If you live near vineyards, you've likely noticed that they receive a haircut around this time of year. Our two vineyards (Elkton Vineyard and Black Oak Vineyard) have been pruned in the past months as well.
It may seem counterproductive to cut off well over 80% of last year’s growth, but it's an integral part of both canopy and fruit management. If not properly pruned, the canopy (leaf cover) can become overgrown and block airflow, risking powdery mildew or mold. If too sparse, the canopy will not provide shade and the fruit can become sunburned; if too dense, the fruit may not get the sunlight needed to ripen properly.
The canes on which fruit is produced grow from existing one-year-old growth. So, to produce fruit, strong canes from the previous year are selected and retained. Conversely, if all one-year-old canes were left un-pruned for another year, they would produce far more fruit than the vine could ripen to satisfactory levels for winemaking. So most of the canes from the previous year are removed in order to give us the quality of fruit needed while maintaining the canopy required.
Pruning is a subject that could merit a newsletter much longer than you'd likely care to read. But if you find this an interesting topic, there is a wealth of knowledge on the Internet. I recommend the following:
Youtube has a number of very informative pruning videos
This WineFolly article on Vine Training styles is very informative
The Extension Office at Oregon State University also has a great primer on grape pruning basics