For many home gardeners, pruning grapevines is a difficult, confusing chore. Fortunately, an understanding of the growth and fruiting characteristics of the grapevine should help simplify the pruning process.
Grapevines produce fruit clusters on the previous season's growth (two-year and older wood is not fruitful). Before pruning, a 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 grapevine 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.
To maximize crop yield, grapevines are trained to a specific system. The most common training systems used by home gardeners are the four-cane Kniffin and six-cane Kniffin.
The four-cane Kniffin system is popular because of its simplicity. It is characterized by four fruiting canes, two on each side of the trunk, trained onto two trellis wires. The grapevine also possesses four very short canes termed renewal spurs. 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 except it contains 6 fruiting canes trained onto three wires, plus 6 one- or two-bud renewal spurs.
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 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. The concept of pruning grapevines based on plant vigor is called "balanced pruning."
The first step in balanced pruning of grapevines is to study the vine and estimate the amount of one-year-old wood in pounds. Start by selecting and retaining the appropriate number of fruiting canes per vine. To aid identification, some gardeners tie brightly colored ribbons or cloth strips on those canes they wish to retain. Leave equal numbers of renewal spurs (canes pruned back to one or two buds). Completely remove all remaining one-year-old canes. Next, 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 buds to be retained on the grapevine, include both the buds on the fruiting canes and those on the renewal spurs.
The balanced pruning concept is illustrated in the following examples. If a grapevine had two pounds of canes removed at dormant pruning, the gardener would leave 30 buds for the first pound of canes, plus an additional 10 buds for a total of 40 buds. Using the four-cane Kniffin system, the four fruiting canes would each have eight or nine buds. There should also be 4 one- or two-bud renewal spurs. The total number of buds should equal 40. If a vine had three pounds of canes removed, the gardener should leave 30 buds for the first pound of canes, 10 for the second pound, plus a final 10 buds 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 as many as needed to obtain the desired bud number.
Tools required to prune grapevines include a hand shears, lopping shears, saw, and a small pocket scale to weigh the pruned plant material. Brightly colored ribbons or cloth strips can be used to identify fruiting canes.
For most home gardeners, pruning grapevines should be relatively easy with a basic understanding of pruning principles, the right tools, and a little courage.
This article originally appeared in the March 10, 2000 issue, pp. 20-21.
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