MSU Extension Grapes

Grapevine cold hardiness is a critical factor in viticulture, especially in regions like Michigan that experience severe winter temperatures. Understanding the mechanisms behind cold tolerance, the factors that influence it, and effective vineyard management practices is essential for grape growers aiming to protect their crops and enhance yields. This article delves into these aspects, providing insights into the physiological processes of cold hardiness, the role of carbon partitioning, and the impact of dormancy on grapevine resilience. Additionally, it presents the latest cold hardiness evaluations for the 2024-2025 season, highlighting potential risks of freeze injury in certain vinifera grape varieties.

Understanding cold hardiness in grapevines

Cold hardiness refers to a grapevine's capability to withstand freezing temperatures during winter months. This trait is essential in regions experiencing substantial temperature drops, impacting both the survival and productivity of vines. Cold hardiness is commonly assessed by identifying the lethal temperature at which 50% of primary buds are killed, known as lethal temperature 50 (LT50). During the dormant season, grapevines undergo physiological changes that enhance their cold tolerance, influenced by environmental cues such as decreasing day lengths and falling temperatures.

Carbon partitioning and mechanisms of cold hardiness

Carbon partitioning is a crucial process affecting a grapevine's cold hardiness, involving the distribution of carbon compounds produced through photosynthesis to various plant parts, classified as sources and sinks. The ability of grapevines to maintain adequate carbon reserves during dormancy is directly connected to their resilience against cold temperatures. Research indicates that achieving optimal cold hardiness requires a balanced allocation of carbon to storage tissues and growth areas. Practices such as proper pruning and canopy management facilitate efficient carbon distribution, ensuring that the plant can withstand temperature fluctuations effectively. Read more in the Michigan State University Extension article, “Preparing vines for winter: Research-driven approaches to cold hardiness in vineyards.”

The process of achieving cold hardiness in grapevines involves several complex physiological mechanisms. As autumn temperatures decline, grapevines enter an acclimation phase, gradually increasing their tolerance to lower temperatures. This phase is characterized by reductions in tissue water content and elevated concentrations of cryoprotectants, such as sugars, which minimize ice formation within cells.

During winter, vines reach peak cold hardiness, enabling them to survive extreme temperatures. This acclimatization continues until spring, when rising temperatures initiate deacclimation (read more in “Michigan grape growers should prepare for potential late winter freeze and early spring frost risks” from Michigan State University Extension), rendering the vines susceptible to frost damage. A comprehensive understanding of these mechanisms is essential for effective vineyard management and enhancing grape productivity in cold climates.

Factors influencing grapevine cold hardiness

Cold hardiness varies significantly across grapevine species and cultivars, making it crucial for growers to select varieties suited to their climatic conditions. For example, Vitis vinifera, a prevalent species in winemaking, exhibits lower cold hardiness compared to North American species like Vitis riparia, which can tolerate much colder environments. Very few Vitis vinifera varieties can thrive in extreme cold regions, necessitating careful varietal selection for successful viticulture under challenging climatic conditions.

Several factors influence grapevine cold hardiness, including genetic traits, environmental conditions and vineyard management practices. Genetic diversity among grapevine cultivars contributes to variations in cold tolerance, with some demonstrating considerably higher hardiness. Environmental factors, such as temperature fluctuations and the duration of cold periods, critically affect a vine's acclimation ability.

Additionally, microclimates within vineyards can significantly impact hardiness. Potential stressors, including diseases, mineral deficiencies and poor vineyard management, can also hinder a grapevine's cold hardiness. Understanding these complexities is vital for viticulturists aiming to optimize their crops' resilience against extreme winter conditions. Read more in the Michigan State University Extension article, “Post-harvest vineyard care: Securing uniformity and abundant yields for your next vineyard season.”

Vineyard management practices for enhancing cold hardiness

Effective vineyard management is essential for improving grapevine cold hardiness, particularly in colder climates. Strategies include selecting appropriate sites with good air drainage and avoiding frost-prone areas that exacerbate cold injuries. Pruning practices, such as utilizing delayed or double pruning techniques, can enhance vine resilience by allowing additional time for acclimatization before cold weather sets in. Management approaches that encourage vigorous growth and well-structured canopies can further boost cold hardiness by ensuring sufficient sunlight exposure and facilitating robust photosynthesis. Read more in, “Post-harvest vineyard care: Securing uniformity and abundant yields for your next vineyard season.”

