1. The grapevines should be trained to reasonably fill the structure but not become overgrown. This is easier said than done, since initially the vine grows fairly slowly, but as it matures, it may become a monster of vegetation. One to two layers of leaves for any area on the canopy is best for flower bud and fruit development.
2. Mature grapevines, by their nature, produce much more wood than they can support. Think of the wild grapevine growing in the forest: it produces a huge amount of wood just to climb up to the sunlight. Your garden grapevine will not need to do that, but they still produce much more wood than is necessary or desirable. Typically, during dormant pruning you need to remove as much as 90 percent of the new growth on a mature grapevine. Plan on leaving about three or four buds per foot of cordon (the horizontal trunk on a grapevine) length. Dormant pruning is usually done in late winter or early spring.
3. Grapes bear their fruit on one-year-old wood.
4. Different grape cultivars have different growth habits. The canes of American cultivars tend to grow in a willowy, downward direction, whereas those of the European cultivars and many French-American hybrids tend to grow directly up. Choose your training system with this in mind.
5. Grapevines vary considerably in their vigor, due to both genetic and environmental factors. Because of this variation, it is difficult to make exact recommendations as to how many buds, or how large a crop, to leave on the vine in any given year. Vigorous vines can support and ripen a larger load of fruit than less vigorous vines. In other words, you can leave more buds for next year's fruit development on a big and robust vine than you can on a little waif of a thing. Therefore, you need to a judgment about how many buds to leave during pruning based on how much growth the plant achieved in the previous growing season, which can be estimated by the amount of wood you have to remove during dormant pruning. As a general rule, plants that are relatively weak growing should have two buds per foot of cordon (row) or 30 buds per plant retained, whereas vigorous vines should have 3 to 4 buds per foot of cordon or 40 to 45 buds per plants retained. This concept is called "balanced pruning" because it balances the crop for next season with last season's growth.
Remember that although our methods of pruning and training are based on science and experimentation, much of grape pruning relies on experience and, for lack of a better word, art. These instructions seem a little befuddling at first glance, but as you engage in the process of pruning, you will understand them more fully and come to appreciate the time spent in your vineyard in winter, in sweet anticipation of the summer to come.
By way of guidance, some traditional training systems employed by commercial and backyard growers are described here. All the various trellis systems refer to a vine in the early spring after dormant pruning.
High-Wire Cordon
American cultivars such as 'Concord' or 'Niagara' tend to produce shoots that grow in a downward direction, so it generally makes sense to put the permanent horizontal branch on a high wire and let the plant drape toward the ground. In the high-wire cordon system, also known as the Hudson River Umbrella, a single horizontal wire is secured at a height of 6 feet, and another is placed at a 3-foot height. The lower wire will be used to hold the vine up during early training, and later, after the plant is trained, it will only be used to secure the trunk in place.
In training a vine to this system, select two strong canes and train them up to the top wire; then, as the canes grow, place them along the top wire, in opposite directions. Be careful not to wrap the canes around the wire, since they will expand in girth over time and can girdle themselves on the wire. It's better to tie the canes to the top wire at first. They will eventually send out tendrils to secure themselves to the wire, but they will need a little help initially. Make sure that you keep an eye on the string that is securing the canes to the wire, since the string can also girdle the canes as they grow. Cordons from one vine should not overlap with those of adjacent vines. Ideally, you would like to train up the first two canes in the first year of growth, but realistically it often takes two years to get the cordons trained to the upper wire. Have patience; they will eventually get there, provided they are in the appropriate climate. As in the case with growing many other perennial small- fruit crops your training and patience in the first years will pay off in future decades.
Once the cordons are established, they should be pruned so that you are left with several shoots or spurs that are five to seven buds long, for a total of 30 to 45 buds per plant, depending on the vigor of the plant, as discussed earlier. These spurs should be spaced 6 to 12 inches apart on the cordon. In addition, for each seven-bud spur, leave one two-bud renewal spur. The smaller spurs will provide the shoots for the following year.
The fruiting shoots will hang like a curtain in groups from the spurs that originate from the arms along the top wire. Shoots should be carefully separated and placed vertically downward from the top wire for a distance of 18 to 24 inches. Positioning should be carried out as soon as the shoots have toughened, usually 2 to 3 weeks after peak bloom (when 50 percent of the fused petals have fallen). Extreme care must be exercised during shoot positioning, as any shoot lost at this time can result in a poorly filled trellis.
In subsequent years, select new canes and leave new renewal spurs from the shoots that developed from the previous year's buds. Remember to balance the number of buds with the vigor of the plant: a high number of buds should be left on vigorous plants than on weaker ones.
Low-Wire Cordon
The idea behind the low- wire, or low-bilateral, cordon training system is similar to that of the Hudson River system just described. The low-wire system, however, is used for grapevines that have a tendency to grow up rather than down, as is the case with the European grapes and most of the French-American hybrids. If you put these forms on a Hudson River system, the shoots would grow up from the top, 6-foot high wire- not a good thing. The low-wire cordon system takes advantage of the plant's natural tendency to grow up, allowing the shoots from the permanent cordons on the lower, 3-foot high wire to fill in the trellis from the bottom up. Of course, gravity tends to work against you, so as the shoots grow up, you need to help them by using catch wires that sandwich the new shoots between them as they grow up. Usually, two sets of catch wires are adequate.
Umbrella Kniffen System
In the Umbrella Kniffen system, the plant's trunk is brought up and tied to a 6-foot high wire, with four or more canes left near the top of the trunk bearing a total of 50 to 60 buds. All other wood except for two renewal spurs is removed from near the head. After pruning to the desired number of buds, retie the trunk to the top wire. The canes should be bent rather sharply over the top wire so that the other outer bark cracks, and the tips are tied to the lower, 3-foot high wire. The renewal buds will develop into shoots that probably will not be fruitful but should be allowed to grow. They are there to be used for next year's fruiting canes. The buds on the main canes will form fruiting shoots that do not need to be tied because the vine has already been trained. Some of those shoots may be well located and can replace the original canes the following season, in which case the renewal shoots are not needed.
Four-Cane Kniffen System
The Four-cane Kniffen system is a variation on the Umbrella Kniffen system in which the canes are selected from both the top and the middle of the trunk. It is often used on low- vigor cultivars. The lower canes are shaded by the upper canes, and the lower ones often produce later-maturing fruit that may be lower in quantity and/or quality than that on the cordons trained on the top wire.
Other Training Systems
The production of 'Thompson Seedless' grapes in California and similar climates requires cane-pruning systems, since the buds closest to the base are not fruitful, yet those are the only buds that are left on the vine in cordon systems. If you trained 'Thompson Seedless' to a low -wire cordon, you would get a lot of leaves and no fruit. The most commonly employed training system for this cultivar is head training. Head training involves tying the main trunk to a vertical post.
Another common training system is the Geneva Double Curtain. This system is good for extremely vigorous vines since it allows the plant to achieve its potential yield in a relatively small space. Weaker growing vines would not fill such a large trellis system.
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