Only a few varieties of American labrusca or non-labrusca bunch grapes and some French/American hybrids grow well in South Carolina's warm humid summers. South Carolina's climate divides between the northern mountains and Piedmont areas, and the sand hills and coastal plains. Growing conditions in these two regions don't favor the same grape cultivars. Problems include fungal diseases of fruit and foliage as well as Pierce's Disease, a bacterial infection. Proper pruning of disease-resistant varieties increases the quality and the quantity of bunch grapes and decreases damage from fungi.
Train newly planted vines to a single-wire trellis by allowing two or three canes to grow vertically along a support stake for the first summer. Select the first to reach the cordon wire, about 5 feet from the ground, and cut back the other canes to the ground.
Cut the leader or trunk cane just past the first cane node above the cordon wire. Tie the cane to the support post.
Select two side shoots to become cordons or branches the following spring as the year's new growth sprouts. Choose canes that join the trunk about 8 inches below the support wire and train them to either side, tying them loosely to the cordon wire with garden twine. Cut off all competing shoots.
Allow both branches to grow 60 to 65 inches to either side of the trunk and then trim back to 48 inches. Cutting back the main branches encourages side shoots to grow. Remove canes that sprout from the top of labrusca cordons. Rub off any flower buds during the 2nd year of growth of either labrusca or non-labrusca grapes.
Prune labrusca vines during the following winter to select strong downward-leading canes. Choose canes 4 to 6 inches apart on the cordons and prune competing canes back to the main branch. Keep canes on the top of non-labrusca cordons, thinning to the same spacing as for labrusca.
Cut back labrusca canes to leave only four to six buds per cane. Trim non-labrusca varieties back to one to three buds per fruiting spur. Leave four to six buds on fruiting canes of hybrid vines.
Train fruiting canes to grow downward in an open airy curtain in the 3rd year. Comb or straighten the vines to grow with an even spacing. Allow no more than one bunch of grapes for every two canes.
Prune mature vines each winter to select new fruiting canes at the proper spacing. Cut out old canes which bore fruit the previous season. Cut back cordons to 4 feet in length each winter. Allow mature vines to set no more than two clusters of grapes on each fruiting shoot.