In Pennsylvania, prune grape vines in February or March. The vines are dormant in winter and pruning stimulates the growth of new fruiting canes in the spring.
Concord grapes, which are native to Pennsylvania, are the most commonly grown grape in the state. Year old canes bear the most fruit. Trim these stems, which are at least as thick as a pencil. Leave about 15 buds on each cane.
Trimming grapes in winter prevents excessive foliage growth and canes that are exposed to more light produce more fruit. Prune heavily when the grape vine is dormant, removing about 90 percent of the newest growth.