The first task in the vineyard each year is to prune off the old shoots from the previous years growth. Each year a grapevine will grow shoots over 5′ long, each producing 2-4 clusters of berries. The nature of the vine is to produce as much growth as possible in a year and expend all its energy. When pruning we try to cut back shoots with buds on them to limit how much new growth a vine can produce in one year. A certain number of buds are left remaining on fruiting spurs to generate new shoots for the current year and to produce the fruit for this years harvest. Every 4-6 inches it is best to cultivate only 2 shoots each producing 2 clusters. So that in an 8′ space there would be 32-40 clusters of fruit from one plant. This type of selective pruning is used to set the potential crop level for the vineyard.
This suggested crop level is based on the plant being vigorous and mature enough to support that much fruit. There is a whole formula to size up each type of vine and gauge whether the average plant in the vineyard can support that much new growth. This is done through a method referred to as balanced pruning. The amount of 1st year wood that is removed each dormant season is a reflection of the magnitude of growth in that vineyard. By taking the total weight of the wood pruned off an average size vine and plugging that into a formula, you will determine the number of buds to leave on a vine depending on its variety. In our case with the average pruning weights of 2.5lbs on our chambourcin vines, means we should leave 35 buds. If some vines appear to be less vigorous, than fewer buds should be retained.
Next it is important to retain the most fruitful and best positioned buds for the best fruit production. Note that grapevines flower and fruit only on one-year-old wood, usually recognized by the reddish-brown bark. This reddish color is also an indication of healthy well exposed growth from the previous year rather than growth that was infected with fungus disease. The most fruitful wood is also 1/4″ to 3/8″ in diameter. Smaller diameter canes are too weak to support good fruit production and excessively large canes will be too vigorous and not produce any fruit.
Pruning dormant vines limits the vegetative growth of the vine and is also a means of maintaining the desired form of the vine. Viable one year old canes can be used to replace old or damaged cordons at this time. This is also a good time to reattach vines to the trellising system and make sure they are adequately supported to bear the weight of a full crop of fruit.
The timing of Winter pruning in the vineyard is a narrow window in the Missouri area since frost can damage new shoots it is best to wait as long as possible before removing excess fruiting wood. However the most important application of fungus protection is a dormant spray of sulphur which must be applied prior to bud break to avoid damaging the new tender shoots. Since handling the vines pruned wood and debris would be uncomfortable after the dormant spray is applied, the timing of winter pruning for us has really become more of an early spring pruning task.
(For more information on vineyard pruning see Principles of Grapevine Pruning by Andy Allen, Viticulturist, Arkansas Tech University)