French and American grapevines differ, although only a trained eye may be able to notice that difference. French grape varieties produce fruit on canes that grow up from main arms on the vines, so they need different trellises then other types of grapevines. Pruning is an important element to growing these grapevines, especially to encourage fruit production, improve flavor and sugar content, minimize disease and control growth.
Let first-year grapevines flourish naturally with no pruning. Keep the canes tied with the wire loosely to the wooden stake, strung about six inches apart. Let as many canes as possible grow in this manner, horizontally.
Pick out the three or four strongest, healthiest grapevine canes that are the size of a pencil diameter. In the first growing season's late winter, cut back all the other canes besides these to the ground. These will be the main lateral cordons that will produce the grapes. In the second growing year, prune off any shoots that are sprouting from below the graft union. Also prune back any flower clusters before they bloom.
Pick the two best laterals during the second year's winter to become the French grapevines permanent cordons. Tie these to the support wire with the wire ties (the support wire is what you originally planted the grape vine on). Prune these laterals to a minimum of 18 inches long. Pick out the strongest looking vertical shoots coming from these laterals, and prune everything within four inches of each of them. These will be the fruiting spurs. Cut off all buds from each spur but two.
Prune back the cordons to 36 inches in horizontal length during the third winter season. Cut all other canes back to leave one or two buds on each. Once the French grapevine starts to flower, thin out the blooms to one per cane in the spring season.