Effects of Vineyard Flooding — SJV Trees and Vines

Karl Lund Viticulture Advisor Madera, Merced, and Mariposa Counties

The 2022 - 2023 rain season has already been prolific. The California Department of Water Resources has a series of precipitation stations across the Sierra Nevada Mountains to track yearly rainfall. The data is broken up into basins, with the San Joaquin Basin and Tulare Basin being relevant to the greater San Joaquin Valley. The San Joaquin Basin runs from Calaveras Big Trees in the north to Huntington Lake in the south and serves as the headwaters for the San Joaquin River. The Tulare Basin runs from Balch Park in the north to Isabella Dam in the south and historically this water would have fed into Tulare Lake. As I’m writing this on April 5, 2023, the San Joaquin Basin has recorded 61.2 inches of rain, well above the 1991-2020 average of 39.9 inches and approaching the 1982-1983 record of 77.4 inches (image 1). The Tulare Basin is at 51.1 inches, well above the 1991-2020 average of 28.3 inches and getting close to the 1968-1969 record of 56.3 inches (image 2). This excessive rain has led to waterlogged vineyards and flooding across the San Joaquin Valley.

Image 1 San Joaquin Basin Precipitation for Oct 1, 2022 - April 5, 2023 (dark blue) compared to average (1991-2020) rainfall (light blue), and various other years. Downloaded from: https://cdec.water.ca.gov/snow_rain.html

The effect this water will have on your vineyard will heavily depend on how much water is in your soil profile and how long into the growing season the water remains. The amount of water in your vineyard soil can range from waterlogged to saturated to field capacity to below field capacity. Field capacity is the state where all the micropores in your soil profile are filled with water, while the macropores are filled with air (including oxygen, which will be important later). As you drop below field capacity the micropores are drained of water until you hit the permanent wilting point. At the permanent wilting point your vines can no longer remove water from the soil, and you need to irrigate your vineyard. Of course, the extra rain this dormant season has sent most vineyards in the San Joaquin Valley in the opposite direction. Saturated soils are soils where both the micro and macropores are filled with water, removing air from the soil profile. Lastly bringing us to waterlogged soils where the soil profile can no longer hold any more water and ponding begins in your vineyard. The amount of water needed to move between each one of these levels of soil moisture will vary dependent on the type of soil and the soil matrix.

Image 2 Tulare Basin Precipitation for Oct 1, 2022 - April 5, 2023 (dark blue) compared to average (1991-2020) rainfall (light blue), and various other years. Downloaded from: https://cdec.water.ca.gov/snow_rain.html

During the dormant season, grapevines are very hardy toward saturated and even waterlogged soil. Flooding a vineyard 40 cm (15.7 inches) deep with water for 40 days was an early (and effective) method of controlling phylloxera outbreaks. Problems with excess rain and flooding during the dormant season come down to soil movement and compaction, as well as vineyard access. Soil erosion during heavy rains can remove, move, or deposit soil in your vineyard. This can lead to vines or vineyard infrastructure being uprooted or buried during these events. Saturated soils can also lead to soil compaction and hardpans. As heavy equipment passes over saturated soils it creates compacted soil layers leading to hardpans within the soil profile. The compacted soils and hardpans make it harder for the vine’s roots to easily spread throughout the soil profile, limiting the growth of your vine’s roots. Compacted soils and hardpans also make it hard for water to penetrate deep into the soil profile. This can make your irrigation less effective at wetting deep soil layers, and leave your vineyard open to worse runoff and erosion when the next heavy rainfall year happens. This will all limit your ability to access your vineyard and conduct normal dormant season vineyard operations.

Table 1 . Wet Feet Tolerance of various rootstocks. Table adapted from Wine Grape Varieties in California

Once the growing season commences, the hardiness of grapevines to saturated and waterlogged soil drops considerably. The issue during the growing season is not with the saturated soil itself, but with soils where the oxygen in the soil profile is severely limited. For root growth to occur the roots need to metabolize sugar either directly from the canopy or from storage in the permanent wood structures (trunk and cordons). Just like us humans, grapevine roots need oxygen to metabolize that sugar. If the soil profile is so saturated that oxygen is limited, root growth will be reduced or stopped. As young feeder roots are the primary uptake point for nutrients from the soil this will have several detrimental effects on the grapevine including reduced shoot growth, leaf chlorosis, and eventually vine death (Williams et al. 1994). The level of resistance your grapevines will have to saturate and waterlogged soils will be determined by the rootstock the vineyard is planted on. For rootstocks “wet feet tolerance” is used to describe the ability to withstand overly saturated soils (Table 1). The two most common rootstocks used in the San Joaquin Valley have vastly different abilities to tolerate wet feet. 1103P is one of the top-performing rootstocks with a moderately high tolerance to wet feet, while Freedom is near the bottom of the list with a low tolerance to wet feet.

For vineyards whose soil is at field capacity, the excess precipitation seen during the 2022-2023 dormant season will still affect how the 2023 growing season will go. Many of these topics were covered in the most recent edition of Vit Tips. As the storm window is still open, a cold front could bring freezing conditions to the San Joaquin Valley. Methods to prevent frost damage were covered by Dr. Justin Tanner’s article Passive Measures to Reduce Impacts of Late Spring Frosts. The excess moisture in the soil will lead to heavy weed pressure this year. Dr. Karl Lund covered this topic in his article Vineyard Weed Control After a Wet Winter. George Zhuang covered several other general topics in his article After A Wet Winter: What You Should Do and Shouldn’t Do. All these articles and more can be downloaded here: https://ucanr.edu/sites/viticulture-fresno/Vit_Tips_Newsletter/

If you have any further inquiries, please contact your county’s UC ANR Viticulture Advisor

Justin Tanner, UCCE Viticulture Advisor San Joaquin, Stanislaus, and Southern Sacramento Counties

209-953-6119, jdtanner@ucanr.edu

Karl Lund, UCCE Viticulture Advisor Madera, Merced, and Mariposa Counties

559-675-7879 ext. 7205, ktlund@ucanr.edu

George Zhuang, UCCE Viticulture Advisor Fresno County

559-241-7515, gzhuang@ucanr.edu

Joy Hollingsworth, UCCE Table Grape Advisor Tulare, and Kings Counties

559-684-3313, joyhollingsworth@ucanr.edu

Tian Tian, UCCE Viticulture Advisor Kern County

661-868-6226, titian@ucanr.edu

Reference Resources:

California Department of Water Resources, California Data Exchange Center Webpage: https://cdec.water.ca.gov/index.html

Williams LE, Dokoozian NK, Wample R. 1994. Grape. In Handbook of Environmental Physiology of Fruit Crops. Schaffer B and Peter C (eds.), pp 115-116. CRC Press, Boca Raton, Florida, USA

Wine Grape Varieties in California UC ANR Publication 3419 pp 12-15