Pruning Grapevines

The training and pruning of grapevines is often confusing to many gardeners. The more common training systems for the home garden are the four-cane Kniffin and six-cane Kniffin. The four- cane Kniffin is popular because of its simplicity. This system is characterized by four canes, two on each side of the trunk, trained onto two trellis wires. In addition, four very short canes (termed renewal spurs) are also retained. The renewal spurs contain one or two buds and are very important in the training system. The buds on the renewal spurs provide shoots and ultimately the canes for next year's crop. The six-cane Kniffin system is similar to the four-cane Kniffin system except that it contains six canes on three trellis wires instead of four canes on two wires. In addition, six one- or two-bud renewal spurs are also retained.

Before pruning grapes, an understanding of the growth and fruiting characteristics of the grapevine is helpful. Grapevines produce fruit clusters on the previous season's growth (two-year and older wood is not fruitful). Before pruning, an average grapevine may have 200 to 300 buds which are capable of producing fruit. If the vine is left unpruned, the number of grape clusters would be excessive. The vine would be unable to ripen the large crop or sustain 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 in late winter or early spring. (Grapevines pruned at this time of year will bleed heavily. However, the bleeding will not harm the vines.) The degree or extent of pruning is dictated by vine vigor. Vine vigor is determined by estimating the amount of the previous season's growth. This concept is called "balanced pruning."

The first step in balanced pruning of grapevines is to size up the vine and estimate the amount of one-year-old wood in pounds. Select and retain the appropriate number of fruiting canes from those on the upper portion of the vine. Remove all the other one-year-old canes, except for the short renewal spurs. Then weigh the pruned canes. The weight of the canes is used to determine the number of buds to retain on the grapevine. To determine the number of buds to leave, use the following balanced pruning formula: "30 plus 10." For the first pound of canes removed, leave 30 buds. For each additional pound, leave an additional 10 buds. When counting the number of buds to be retained on the grapevine, include the buds on the fruiting canes and also on the renewal spurs. The balanced pruning concept is illustrated in the following examples.

A grapevine had two pounds of canes removed at dormant pruning. The gardener would therefore leave 30 buds for the first pound of canes, plus an additional 10 buds for a total of 40 buds. If the four-cane Kniffin system is utilized, the four main canes would each have 8 to 9 buds. Four renewal spurs containing one or two buds would also be retained. The total number of buds should equal 40. If a vine had three pounds of canes removed, the grower should leave 30 buds for the first pound of canes, 10 for the second pound, plus a final 10 for the third pound, or a total of 50 buds. For the training systems common in Iowa, the maximum number of retained buds on a grapevine is 60. If too many buds are present after the initial pruning and weighing, remove additional buds to obtain the desired bud number. A grapevine trained to the four-cane Kniffin system after pruning.

This article originally appeared in the March 3, 1995 issue, p. 17.

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