MSU Extension Grapes

The transition into winter is a pivotal phase in vineyard management, as it directly impacts the health, resilience and productivity of grapevines in the coming growing season. Proper winter preparation is essential to safeguard vines against harsh conditions, mitigate risks of damage, and ensure they emerge ready for vigorous growth in spring. This article presents a comprehensive guide to critical winterization steps, underpinned by research and industry best practices, to help vineyard managers protect their investment and maximize vineyard performance.

Cold acclimation of the vines

Cold acclimation is a complex physiological process that prepares grapevines to survive freezing temperatures by transitioning into dormancy. This adaptation is essential in regions with harsh winters, such as Michigan and other northern climates, where temperatures can drop to levels that severely damage or kill unprepared vines. This article draws on the extensive studies of Imed Dami, PhD, of Ohio State University, professor of viticulture and extension specialist, a world-renowned expert in the field of grapevine cold hardiness. Dami provided invaluable insights into the mechanisms and management strategies for optimizing grapevine resilience against extreme winter conditions.

Author’s note: Tragically, Dami passed away recently, leaving a profound void in the viticulture community. His contributions were unparalleled, and his loss is deeply felt by both growers and researchers alike. Dami was frequently invited to Michigan to share his expertise with the grower community. His knowledge, guidance and approachable nature were always met with great appreciation. This article is dedicated to his memory and celebrates his extraordinary academic and practical impact on the grape and wine industry.

Cold acclimation involves several phases

Induction phase: Triggered by shorter day lengths and declining temperatures, in the late summer grapevines initiate biochemical and structural changes that prepare them for colder conditions. Studies by Dami and colleagues highlight that photoperiod sensitivity plays a significant role in native North American grape varieties, while European varieties rely more on temperature cues, making them more vulnerable to early fall frost.

Deep dormancy: During this phase, vines achieve their maximum cold hardiness. Changes at the cellular level, such as the accumulation of soluble carbohydrates and antifreeze proteins, protect vine tissues in the winter from intracellular ice formation. Dami’s research underscores the importance of carbohydrate storage in roots, trunks and canes, which not only contributes to cold tolerance but also supports vigorous and uniform regrowth in spring.

Deacclimation: As temperatures rise in late winter and early spring, vines lose their cold hardiness. Proper management practices during the acclimation phase can delay premature deacclimation, reducing the risk of damage from late frosts.

Factors influencing cold hardiness

Genetic variability: Native North American varieties such as Concord and hybrids tend to develop cold hardiness earlier and to a greater extent than European Vitis vinifera varieties like Chardonnay and Merlot.

Carbohydrate reserves: Accumulation of soluble sugars in vine tissues is crucial for cold acclimation. These sugars act as cryoprotectants, lowering the freezing point of cell sap and preventing ice formation. Dami’s research demonstrates that limiting late-season nitrogen and ensuring optimal leaf canopy management significantly enhances carbohydrate reserves.

Environmental triggers: Photoperiod and temperature are primary environmental factors influencing cold acclimation. Dami’s studies reveal that exposing vines to gradual decreases in temperature enhances cold hardiness, while sudden drops can result in incomplete acclimation and tissue damage.

Best practices for enhancing cold acclimation

Studies by Dami demonstrate that proactive management during the acclimation period not only mitigates winter stress but also lays the foundation for healthier, more productive vines in the growing season ahead. Ensuring grapevines are well-prepared for winter is essential to their survival and productivity. Several best practices, supported by research, can help vineyard managers enhance cold acclimation and optimize vine health.

Early cessation of fertilization and irrigation

• Nitrogen management: Cease nitrogen fertilization by mid-summer to avoid stimulating late-season growth of tender, frost-prone tissues. Excess nitrogen delays lignification (the hardening of vine tissue into bark; Figure 1), leaving vines susceptible to winter damage. Dami's research underscores the risk of over-fertilization in reducing cold hardiness and emphasizes balanced nutrient application as a cornerstone of vineyard health.
• Irrigation practices: Gradually reduce irrigation in late summer and early fall to mimic natural environmental cues, signaling vines to slow vegetative growth and prioritize carbohydrate storage. Maintaining moderate water stress during this period has been shown to improve cold hardiness.

Canopy management

• Leaf area optimization: Efficient photosynthesis is essential for carbohydrate accumulation, which supports vine survival during dormancy. Managing canopy density ensures light penetration and air circulation, preventing excess shading that can hinder carbohydrate production.
• Shoot thinning: Thinning excessive shoots earlier in the season prevents overcrowding, helping to balance vegetative growth and improve the quality of reserves stored for winter.

Pruning strategies

• Timing: Avoid heavy pruning in the fall as it stimulates late growth that interferes with cold acclimation. Pruning should be done in late winter or early spring, after the risk of extreme cold has passed.
• Preserving old wood: Retain sufficient old wood during pruning to serve as a reservoir for stored carbohydrates and protect vines from extreme cold events.

Soil and cover crop management

• Cover crops: Planting cover crops such as rye, clover or legumes enhances soil health and competes with vines for nutrients, reducing excessive vine vigor. Research suggests that cover crops can also improve soil structure and microbial activity, aiding in nutrient cycling.
• Soil insulation: Mulching with organic material or soil mounding around the vine base insulates roots and protects graft unions from freezing temperatures.

Measuring and monitoring cold hardiness

• Differential thermal analysis (DTA): This technique measures the freezing tolerance of vine tissues, enabling precise monitoring of cold hardiness. DTA data helps vineyard managers identify the timing of dormancy and assess risk factors for frost damage.
• Tracking temperature patterns: Use weather data to monitor temperature fluctuations and adjust vineyard practices accordingly. Research has shown that gradual cooling periods promote deeper dormancy compared to sudden temperature drops.

Vineyard fall fertilization

• Macronutrient applications: Apply essential nutrients like phosphorus, potassium and micronutrients post-harvest to replenish soil fertility. Phosphorus is critical for energy transfer and root development, while potassium strengthens cell walls, improving cold tolerance.
• Precision fertilization: Utilize tools like satellite systems, prescription maps and soil testing to ensure fertilizers are applied precisely and efficiently. This targeted approach minimizes waste and environmental impact while ensuring optimal vine nutrition.
• Timing: Apply fertilizers before leaf fall to allow vines to absorb nutrients efficiently and build reserves for winter dormancy.
• Addressing nutritional deficiencies: Monitor vines for signs of deficiencies, such as leaf yellowing, necrosis or stunted growth. Early intervention with balanced fertilization ensures vines have adequate nutrition for cold acclimation.
• Carbohydrate reserves: Dami research highlights the role of post-harvest nutrition in building carbohydrate reserves, which are essential for both dormancy survival and uniform spring bud break.

Preparing vineyards for winter is a multifaceted process that combines scientific knowledge and strategic practices. From fostering cold acclimation and optimizing fertilization to planting cover crops, each step is vital to protecting vines and enhancing their potential for the next growing season. By adopting evidence-based methods, including precision agriculture, vineyard managers can ensure robust vine health, sustainable practices and a thriving harvest year after year.