Understanding and Addressing Uneven Ripening in Grapes

As we approach harvest, many Wisconsin grape growers have reported uneven ripening across clusters and even within individual clusters. This issue, though not uncommon, presents significant challenges in managing harvest timing and ensuring consistent fruit quality. This season, the problem seems to be particularly severe due to a combination of weather conditions during pollination and other environmental factors. Let’s take a closer look at the possible causes and strategies to address uneven ripening.
Causes of Uneven Ripening
1) Weather Conditions During Pollination: One of the primary causes of uneven ripening this year has been the weather we experienced during the flowering period. Cool and rainy conditions disrupted pollination, resulting in Millerandage, a condition where some berries in a cluster develop normally while others remain small and seedless. This lack of uniform pollination leads to the formation of smaller, underripe berries, which can significantly affect the overall quality of the crop. These small, underdeveloped berries ripen much later than their larger counterparts, making it difficult to achieve uniform ripeness across clusters.
2) Nutrient and Water Imbalances: In addition to weather, imbalances in nutrients such as nitrogen, potassium, and boron can also contribute to uneven ripening. These nutrients play critical roles in berry development and ripening. When vines are not receiving the right levels of nutrients, or when water stress occurs—especially during critical growth stages like bloom and berry set—uneven ripening is likely to follow. Sandy soils, which are prevalent in some areas of Wisconsin, tend to exacerbate these issues by providing poor nutrient and water retention.
3) Cluster Architecture and Canopy Management: Dense grape clusters or canopies can create microclimates within the vineyard that lead to uneven ripening. Berries shaded by leaves or other clusters may receive less sunlight and airflow, delaying their development compared to berries that are more exposed. Proper canopy management, including balanced pruning, trellising, and leaf removal, is crucial to ensure that all berries receive adequate light and air exposure.
Addressing the Problem
Although we cannot change the weather conditions from earlier in the season, there are steps we can take to manage uneven ripening more effectively and mitigate its impact.
1) Nutrient and Irrigation Management: Conducting regular soil and tissue tests can help identify nutrient imbalances and allow for the implementation of a targeted fertilization program. Ensuring that your vines are getting the right balance of nutrients, especially boron and potassium, can support more even ripening in future seasons. Additionally, monitoring soil moisture levels and using appropriate irrigation techniques, such as regulated deficit irrigation, can help regulate the ripening process.
2) Canopy Management: Managing your vineyard’s canopy by removing excess foliage and ensuring balanced sunlight exposure can also help improve uniform ripening. Aim for a balanced shoot-to-fruit ratio and adequate spacing between clusters to promote even air circulation and sunlight penetration.
3) Selective Harvesting: In situations where uneven ripening persists, selective harvesting may be the best strategy. By making multiple passes through the vineyard, harvesting only the fully ripened berries, and leaving others to mature, you can optimize fruit quality. This approach, while labor-intensive, helps ensure that only the best quality fruit is harvested.
Uneven ripening is a complex issue that can be caused by a combination of weather, nutrient imbalances, and vineyard management practices. This season, poor pollination due to cool and rainy weather during flowering has been a significant contributor. However, through careful management of nutrition, irrigation, and canopy structure, we can work toward minimizing uneven ripening in future harvests. Selective harvesting can also help manage the impact of this issue as we prepare for this year’s harvest.
Feel free to reach out if you have any questions or need further advice as you manage your vineyard this season!
This article was posted in Grapes and tagged Amaya Atucha, Grapes, uneven ripening.