Mid-Season Irrigation Management in Wine Grapes

By Travis Goldman

Managing wine grapes is an art form and no two ranches are same. Different climates, varieties, and soils all play into vine characteristics, but proper irrigation management is what brings them all together.

As we do in our spring program, we will match the needs of our vines as we move further into summer.

Summertime irrigation management plays a critical role in canopy health and optimizing fruit size and quality for harvest. Irrigation stress points can be identified using soil tension within our root zone using Hortau’s real-time soil tension data. With this data, we can accurately monitor root pull and identify when water needs to be applied.

Transitioning out of spring, we begin to see stronger root pull from the vines as leaf maturity supports fruit development.

This steady uptake of water is best managed while letting our soil tension rise and apply what is needed. A healthy balance of water and oxygen in our soils gives our vines the best advantage for efficient growth without wasting inputs.

Over-applying water and nitrogen-based fertilizers at this stage can encourage canopy vigor and lead to an excessive canopy and unneeded vegetation. A larger canopy will add more leaf surface for transpiration, driving more water use and restricted airflow. On the opposite side of the spectrum, insufficient irrigation practices will lead to nutrient deficiencies and stunted fruit.

May fertility and irrigation management really go hand in hand. Some irrigations might have to run a little longer than needed to get material out, but with proper planning we can reduce leaching and excessively wet roots.

In the example below, we can see that after our initial irrigation of 12 hours, we realized that we could step back to 7.5 hours. On May 12, we were a little bolder and reduced the irrigation down to just 5 hours. This irrigation was insufficient in fully rehydrating the wetting pattern and gave the grower 5 days to get around to other blocks with the pump. With these rather short sets, we were able to efficiently build the proper canopy to support the fruit load on the vine.

In a vineyard that established too much vegetative growth, we can look at its water and fertility program from May.

Previously, this block was stunted due to excess water standing in the root zone. Soils came back slightly high in nitrogen but due to the slower growth the production team decided on apply more water and fertilizer to get the vines to pick up.

In this management program, the vines have very little time to stay within the desired comfort zone and barely have time to get away from the oxygen-deprived soils. Later in May, the root system began to take on this luxury abundance of nutrition and the vines vegetative stage took off. This excess vegetation hurt the airflow of the block and powdery mildew became an issue. A series of sprays and mechanical pruning were required to correct this issue but the canopy was still larger than needed for the fruit set of the vineyard.

On a more cautious side, some vines like estate wine grapes are stressed to prevent excessive vegetation. This management plan will work but only if the canopy footprint is not fully set in. In this example, a very nice estate wine is being managed to control plant vigor and production. Because of the smaller canopy, we see much less root pull from the vineyard and thus less water is needed. As our shallower soil profile drifts out of the “comfort zone,” the vine will begin to slow down. The vine will also begin to pull water from areas with available water seen in the deeper soil horizons. To maintain this controlled stress, water should be applied to rehydrate the 18” root zone. A 6-hour set on May 17 was scheduled, but due to pump issues the irrigation was stopped and restarted on May 19. Having a continuous irrigation would have rehydrated the 18” profile before reaching the 24” and 36” soil depths. An additional 3.5 hours would have been advised on May 19 to achieve what was needed on May 17. By the end of May, the 24” profile has pulled out of the “comfort zone” and a larger irrigation will be required to rehydrate the soil. With this stress, the canes will show shorter internodes compared to a less stressed vine.

Moving into the heat of the summer months of June and July, we have to take care of the fruit and the canopy that we have established. Water uptake is critical for cell expansion and nutritional flow for fruit and for the maintenance of leaf chlorophyll. By matching plant demand with proper irrigation, we encourage nutritional flow without pushing cell expansion too quickly. Excess water at this stage will lead to reinvigorating the canopy and larger diluted fruit that runs the risk of rupturing. Vines that are water stressed at this stage of the year will have a reduced fruit size, shorter cane internodes and an onset of chlorosis within older leaves.

In an overgrown canopy we will see rapid water update. In the below example, we are kind of stuck with feeding a grizzly bear that needs 16-hour sets every four days. The right response to this is to give the vineyard what it needs and nothing more. During this time span, the grower expected to go more than three days without water by applying large and frequent irrigations. Because our soils can only hold a finite amount of water, we see hours of leaching.

By dissecting these irrigations and soil tension curves, we can confidently say that 147 hours of water was not needed to sustain the block during this period. In an irrigation schedule, I would have suggested that the grower irrigate for 16 hours on July 9, 13, 17 and 22. Another repercussion from overwatering on this block was the fruit grew a little faster than needed causing thinner skins and a crowded cluster. By August botrytis began to show up as the crowded fruit began to split.

Under ideal growing conditions, we would be transitioning into July with a controlled canopy that is able to support developing fruit. In the example below, we were able to match crop water requirements and minimize down time below and above optimal growing conditions. By matching the vine’s uptake and waiting for the tension to rise to a higher threshold, the grower was able to maintain crop weight without compromising skins and crowding. This time also gives the grower the freedom to run up to two irrigations per head of water and fit in an additional seven irrigation sets on the ranch. In comparison to an over-watered block, we can identify better root pull throughout the entire root zone because of beneficial soil/air/water conditions.

Moving back to our high value wine grapes we see two different practices. Practice 1 allows the vine to slowly stress by not meeting crop irrigation demands and periodically applying water. This fruit will tend to be much smaller, but botrytis is almost never an issue unless pests are feeding on the fruit. Later in the summer, the canopy will start to express deficiencies of water and nutrition. One larger pest issue is mites and the possibility of defoliating the canopy if pest pressure occurs.

Practice 2 still takes on a deficient irrigation practice but rehydrates the soil after periods of stress. In the example below, we can see that the 11.5-hour irrigation on July 10 was sufficient in rehydrating the entire wetting pattern  and that the irrigation on July 19 should have run for an additional 6 to 7 hours. The intentional stress added to the vine from July 13-19 helped control fruit size as well as restrict canopy vigor. During this month, I would have rather run an 11-hour irrigation on July 3, 14 and 25.

Irrigation management for wine grapes in the summer months can take on multiple approaches, but my best advices is to efficiently take care of the canopy and fruit set that you are dealing with. Irrigation practices that benefit roots and canopy can be found in both high and low volume producing blocks as long as they are routinely monitored. Root development and root health is significantly improved if we allow the vine to cycle through water and apply what is needed. Equally so with good irrigation practices we can meet the demand of the developing fruit and improve quality.

For more questions on vineyard irrigation management, contact your local Hortau Irrigation Advisor today.

About the Author

Irrigation Management Advisor Travis Goldman was born and raised in Watsonville and currently provides irrigation scheduling services to growers in California’s Central Valley. He graduated from UC Davis and has worked throughout California as a service technician, grower support specialist and irrigation management advisor to learn and understand every aspect of Hortau’s irrigation management platform.

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