Low-cordon VSP is a typical system for upright-growing Vitis vinifera cultivars such as Pinot Gris and Riesling. High-wire cordon is more commonly associated with cold-climate hybrids such as Marquette and Marechal Foch, whose shoots tend to grow more procumbently or downward. This distinction should remain in mind throughout postharvest management because the location of perennial wood, the direction of shoot growth, vine capacity, cold tolerance and replacement strategy all affect what the grower should look for after harvest.

Historic Michigan research is particularly informative here. In long-term experiments involving nine cultivars and four training systems, Michigan State University (MSU) researchers compared low head, high head, low cordon and high cordon systems. High-cordon vines generally exceeded low-cordon vines for vine size, yield, fruit sugar accumulation and bud and cane cold hardiness. The experiments also demonstrated that increasing permanent wood changed vine carbohydrate dynamics and overall vine performance. These results should not be interpreted as meaning that high cordon is universally superior, but they demonstrate that training system is a physiological treatment, not merely a way of holding vines off the ground.

Moreover, “hybrid” should never be interpreted as one physiological category. Vidal, Chambourcin, Traminette, Marquette and Frontenac differ greatly in cold tolerance, vigor, disease susceptibility, fruitfulness and growth habit. The same caution applies to vinifera. Pinot noir, Riesling and Cabernet Franc cannot necessarily be managed identically. The guiding principle for the postharvest season should therefore be treating the vineyard according to the biology of the cultivar and the architecture of the training system, not simply according to the calendar.

The Post Harvest window: Valuable, but not Equally Valuable for Every Cultivar

After fruit removal, the vine does not immediately stop functioning. Healthy green leaves can continue assimilating carbon, while carbohydrates and nitrogen are redistributed toward roots, trunks and other perennial organs. Those reserves become important the following spring because early shoot development begins before the new canopy can fully support itself through photosynthesis. MSU research in Chardonnay has demonstrated that functional postharvest leaves contribute to carbohydrate accumulation and that well-watered vines can continue producing new roots after harvest, whereas severe postharvest water stress accelerates leaf loss and reduces new-root formation. The length and importance of this postharvest period, however, differ strongly among Michigan cultivars. An early-harvested hybrid or early vinifera may retain several weeks of healthy canopy after the fruit has been removed. A late-harvested Riesling or Cabernet Franc may have only a short period before natural senescence, frost and cold temperatures largely terminate canopy function.

This distinction is particularly important when comparing hybrids and vinifera. Many cold-hardy hybrids can tolerate winter temperatures substantially below those tolerated by vinifera. Michigan references place most vinifera cultivars in tender to moderately tender categories, although Riesling, Cabernet Franc and Pinot noir may be somewhat hardier. Many traditional hybrids fall into moderately hardy categories, while cultivars derived substantially from V. riparia, including several northern cold-hardy hybrids, can tolerate considerably lower midwinter temperatures.

Consequently, maintaining good reserve status and avoiding late vegetative growth are important for all cultivars, but the consequences of incomplete acclimation can be much more severe in a susceptible Chardonnay or Merlot block than in a well-acclimated Frontenac or Marquette block.

Training system further modifies the response. In low-cordon VSP vinifera, the perennial cordon and fruiting zone are relatively close to the ground. The system facilitates vertical shoot positioning and fruit-zone manipulation, but the permanent structure can be lost when trunks or cordons suffer severe winter injury. In high-wire hybrids, the cordon is substantially higher and shoots grow downward.

There is generally less need to repeatedly position shoots vertically between catch wires, but excessive vigor can produce a dense hanging curtain that shades the interior canopy and interferes with airflow and spray penetration. The postharvest objective for both systems is the same, maintain functional mature foliage without stimulating renewed growth, but the visual signs of imbalance are different.

Read more about the essentials of post harvest vineyard care here.

—Story by Paolo Sabbatini, Michigan State University Extension