohibition within the United States, includes achievements such as: devising a method for classifying the climates of wine-growing regions (The Winkler Scale) and publishing the internationally acclaimed viticultural reference bible General Viticulture.

Winkler’s most important work in viticulture began in the late 1930’s, after Prohibition was repealed, however, by that time, he had already gained a doctorate in horticulture at UC Berkeley and developed a method for the preservation of grapes using Sulphur Dioxide gas. This in-turn created a whole new market for California-grown table grapes on the East Coast.

The Winkler Scale

While working at UC Davis in the late 1930’s , Winkler, alongside Maynard Amerine, wanted to determine which grape varieties would produce the best wine throughout different areas of California. The system they devised split the state into five different temperature-based regions (numbered I - V) and this division later became known as the Winkler Scale.

Degree Days

The Winkler Scale, otherwise known as the heat summation method or Winkler Index, lays its foundations on the assumption that grapevines require a minimum temperature of 50°F to develop. Each day within the plant’s growing cycle is awarded a "degree day" value, whereby 1 degree day is added for every °F above 50 for the day’s average temperature.

The aforementioned cycle refers to the period between April 1st & October 31st in the Northern Hemisphere and October 1st & April 30th in the Southern Hemisphere. At the end of these cycles, all days are added up with the total used to define which region on the Winkler Scale the area being studied should be classified into.

The Winkler Index Regions

The Winkler Index is classified into five regions ranging from 2,500 days to 4,000 or more, known as Heat Summation Regions, as follows:

  • Region I - 2,500 degree days or less
  • Region II - 2,501–3,000 degree days
  • Region III - 3,001–3,500 degree days
  • Region IV - 3,501–4,000 degree days
  • Region V - Greater than 4,000 degree days

Using the same principles, it is possible to work out optimum degree day values for different grape varietals (based on the average daily temperature required to reach maturity). With this information, vineyard professionals can accurately select the best area to plant specific grape varieties and subsequently attain the highest possible quality of wine.

In general, regions I and II produce the highest quality wines and are ideal for the growing of grapes such as Chardonnay, Pinot Noir, Cabernet Sauvignon and Sauvignon Blanc. Region I in California, which includes Oakville in the Napa Valley, can be described as comparable to Burgundy in France when looking at degree day totals. Region II, on the other hand, which includes most of Napa and Sonoma counties, can be compared to Bordeaux.

Practical Use of the Winkler Index

During the period when Winkler and Amerine were carrying out their research, it was widely recognized within the US that national wine was inferior to European wine, and in particular, French imports. However, now Californian vintners and vineyard owners had a tool which they could use to vastly improve the quality of their wine.

Vines, which had before been planted largely at random, were ripped up and replaced with area-specific varietals. The improvements to the quality of Californian wine were immediate and the period that followed was one of further development and study, all masterminded by Winkler, who continued his work at UC Davis until 1963.

The Winkler Scale is used to this day throughout the California, Oregon and Washington states as well as various wine growing regions outside of the US.

Albert J. Winkler's Published Work

Albert J. Winkler & His Contributions to Viticulture

Winkler’s most notable written work, General Viticulture, was first published in 1962 and then later revised in 1974. To this day, it is considered to be "THE" English language viticultural reference. Since the publication of the 1974 2nd edition, the book has been translated into a number of different languages including Spanish, Italian, Hindi and Russian.

Besides General Viticulture, Winkler has over 150 published essays to his name, and his research and writing continued long into his retirement.

On August 29, 1989, Albert J. Winkler died of natural causes. He was 95 years old. His death came less than one year after that of his childhood sweetheart and wife of 69 years, Pearl Buehrer Winkler. His obituary and recognition is noted in the respective UC Calisphere page along with his life's work.