MSU Extension Grapes

Michigan viticulture at the crossroads of climate and risk

In Michigan’s thriving wine regions, stretching from the sandy soils of the Lake Michigan Shore American Viticultural Area to the cool-climate slopes of Leelanau and Old Mission Peninsulas, grape growers are no strangers to the volatility of Midwestern weather. Yet among the many challenges they face, hailstorms present one of the most abrupt and devastating threats to vine productivity and longevity. Unlike frost, which tends to strike early and predictably, hail can arrive without warning during any phase of the growing season, wreaking havoc on green tissue, inflorescences, clusters and even woody structures in a matter of minutes.

On Saturday, June 21, a significant hailstorm went through several of Michigan’s key winegrowing regions, delivering potential damage to developing canopies and young clusters. While the full extent of the damage is still being evaluated by growers across the state, early reports indicate varying degrees of injury to shoots, leaves and fruit, depending on location, storm intensity and vineyard exposure. The Michigan State University Extension viticulture team is actively working alongside industry partners to assess damage patterns, document vineyard conditions and guide growers in making timely recovery decisions.

At this stage, every hour counts. The first seven days following a hail event represent a critical window during which targeted interventions, such as strategic pruning, disease protection and canopy management, can dramatically influence the vine’s ability to recover both this season and next.

What follows is a concise, science-based outline of the most important steps to take immediately after a hailstorm. These actions, if implemented promptly, can help stabilize the vine, prevent disease entry and preserve yield potential where possible. The risk is not only immediate but growing. Recent climate models indicate that hail-prone storm cells are becoming more frequent and more severe in temperate viticultural zones, including across Michigan.

A 2020 study found that hailstorm frequency in the Midwest increased by over 17% since the early 2000s, and a 2024 review emphasized that climate-induced atmospheric instability is likely to raise that trend further. In recent growing seasons, hail events have inflicted significant losses on Michigan vineyards, from Grand Traverse County to Van Buren County. In 2023 alone, growers in several regions reported yield losses of over 50% following early summer hailstorms. The long-term impacts of such damage, on both fruit quality and vine canopy, can be severe if not properly managed.

But recovery is possible. By applying timely, research-informed management strategies, growers can not only salvage the current crop where feasible but also protect vine health and future productivity. This advisory provides a comprehensive, science-based guide to managing vineyards after hail events, integrating current research from international viticultural studies and adapting them to Michigan’s regional context.

Assessing damage and taking action in the first 48 hours

The window for effective hail recovery begins immediately. Within the first 24 to 48 hours, growers must thoroughly inspect vines to assess the full extent of the damage. This inspection must go beyond superficial canopy loss. In fact, one of the greatest challenges following hail is that vines begin to regrow quickly, particularly under warm and moist conditions, masking damage to the vascular system or woody structures. A careful survey should look for leaf shredding and defoliation, shoot tip breakage, shattered petioles, bruised or ruptured berries, and bark injury to the trunks or cordons (Figure 1). Photos and spatial records are not only useful for insurance claims but for understanding spatial variability in damage across blocks.

The phenological stage at the time of the storm critically influences the vine’s potential to recover. When hail occurs early in the season, between budbreak and bloom, grapevines, especially healthy, mature ones, can regenerate remarkably well. Secondary and latent buds often push new shoots, allowing partial canopy and even yield recovery. Recent research on several cultivars reported that vines completely defoliated in early spring were able to form new canopies and achieve up to 40% of their expected yield when managed properly.

The situation becomes more complicated in mid-season, from bloom through fruit set. Damage during this window typically destroys emerging clusters and can lead to uneven regrowth. Still, if canopy function is restored quickly, vines may accumulate enough carbohydrates to sustain next year’s fertility.

Hail after veraison, however, is especially dangerous (Figure 2). At this stage, leaves are the engine for sugar accumulation in the berries and for replenishing starch reserves in roots and trunks. When these leaves are lost or clusters are bruised or burst, not only is that season’s fruit compromised, but the vine’s reserve capacity for overwintering and bud fertility may be diminished. According to Frioni et al. (2023), vines that lost more than 60% of their canopy post-veraison showed delays in ripening and lower bud fertility in the following season, even when fruit was removed.

Pruning for recovery: The role of crown buds and trunk renewal

One of the most effective post-hail interventions is strategic pruning. Rather than attempting to leave damaged shoots in place, growers are advised to prune back to basal nodes, a technique known as basal stub pruning. By cutting just above the crown bud at the base of the damaged shoot, growers’ direct energy into strong, well-placed regrowth. These shoots are more likely to develop into usable fruiting canes for the following season, and in some cases, may even produce a modest crop. This technique also minimizes the risk of shoot dieback and secondary infection.

