By Richard Jauron
Extension Horticulturist
Standing in front of a mass of tangled grapevines and wondering what to do can be a scary experience for the novice or even the more experienced gardener. Pruning grapes becomes easier after understanding the growth and fruiting characteristics of the grapevine. Horticulturists at Iowa State University Extension and Outreach answer questions and give tips about grapevine care.
To have additional questions answered, contact the ISU Hortline at 515-294-3108 or at hortline@iastate.edu.
When should I prune grapevines?
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 does not harm the vines.
What is the proper way to prune grapevines?
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.
Grapevines are trained to a specific system to facilitate cultivation, harvesting and pest control. The most common training system used by home gardeners is the four-cane Kniffin system. In a four-cane Kniffin system, the canes of the grapevine grow on two wires, one located three feet above the ground and the second six feet high.
If utilizing the four-cane Kniffin system, select four canes on the upper wire, two going in each direction. Also, select four 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 one or two buds. These short one or two 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 eight 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 eight 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 grapevine are grown on three wires. The wires are positioned two, four and six feet above the ground. After pruning, a grapevine trained to a six-cane Kniffin system consists of six fruiting canes and six 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.