Canopy management
At some point every year a grapevine will likely produce too many shoots to effectively farm a vineyard. Left unmaintained, a vine's shoots will form a dense mass of foliage that prevents spray, sunlight, and air penetration. In a vigorous growing region like the Mid-Atlantic, an unmaintained vineyard can resemble a jungle. This is because a grapevine's natural tendency is to grow and spread as much as possible while producing only as much fruit (and only as sweet) as required to be attractive to animals that eat its fruit, spread the seeds, and produce new grapevines. Viticulturists, however, want to grow grapevines like little trees that produce a large quantity of very sweet fruit. We have accomplished this goal through a variety of methods: breeding, site selection, devigorating rootstocks, training/trellis, and canopy management. Cover crops and close vine spacing have been tried to promote competition and reduce vigor, though I'm skeptical this is effective. With an established vineyard though, our ability to manage vigor rests largely only with canopy management.
In theory, winter time pruning is the means by which we establish the number of shoots that are produced in the growing season. Each node on an active shoot is a bud position the following year that should produce a shoot of its own. In practice however, buds can produce up to three shoots, and this just from the buds you intended to push new growth. Shoots can emerge from the trunk, cordon, and even the rootstock. Our goal is only to encourage fruit bearing shoots from positions established in winter time pruning. Once the vines show aggressive shoot growth as the weather warms in early June, we perform the first stage of canopy management by removing these extra shoots, called shoot thinning (and suckering when it involves removing shoots arising from the trunk).
Later in the season we're concerned more with lateral shoots (shoots growing from shoots), and sometimes even laterals growing from laterals, in addition to the primary shoots we intended to grow. You'll also find secondary shoots the vine pushed after your initial shoot thinning operation. While you may find that secondary shoots produce fruit, you don't typically rely on this fruit for wine production. So all these shoots found on the grapevine, that aren't primary shoots produced from a bud position chosen during winter pruning, are superfluous. Not only that, but they crowd the shoots you want and rob them of sunlight, pesticide spray, and air penetration.
The photos below show hedging and aggressive lateral shoot removal to address a crowded canopy issue, common during this time of the season. This vineyard didn't have much secondary shoot production so essentially all the shoots removed were laterals. This was done to illustrate just how much of the canopy is comprised of shoots that don't produce fruit (and don't need to be there at all). In reality, lateral shoots only need to be removed to the extent they impede sunlight, air, and spray penetration. In fact, mature leaves on laterals contribute to carbohydrate production that is exported to the primary (fruit bearing) shoot and contributes to fruit ripening. However, in wet years (which is arguably every year in the Mid-Atlantic) vines continue to grow well into veraison, and active shoot growth produces new leaves that act as an energy sink and not as a producer of energy for the vine and fruit. Removal of immature leaves from any shoot in a never-ending growing season is a level of detail few growers perform.
In commercial wine grape production canopy management is typically performed about twice per season - when the vines experience rapid growth and later in the season for hedging and as a second pass as the vines continue to push new shoots. Many vineyards get only one pass through. The proper amount of canopy management is specific for your site. And while you could justify an incredible amount of hands-on canopy management, your goal is ripe fruit free of fungal disease, not necessarily a manicured vineyard.