Research from the Australian Wine Research Institute and NZ Winegrowers shows that crown bud pruning following hail can restore productive architecture more effectively than partial shoot cuts. In contrast, mid-shoot pruning, where damaged shoots are trimmed halfway, leads to weak lateral regrowth that is poorly aligned and often unsuitable for training.

When damage extends to the cordons or trunks, particularly common in young vines, retraining may be necessary. If bark is split or the vascular tissue is compromised, growers should select a vigorous sucker or basal shoot, remove competing growth and begin vertical retraining. This approach ensures long-term vine survival and avoids the complications of internal rot and trunk disease associated with hail wounds. Studies from Michigan State University (MSU) confirm that retraining yields higher long-term success rates than attempting to use partially damaged shots and canes.

Understanding the physiology of wounding and vine recovery

The ability of grapevines to recover from hail damage is rooted in their physiological resilience. Even when a significant proportion of the canopy is lost, vines can maintain photosynthesis through lateral shoots and often avoid reductions in berry sugar accumulation or bud fertility if water, nutrition and pest control are optimized. A recent field study in Spain by Ghirardelli et al. (2021) demonstrated that vines losing up to 70% of their primary leaf area rapidly regrew lateral shoots with comparable photosynthetic rates. Total soluble solids in the fruit continued to accumulate at near-normal rates, and cluster thinning post-storm even enhanced fruit uniformity in some cases.

Wound healing, however, presents a different challenge. Grapevines do not regenerate vascular tissue. Instead, they compartmentalize damage by forming callus tissue around wounds. The speed of this process is dependent on the vine’s energy reserves, pruning technique, and environmental conditions. Warm, dry weather promotes wound desiccation and healing, while excessive moisture or over-vigorous regrowth can slow the process and increase infection risk.

To aid healing, growers should apply balanced fertilization, particularly focusing on potassium, calcium and boron, which support tissue integrity and avoid excessive nitrogen, which promotes vegetative overgrowth. Irrigation should be moderate, aiming to maintain vine function without encouraging excess lateral development.

Disease management: Fungicides, wound care and latent threats

Hail injuries, particularly to clusters, open the door to a wide range of fungal pathogens. Among these is Botrytis cinerea, the causative agent of bunch rot, which colonizes ruptured berry skin. In warm, humid conditions, infection can spread rapidly, especially if damaged clusters are retained. Applying a fungicide within 24 to 48 hours of a hailstorm is critical. MSU Extension trials confirm the effectiveness of several products when applied early and with sufficient canopy penetration.

In addition to bunch rots, growers must be vigilant for wood pathogens. Hail-inflicted wounds can serve as entry points for Eutypa lata, Botryosphaeria spp. and Agrobacterium vitis (crown gall). These diseases often remain latent, appearing in subsequent years as reduced shoot vigor, cankers or gall formations at the crown. To reduce infection risk, large wounds on trunks or cordons should be sealed with protective compounds. Pruning should be delayed until the vine stabilizes unless necessary to remove dead or decaying tissue. Sanitation of tools and the vineyard floor is also critical to reduce inoculum pressure. See “Preventing grapevine decline in Michigan: Pruning wound management and best vineyard practices” for details.

Building resilience: Shade nets and adaptive pruning

While hail cannot be prevented, its impact can be mitigated through preventive measures. In high-risk areas or blocks, anti-hail nets made from high-density polyethylene have shown excellent results in trials across several viticulture regions of the world. These nets reduce not only the physical injury to vines but also sunburn risk and bunch rot incidence in seasons with frequent storm activity.

Post-hail summer pruning must be approached strategically. Removing damaged shoots and excess lateral growth can improve airflow and reduce disease pressure, but over-thinning can exacerbate stress, especially in dry or heat-prone seasons. Canopy thinning should focus on removing necrotic or non-productive tissue while preserving functional photosynthetic surface area. Research by Poni et al. (2023) cautions that aggressive leaf removal post-hail may expose clusters to sunburn, especially under clear, hot conditions common in Michigan’s post-storm weather patterns.

Resilience through informed action

Hail damage presents a challenge for Michigan grape growers, but timely, targeted and informed interventions, rooted in a sound understanding of vine physiology, disease ecology and regional growing conditions, can make the difference between a total loss and a partial recovery. Growers who act within the first week, prune with purpose, apply fungicides strategically, and monitor their vines closely throughout the season are far more likely to restore vine function and ensure strong performance in the following year.