Pruning Vines for Table Grape Production

Pruning is the removal of unwanted parts of a vine. A manageable growth form of a vine is the most visible result of pruning, but the most important reason for pruning is to control crop level. Unpruned vines will at first overbear to produce large crops of poor quality fruit. Vines left unpruned for several years develop alternating cycles of large and small crops. New grape growers often fear that pruning will injure a grapevine. The reverse is true. The more severely a vine is pruned, the smaller the crop that will develop. That reduces the stress on a vine to ripen the crop so that the vine grows more vigorously. Even a poor job of pruning maintains a vine's health better than no pruning.

Pruning is performed all winter in many large commercial vineyards because this big job requires many hours. Spring is the ideal time to prune temperate-climate vineyards, however, because the extent of winter injury to vines can be determined. If injury has occurred, pruning severity can be adjusted. If winter injury to vines is suspected, evaluate the extent of that injury before beginning to prune vines in the spring. Cut a cross-section of about 10 nodes on canes that are the same quality as those that will be chosen for fruiting. If three or more of those nodes have dead primary buds, as indicated by their dark color (Fig. 2b), then more thoroughly investigate node mortality. Collect 10 canes with 10 nodes each and cross-section those 100 nodes. Prune vines according to the primary bud mortality found as follows: 0 to 15% mortality — prune vines as usual; 20 to 50% mortality — add additional fruiting canes to compensate for the percentage of dead primary buds found; greater than 50% mortality — prune out only unmanageable growth at the bases of vines; delay pruning until after bud break to determine which buds are still alive, then perform a light pruning to retain the desired number of live nodes.

Shoots are easily damaged or even detached from vines as they begin to grow. Under normal circumstances, pruning, trellis repair and tying of vines should be completed before vines begin to grow. In two situations, however, pruning should be delayed until after shoot growth has begun. One, as just mentioned, is after severe winter injury. The other is if vines are planted in a location highly susceptible to spring freezes. When nodes open to expose the first leaves on shoots, winter hardiness is lost and vines become susceptible to spring freeze injury. Unpruned grapevines experience bud break more slowly than pruned vines. Therefore, when vines have been plant ed in a location vulnerable to frequent spring freeze injury, leaving them unpruned until after shoots have begun to grow gives a several-day advantage in maintaining vine hardiness. Pruning during the early stages of vine growth often causes bleeding of sap from pruning wounds. Contrary to folklore, this is not harmful.

Pruning Severity

Approximately 85 to 90% of the previous season's growth should typically be pruned from a grapevine. This may seem drastic for those unaccustomed to this task, but such pruning is especially important for the table grape grower. If too many fruiting buds are left on the vine, it will produce numerous small, unattractive clusters.

Pruning formulas guide commercial pruning practices by relating the size of a vine to the crop level it can ripen. In this process, canes pruned from a vine are weighed. The weight of the canes is an estimate of a vine's ability to develop the leaf surface area needed to ripen a crop. The amount of useful leaf area that can be displayed by a vine space is limited because only the outer one or two layers of leaves on a grapevine are functional. Therefore, there is a corresponding upper limit for the number of fruiting buds that should be left on the vine after pruning to fill its vine space, regardless of how large the vine grew the previous year.

The main priority of pruning is to keep the appropriate amount of fruiting potential on the vine to produce this year's crop. The more fruiting nodes (buds) saved on a vine, the greater the crop potential. Too much crop, however, will result in poor fruit quality. In commercial practice, the weight of cane prunings on a vine is related by a formula to the number of fruiting nodes retained on the vine. To avoid the complexity of pruning formulas and the need to weigh the canes, the following guidelines for vine pruning severity are made for table grape production in a temperate climate. They are based on visual estimates of vine size. It is assumed that vines are spaced 7 to 8 feet apart.

1. VERY SMALL VINES: Description — Vines fill one-third or less of their vine spaces with growth by the end of the growing season. Mature canes seldom exceed 6 feet in length and often may be

2 feet or less. If all cane prunings were weighed, they would be less than 1 pound.

