For some home gardeners, pruning grapevines is a difficult, confusing chore. However, it's not really difficult if you understand the basic pruning principles and have the right tools.
Grapevines produce fruit clusters on the previous season's growth. Before pruning, a grapevine may have 200 to 300 buds capable of producing fruit. If the vine is not pruned, the number of grape clusters would be excessive and the grapevine would be unable to ripen the large crop or produce adequate vegetative growth.
The purpose of pruning is to obtain maximum yields of high quality grapes and to allow adequate vegetative growth for the following season.
The most desirable time to prune grapevines is late winter or early spring. In Iowa, pruning can begin in late February and should be completed by early April. Grapevines pruned at this time of year may "bleed" heavily. However, the loss of sap will not harm the vines.
To maximize crop yields, grapevines are trained to a specific system. The most common training system used by home gardeners is the four-cane Kniffin system. The four-cane Kniffin system is popular because of its simplicity. In a four-cane Kniffin system, the canes of the grapevine grow on 2 wires, one located 3 feet above the ground and the second 6 feet high.
When utilizing the four-cane Kniffin system, select four canes on the upper wire, two going in each direction. Also, select 4 canes on the lower wire. To aid identification, some gardeners tie brightly colored ribbons or strips of cloth on those canes they wish to retain. All remaining one-year-old canes should be completely removed.
Going back to the upper wire, select two of the remaining four canes (one going in each direction). Prune these canes back to 1 or 2 buds. These short 1 or 2 bud canes are referred to as renewal spurs. The renewal spurs provide the shoots or canes that will produce next year's crop. Prune the remaining two canes on the upper wire back to 8 to 13 buds. The number of buds left on the fruiting canes is determined by plant vigor. If the grapevine is vigorous, leave 13 buds per cane. Leave only 8 buds per cane if the grapevine possesses poor vigor.
Prune the four canes on the lower wire the same as those on the upper wire. When pruning is complete, no more than 60 buds should remain on the grapevine. When counting the number of buds on the grapevine, include both the buds on the fruiting canes and those on the renewal spurs.
The six-cane Kniffin system is another training system occasionally used by home gardeners. In the six-cane Kniffin system, the canes of the grapevines are grown on 3 wires. The wires are positioned 2, 4, and 6 feet above the ground. After pruning, a grapevine trained to a six-cane-Kniffin system consists of 6 fruiting canes and 6 one- or two-bud renewal spurs. As with other training systems, the maximum number of retained buds is 60.
Tools required to prune grapevines include a hand shears, lopping shears, and saw. Brightly colored ribbons or cloth strips can be used to identify fruiting canes and renewal spurs.
Additional information on pruning grapevines can be found in ISU Extension pamphlet RG-502 'Pruning Grapvines.'