The critical role of dormancy in cold resilience

Dormancy is a fundamental phase in the grapevine lifecycle, during which metabolic activity significantly diminishes to withstand harsh winter conditions. This phase can be categorized into three distinct stages: acclimation, mid-winter dormancy and deacclimation. Acclimation begins in the fall with decreasing temperatures, prompting grapevines to enter a heightened state of cold hardiness. Physiological changes occur during this period, including the accumulation of protective compounds and modifications in cell structures to prevent ice formation. By mid-winter, grapevines achieve maximum cold hardiness, enabling them to endure temperatures well below their lethal thresholds.

However, the transition from mid-winter dormancy to deacclimation carries significant risks, particularly during extreme cold events. As warming temperatures signal early spring, grapevines may lose their cold hardiness in a process known as deacclimation. This gradual reduction in tolerance can leave buds vulnerable to late frosts, particularly when unseasonably warm spells encourage early bud break. If active buds face a severe cold snap shortly thereafter, substantial damage can occur, resulting in reduced yields and compromised fruit quality.

The risks related to extreme cold during the deacclimation phase underscore the importance of closely monitoring temperature variations. Sudden drops in temperature can lead to "frost kill," wherein ice crystals form within delicate bud tissues, causing cellular damage and death. Growers have reported significant losses under such conditions, especially in traditionally milder regions experiencing unexpected cold snaps due to climate anomalies. Read more in “Michigan grape growers should prepare for potential late winter freeze and early spring frost risks.”

Furthermore, different grapevine varieties respond variably to temperature changes during this critical zone. Research indicates that certain cultivars possess varying tolerances to both cold and frost events, creating discrepancies in how efficiently each variety manages abrupt climatic shifts. Therefore, growers must select grapevine varieties based on their specific terroirs and resilience to extreme temperatures.

We have integrated daily minimum temperatures from across Michigan's grape-growing regions with a cold hardiness table for grapevines, which categorizes varieties into six levels of cold tolerance, ranging from very cold tender to very cold hardy. Table 1, developed by Dami et al. in 2007, provides a comprehensive framework for assessing freeze risk. The combined chart in Figure 1 indicates that freeze damage is expected in all very cold tender and cold tender vinifera grape varieties. Additionally, the chart suggests the possibility of some freeze damage in varieties classified as relatively cold hardy.

Note: Positive LT50 values indicate temperatures above freezing, while negative values indicate sub-freezing temperatures.

The lethal temperature for 50% bud mortality (LT50) is a critical metric for assessing grapevine cold hardiness. A lower LT50 indicates greater cold tolerance. The data provided spans from Nov. 15, 2024, to Jan. 16, 2025, showing the progression of cold hardiness across various grapevine varieties.

Observations of cold hardiness

The LT50 temperatures show a clear trend of varying cold hardiness across the different grape varieties evaluated.

Resistance variability

Chardonnay and Pinot Gris appear to have a relatively porous resistance to cold temperatures, with their LT50 values dipping significantly below freezing as the dormant season progresses.

Riesling shows moderate cold resilience, although it also experiences significant temperature drops by mid-January.

Merlot exhibits notable fluctuations, showcasing substantial vulnerability by reaching a LT50 of -16.1 degrees Fahrenheit, which indicates its sensitivity to colder temperatures.

Most and least cold-hardy varieties

Pinot Noir and Cabernet Franc (particularly in high tunnel conditions) display considerable cold hardiness, with Cabernet Franc pictured to better withstand cold conditions earlier in the season compared to the latter.

In contrast, Refosco shows high LT50 values early in the season, indicating that it may not withstand as severe conditions as others can.

General trends

There is a distinct pattern of decreasing temperatures as time progresses, with most varieties experiencing colder LT50 values in January compared to their initial assessments in November. This is indicative of the increasing cold acclimation process typical in vines as they prepare for winter.

Understanding grapevine cold hardiness is essential for successful viticulture, particularly in regions susceptible to severe winter temperatures like Michigan.​ The physiological mechanisms behind cold tolerance, the importance of carbon partitioning and effective vineyard management practices are pivotal in enhancing the resilience of grapevines against extreme cold. The analysis of cold hardiness assessments for the 2024-2025 dormant season reveals significant variability among grapevine varieties, emphasizing the need for careful varietal selection and targeted management strategies.

Moreover, recognizing the critical role of dormancy and the potential risks of freeze injury during deacclimation is paramount for grape growers. The data highlights that specific vinifera varieties may face substantial risks during winter months and early spring, necessitating proactive measures to mitigate potential losses. Ultimately, by integrating knowledge of cold hardiness traits, environmental factors and best practices in vineyard management, growers can optimize grape production and improve the sustainability of their operations amidst the challenges posed by climate fluctuations. Through informed decision-making and adaptive strategies, viticulturists can safeguard their crops against extreme temperature events, ensuring fruitful yields and high-quality grapes for the future.