Reasons and Rules of Pruning Grapevines
UCCE Farm Advisor, Kern County
Viticulture - Grapes & Kiwis
December 24, 2001
Reasons and Rules of Pruning Grapevines
Although the holidays are here and most of us are enjoying our family and friends, a few cultural practices remain in the vineyard. The most important operation during the dormant season is pruning. Pruning of grapevines is recommended anytime after leaf fall, which may occur late fall or throughout the winter. Once the leaves fall, the vascular system becomes inactive and plugs up. Before this time, minerals and carbohydrates are transferred from the leaves into the permanent, woody structures of the vine for winter storage. For this reason, pruning before leaf fall can affect storage leading to mineral deficiencies and poor bud maturation, which can affect the growth of the vine and the crop in the following season. Timing of pruning within the dormant season may also affect the time of bud break. Vines pruned very late in the season usually start spring growth slightly later than those pruned mid-dormancy. Such a delay in bud break may be desirable in frost prone areas. The following are several basic reasons to prune:
1. Maintain vine form. Grapevines are referred to as lianas, or woody tree climbers. They climb by several adaptive mechanisms including stem twining and by attaching themselves to supports with tendrils. In commercial grape production, vines are trained and pruned upright onto a trellis support structure. This enables the vine to retain a similar shape year after year in order to facilitate cultural operations including harvest.
2. Regulate the number and positions of shoots on a vine, and cluster number and size. During pruning, one removes buds that would otherwise become new shoots, with new clusters in the spring. By regulating the total number of buds, one is concentrating growth into remaining shoots and clusters.
3. Improve fruit quality and stabilize production over time. A given vine in a given season can only produce a certain quantity of fruit. Its capacity to do so is largely dependent on the amount of leaf area and photosynthetic activity. By consistently limiting the number of shoots and leaves by dormant pruning, one is also working to produce the maximum crop without delaying maturity year after year.
4. Improve bud fruitfulness by bud selection and placement. Improvement of bud fruitfulness occurs generally when one selects healthy wood with plump bubs that have been exposed to sunlight. Think of it as an investment in next years crop.
There are several ways to prune grapevines. This article will concentrate on two of the most common, spur-pruning and cane-pruning. With spur-pruned varieties, the ideal shoot density is about 15-18 shoots per 3 foot of cordon. The "rule of appendages" is a good way to remember how many buds to leave on each spur. In general, one should leave 3 buds if the spur is thumb-size in diameter, 2 buds if the spur is the size of a middle finger and 1 bud if the spur is pinkie-size.
Cane-pruned varieties, such as Thompson Seedless, are always more difficult and expensive to prune because great care must be taken in selecting wood. One generally starts with 4-8 mature canes depending on the vigor of the vine and the trellis system. Each cane is pruned back to about 10-15 well placed buds. More important than the number of canes is the quality of canes. One should choose medium diameter, round, well-browned canes with buds about 3-3.5 inches apart. The buds should be plump and one should avoid selecting flattened canes with long internodes and poor color. One should also choose canes that have been exposed to sunlight, that is, having developed outside the canopy. Avoid canes that have developed in the shade, as they are not as fruitful. In addition to the canes, one also must leave about 4-6, 2 bud renewal spurs on the head of the vine to produce the fruiting canes for the following season.