The appropriate time to prune grape vines is in late February or March, and pruning must be complete by early spring to promote new growth. Pruning times are the same for grapes grown on an arbor, fence or stake, although pruning methods differ.
Mature vines that are at least three years old require extensive pruning, and it's appropriate to cut away up to 70 percent of these vines each year. To avoid accidental cuts, experts recommend tying colored ribbon or yarn around retained canes. Pruned vines often lose sap, a process called bleeding, but this isn't harmful to the plant.