Recommendation — Retain 15 nodes on the vine. When shoots average 12 inches in length, defruit the entire vine to promote vine growth so it can bear larger crops in future years.

2. SMALL VINES: Description — Vines fill approximately half of their vine spaces with growth by the end of the growing season. If all cane prunings were weighed, they would total 1 to 1.5 pounds.

Recommendation — Retain 20 to 25 nodes on the vine. When shoot growth averages 12 inches in length, retain a maximum of one cluster per shoot on 15 shoots. Remove all other clusters. This allows some cropping while promoting vine growth for larger crops in future years.

3. MEDIUM VINES: Description — Vines fill about three-fourths of their vine spaces with growth by the end of the growing season. If all cane prunings were weighed, they would total 1.75 to 2.5 pounds.

Recommendation — Retain a maximum of 30 to 35 nodes on the vine.

4. LARGE VINES: Description — Vines completely fill their vine spaces with growth by the end of the growing season. If all cane prunings were weighed, they would total 2.75 pounds or more.

Recommendation — Retain a maximum of 40 to 50 nodes on the vine.

Adjust these guidelines as you gain experience. For example, if you left 30 fruiting nodes on a vine and the crop ripened well and some canes grew very long, then you should retain more than 30 nodes on the vine the following year. On the other hand, if the fruit did not mature well, shoots were relatively short, or both, then leave fewer than 30 nodes on the vine the following year. Never exceed 50 nodes per vine for vines producing table grapes. If you leave 50 nodes on

Section III -

Cultural Practices for Managing Mature Grapevines a vine and the vine still grows excessively, reduce vine stimulation by reducing or eliminating fertilizer and/or by establishing a sodded vineyard row middle.

Manage crop size on medium to large vines through a combination of dormant pruning and adjustment of the number of clusters per vine during the growing season. Crop adjustment strategies are discussed later.

Selecting Fruiting Canes

The length of fruiting canes saved on a vine is influenced by the choice of a vine training system. They should be long enough to be conveniently tied to the trellis. The use of 8-, 10- or 12-node canes is common (Fig. 13). When a cane length with a specific number of nodes per cane has been chosen, use this cane length consistently to prune all the vines in a specific vineyard. For example, a medium-sized vine requires 30 to 35 nodes (see pruning severity). It could be pruned to four canes with eight nodes each (Fig. 13).

A quality fruiting cane has a bright, shiny appearance; ranges in color from straw yellow to dark brown, depending on the variety; and has the diameter of a pencil or slightly larger. At times a pruner is forced to use large-diameter canes with lateral canes branching from them. The nodes on lateral canes are highly fruitful; the nodes on a primary cane at the base of a lateral cane are poorly developed and often winter-kill easily. Therefore, large-diameter canes will be satisfactory for fruiting as long as the lateral canes are pruned to one- or two-node spurs whenever they occur (Fig. 1f).

Choose fruiting canes in about equal numbers on each side of a vine space so the fruit is distributed well within the vine space. When tying canes to the trellis, do not cross canes over from one side of the vine to the other. This creates congested growth. For example, when using a modified 4-arm Kniffin training system (Fig. 3b), place two fruiting canes on each side of the vine space. Select canes so they originate and are tied on the same side of the vine. When vines are dou-ble-trunked, fruiting canes for each side of the vine space should arise from the respective trunk on that side (Fig. 3b). A useful way to remember the important aspects of grapevine pruning was devised by Trenholm Jordan, former Extension viticulturist in New York. He used the letters QND: choose Quality canes as indicated by good color and diameter; choose the correct Number of canes in relation to the size of the vine (see the section on pruning severity); and Distribute canes well within the vine space (see the section on vine training systems).

Before pruning can begin, you must decide on the vine training system to be used. With that knowledge, pruning can begin.

Steps in Pruning

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  • cedivar hornblower
    How many buds should be left when pruning american grapes?
    2 years ago

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