Vineyard Post-harvest Management Tips

George Zhuang, UCCE Fresno County, from Vit Tips December 2019

The end of October generally aligns with the end of the harvest season. While this might start the offseason for grape growers, grapevines still require management beyond pruning after harvest. Successful management strategies postharvest can help to maximize the following year’s yield and fruit quality while reduce disease pressure.

Post-harvest Irrigation Management

Depending on production type (raisin, wine, or table grapes), irrigation type (flood or drip), management practices (deficit irrigation), and harvest type (hand vs mechanical) grapevine canopies can end the season stressed and damaged (Figure 1). Therefore, post-harvest irrigation is critical to relieve the vines from stress. This will help maximize the post-harvest canopy photosynthetic activity to accumulate carbohydrate (stored as reserve), to prepare cold hardiness (Greven et al. 2016) and hydrate the vine tissues including root to eliminate embolism (Brodersen et al. 2010). The goal of post-harvest irrigation is to avoid delayed, erratic bud break and ensure canopy early growth with sustainable yield and fruit quality on the following season. How much water is a key question for farmers when applying post-harvest irrigation. For late-season varieties the goal is to apply adequate water to maintain photosynthetically functional canopy, while avoiding too much water such that vines push new growth.

Figure 1. Wine grape canopy after mechanical harvest

According to Dr. Larry Williams, University of California at Davis, at least 10% of seasonal irrigation amount should be applied after harvest. So that is approximately equals to 2-3 acre inches of water (assuming the total water budget for a typical Fresno vineyard requires 2 acre foot per year). For early season varieties, with longer postharvest growth periods, the same goal of maintaining photosynthetic activity, while avoiding excessive growth still holds. However due to the extended length of time, and the possibility of more extreme heat the irrigation needs will be much larger. In either case, growers should continue to irrigate until leaf senescence and dormancy starts, or fall rains begin.

Drip irrigation can satisfy the need for post-harvest irrigation, although dry winter or the need of salt leaching might require furrow or flood irrigation (Figure 2) to recharge the soil profile and move the salts

Adequate soil moisture post-harvest and during dry winter, will hydrate vine tissues (cordon, trunk and root) preparing cold hardiness and help the budbreak on the following season. Different types of damages can result from lack of soil moisture during the winter and early spring including:

Figure 2. Post-harvest flood irrigation

• Winter freeze damage

• Erratic spring bud break

• Delayed early shoot growth

• Poor fruit set

• Small cluster size

• Low yield

• Poor fruit quality

Post-harvest Nutrient Management

Post-harvest is a good time to calculate your vines losses from this year’s production. Annual loss from harvest are approximately 3 lbs. of N and 5 lbs. of K from each ton of green grapes removed. Assumingno N or K in a vineyard’s irrigation water, these nutrients will need to be applied back to the vineyard in the form of fertilizer. It is recommended that yearly bloom petiole or leaf tissue analysis be conducted to monitor vine nutrient status to avoid deficiencies and provide feedback on the effectiveness of current nutritional program. An active canopy is necessary for the vines to assimilate N and K. Therefore, it is best to wait until spring to apply these nutrients. An effective timing of N application is one month after bud break and after fruit set. If the variety is early season, a postharvest application may work if the canopy has an extended period of time that it will remain active. Timing of K application is less restricted and can be applied before or after fruit set.

Figure 3. Soil sulfur band application post-harvest

Post-harvest is a good timing to apply soil amendments (Figure 3), e.g., sulfur and gypsum, to adjust soil pH and improve infiltration.

Dormant Pruning Strategy

Pruning is one of the most important tasks post-harvest. Although it might still be more than a month away from pruning season, growers should take into the considerations now how and when to prune the vines. With the scarcity of labor and increased labor cost, the number of growers using, or interested in adopting mechanical pruning has increased. Many growers have adopted mechanical pre-pruning to save labor cost and improve efficiency of a follow-up hand pruning. In this article I will focus on mechanical box pruning with the possibility of one follow up hand pass to make final adjustments. However, several factors should be considered before switching to mechanical pruning:

Figure 4. More than double number of spurs yielded from mechanical pruning than hand pruning

• Grape type/variety

• Labor

• Trellis

• Production goal

• Disease pressure

Typical box pruner will yield 4 to 5 inches of spur height above and sideways of the cordon wire. Therefore cane-pruned varieties, e.g., most raisin varieties, normally are difficult to be mechanically pruned. New mechanical pruners have been developed to conduct mechanical cane pruning; however, they usually require a specific trellis. Table grapes are more challenging to mechanically prune due to the trellis type, canopy management needs, fruit quality requirements, and disease pressure, however some table grape growers are experimenting with mechanical pre-pruning of spur-pruned varieties. Simple trellises, like two-wire California sprawl and single high wire, will be the easiest to mechanically pruned, and likely wine grape is the easiest choice among grape types.

Mechanical pruning normally leaves more than double the number of buds per vine as compared to hand pruning (Figure 4). This can translate to more clusters per vine with smaller cluster size due to less fruit set, along with smaller berries due to vine self-regulation. Loose clusters with small berries can be beneficial for fruit quality of wine grapes, e.g., bigger skin/pulp ratio, and reduced cluster compactness in bunch rot prone varieties, e.g., Petite Sirah and Zinfandel (Figure 5). Mechanical pruning might increase the yield per vine initially. However, once the vines are adapted to mechanical pruning, the yield of mechanical pruned vines is generally close to hand pruned vines.

More spurs left by mechanical pruning mean more pruning wounds for trunk disease pathogens to attack. Mechanical pruning early might pose a greater risk of trunk disease, however, less labor input through mechanical pruning means growers have greater flexibility to choose the timing. Late mechanical pruning can be an effective way to reduce trunk disease during the dormant season. 

Figure 5. Compact Petite Sirah cluster and bunch rot

Early season irrigation management can be critical for mechanically pruned vines. Mechanically pruned vines result in more rapid early growth than hand pruned vines. Growers might want to apply water and nutrients earlier to satisfy this early growth demand.

Summary

Post-harvest vineyard management is not the finish to this season, but the start of maximizing the yield and fruit quality for the following year, while reducing the disease risk and abiotic stress on vines.

Reference

Greven, M. M., Sue M. N., D. Stuart T., Helen B., Jeff B., Maria C. V. 2016. Effect of Postharvest Defoliation on Carbon and Nitrogen Resources of High-Yielding Sauvignon blanc Grapevines. Am J Enol Vitic. June 2016 67: 315-326.

Brodersen, C. R., Andrew J. M., Brendan C., Mark A. M., Kenneth A. S. 2010. The Dynamics of Embolism Repair in Xylem: In Vivo Visualizations Using High-Resolution Computed Tomography. Plant Physiology. November 2010. DOI: https://doi.org/10.1104/pp.110.162396