The best time to do a full prune on a grapevine is in the late winter, approximately one month before the end of the last frost. Pruning too much during the warmer seasons can result in freeze damage to the buds. However, some growers do a basic maintenance trim towards the middle of summer. This maintenance trim will direct the grapevine to put its energy into growing fruit instead of growing longer vines.
Examine the new growth on your grapevine. If the new growth has caused the vines to grow longer than 3 feet, you can prune them back.
Cut off the growth with pruning shears. This should leave the length of the remaining vines approximately 2 to 3 feet long.
Encourage the part of the vine you cut off to produce roots by dipping it in some rooting powder, planting it in moist potting soil and putting a clear, plastic bag over the top of it. In approximately three weeks, you will have a new grapevine plant.