By Christopher Chen, Ph.D., UC Cooperative Extension

It’s been a while since we’ve been able to say that we had a ‘normal’ growing season in vineyards around Sonoma County. Whether it is early-spring frost events, consecutive years of drought, or higher summer temperatures, it seems that there are many obstacles to cultivating wine grapes lately. The impact of these weather events on north coast wine country may be viewed as a direct impact of changing climates on agriculture. A rise in the frequency and intensity of ‘extreme’ weather events has been observed across the state of California in recent decades; these include large wildfires, freezing temperatures, prolonged drought, and is leading to the migration of agricultural pests and diseases. It is the increasing frequency of these events that have kept the grape growers of Sonoma County on their toes in recent years. Since April 2013, day-to-day precipitation events have been recorded at little more than a tenth of an inch or less regardless of the time of year. The lack of winter precipitation has resulted in a prolonged drought which has exacerbated the concerns of vineyard managers. However, winter 2022-2023 has seen a huge increase in daily precipitation compared to the previous nine years, breaking the trend for now.  

Unpredictable precipitation has been complemented by unexpected frost conditions across the county, with some regions more strongly impacted from year-to-year. In direct contrast to frost events, our climate is somehow also getting warmer over time with the general trend headed towards a hotter, drier region. Heat accumulation hours or Growing Degree Days have been accumulating more rapidly and to a higher maximum value across the county since 2013. This acceleration of heat accumulation has seen vineyard health and development impacted. Key phenological events like flowering and véraison generally occur sooner in vineyards than previously recorded; vines are more likely to succumb to disease and other extreme stressors when water is lacking than with well-watered conditions. 

With changing climates, researchers know what kind of changes to expect in our weather, but not always when and where these changes will occur. The north coast viticultural areas of California are extremely varied when it comes to their weather patterns and local microclimates. It is this variability which may lead research on climate-adaptive viticulture to come to broad, sweeping conclusions which do not apply to some areas in the coastal north. To prepare for changing climates and to properly adapt our viticultural practices, it becomes necessary to adopt a hyper-localized approach when looking for solutions.

The UCCE North Coast Viticulture program has taken steps to address the growing impacts of climate change on grapevines. A new study, focusing on the tolerance of different rootstocks to drought conditions was launched in 2022. This study has begun to identify unique microclimates across Sonoma, Mendocino, and Lake Counties in order to quantify the regional drought response of the five, most-popular rootstocks planted in the north coast. The project, entitled “Rootstock Selection Under Drought” puts a strong focus on the impact of place on the vine drought responses. 

Seven partner vineyards representing six different American Viticultural Areas (AVAs) have agreed to allow UCCE researchers into their vineyard blocks in order to quantify the conditions grapes are exposed to throughout the year at those unique locations. In an attempt to identify the underlying characteristics of a site and quantify térroir, each experimental site will be classified by coastal-influence, elevation, soil properties, and weather patterns. These data will help sort each site into a representative microclimate which may be used as a basis for comparing grape rootstock performance under drought conditions. The study has focused on the two largest scion varieties in California, Cabernet Sauvignon and Chardonnay; these varieties represent nearly 40% of the planted wine-grape acreage in California and have been selected to ensure consistent results from the study.

Information provided by collaborating growers and nurseries has allowed the project researchers to select five, well-represented grape rootstocks to test. Across California, some rootstocks are widely-planted in a limited region, often for specific benefits. In the San Joaquin Valley, the rootstock “Freedom” is often planted. However, Freedom is not a common choice of grape rootstock in the north coast. Based on popularity and variations in previously reported drought tolerance, the five rootstocks selected for this study are 1103 Paulson, 110 Richter, 101-14 Mgt, 3309C, and SO4. Combined, these five cultivars account for over 70% of wine-grape acreage planted in the north coast since 2018.

The goal of the “Rootstock Selection Under Drought” project is to better understand how our rootstocks will be useful or detrimental under the varying potential outcomes of climate change. By looking at their responses to drought conditions in the niche microclimates of north coast AVAs, we will gain a better understanding of grape rootstock performance under water stress. Our goal is to classify the microclimates included in this study into categories; categories which can be referenced by growers to help decide on cultivars for future vineyard plantings. The hyper-localized impacts of climate change will need to be confronted at a hyper-local level. Changes in precipitation may differ between neighboring valleys. One of the most actionable items any grape grower can take is to try planting uncommon cultivars. While the growth characteristics of many scion varieties are well known (i.e., early or late budbreak, vigor, early or late maturity, etc..), growers in a given AVA may have never planted varieties which may be able to maintain desirable berry characteristics in new climate conditions. The same concept is true for grape rootstock cultivars. To be a truly, climate-adaptive vineyard, growers will need to identify which grapevines work best in their new climates and which should be avoided.

The coming period of ‘trial and error’ may be difficult for some grape growers. Not everyone can afford to replant a vineyard to a little known cultivar on an unproven rootstock. Our goal, as a research and extension institution, is to help growers find the answer sooner, rather than later, and with as little cost as possible. If you are interested in supporting our efforts in climate-adaptive viticulture, please reach out to your nearest University of California Cooperative Extension office.