Pruning and Training Grapevines
Pruning Grapevines
Dormant pruning is a critical component of wine grape production, providing a mechanism controlling the size and the form of the grapevine, optimizing the production potential of the grapevine, and maintaining the balance between shoot and fruit growth. If the vine is left unpruned, the number of grape clusters would be excessive resulting in a reduced rate of sugar accumulation, reduced pigmentation in the berry skins, and decreased synthesis of flavor and aroma constituents. Other negative effects include poor wood maturation and a reduction in vigor and vine size.
Principles of Grapevine Behavior
Some of the principles of grapevine behavior that one must consider when pruning vines are discussed below enunciated by Winkler et al. (1974).
Cane Pruning Versus Spur Pruning
One of the most critical steps in viticulture is pruning the grapevines. This practice, honed over centuries, is essential to ensuring that the vines produce the highest-quality grapes. Proper training of grapevines is necessary to maintain plant size, shape, and productivity. If left unattended, grapevines can become unruly, and fruiting will be poor due to excessive vegetative growth. There are two main pruning methods for grapevines: cane pruning and spur pruning.
Time of Pruning
Vines can be pruned during the dormant season between leaf fall and bud break the following spring. Early pruning in wet winter climates exposes the vines to greater risk of infection by wood canker diseases (e.g., Eutypa dieback).
Vine Balance
Vine balance is defined as the state at which vegetative and reproductive growth can be sustained indefinitely while maintaining healthy canopy growth, adequate fruit production, and desired fruit quality, namely sugar levels, acid balance, and flavor compounds. In the most straightforward sense, vine balance has been defined and calculated as the ratio of vine yield to vine size, representing reproductive and vegetative production, respectively. This relationship is known as crop load and is calculated by taking a vine’s yield and dividing it by the dormant pruning weight. Crop load ratios that are lower than the optimum range are under-cropped (low yields and larger vine), while numbers at the high end of the spectrum are over-cropped (more fruit and smaller vine). Ultimately, being at either end of the spectrum can lead to unsustainable vine growth and production. There are no prescriptive guidelines for achieving balanced vines across all vineyard sites.
Overcropping
If too many fruitful buds are left during winter pruning, the photosynthetic capacity will be inadequate to ripen the fruit clusters. This situation is referred to as overcropping.
Undercropping
Undercropping involves retaining too few buds resulting in a vigorous canopy at the expense of fruit production. Undercropping just doesn’t lower yields, but as a consequence produces an excessively shaded canopy that provides a poor fruit-ripening environment.
Methods in Determining Vine Balance
There are a number of methods used in determining vine balance, but some of the most common are:
Pruning Formulas: Pruning formulas for many varieties have been developed to calculate the number of buds to be retained for a given pruning weight. A pruning formula of [30 + 10], for example, would require leaving 30 buds for the first pound of canes removed, plus an additional 10 buds for each additional pound of prunings.
Yield-to-Pruning-weight Ratio: Another pruning strategy that utilizes the vine balance concept is to adjust pruning levels based on the yield-to-pruning-weight ratio, also known as the Ravaz Index. It is calculated by dividing the pounds (kgs) of fruit per vine in a given row or block at harvest by the pounds (kgs) of pruned canes per vine taken from the same area the following winter.
Average Cane Weight: Mean shoot weight is a good index of vigor. Counting the number of dormant canes per vine before pruning and dividing by the number of canes pruned gives the average cane weight.
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