Dec 3, 2012Dormant pruning regulates North Carolina winegrapes
Adapted from the North Carolina Winegrape Grower’s Guide, a publication of North Carolina State University.
Dormant pruning is the primary means of regulating the crop. If other factors do not limit productivity, vines pruned correctly are likely to produce large crops of high-quality fruit. Pruned incorrectly, vines and crop will ultimately suffer. It is important to understand how many nodes to retain, as well as which nodes are associated with good cold hardiness and fruitfulness.
A mature, un-pruned grapevine can have more than 400 buds. Overcropping would occur if all of these buds were allowed to grow and bear fruit. There are both immediate and long-term effects of overcropping grapevines. Immediate effects are observed in the current year. Symptoms can include reduced sugar accumulation in fruit and reduced pigmentation in berry skin. Rather than maturing into woody canes, the shoots of overcropped vines typically die back completely to older wood, or they may mature only one or two basal nodes (toward the base of the shoot). Poor wood maturation occurs because the maturing fruit competes for the necessary carbohydrates.
The long-term effect of overcropping is reduction of vine vigor (rate of shoot growth) and vine size (pruning weight). Vine size reduction due to overcropping can occur without a noticeable degree of cane dieback. Although wood might appear to be mature, stored starch reserves in vines stressed by overcropping can be so low that the next year’s vegetative growth and crop will be severely reduced.
Although dormant pruning is the primary means of controlling the crop, it will not provide adequate control in all situations. Additional control through thinning of flower or fruit clusters is generally required with young vines (two years old or younger), with very fruitful varieties such as some of the interspecific hybrids, and in any case where the vine vigor and vine size are insufficient to fill the available trellis space.
Number of nodes to retain
Eighty to 90 percent of the one-year-old wood is removed from vines at dormant pruning. Before pruning mature grapevines, the vineyardist must decide how many nodes to retain. Overcropping and excessive canopy density will occur if too many nodes are retained. On the other hand, the crop will be needlessly reduced if too few remain.
Furthermore, severely pruned vines are apt to produce excessively vigorous shoots because all of the stored energy in the trunks and roots is available to relatively few growing points. Excessive shoot vigor can reduce fruit set and delay shoot maturation in the fall.
Balanced pruning was developed to help vineyardists determine the appropriate number of nodes to retain. This method is based on the concept that a vine’s capacity for vegetative growth and fruit production is a function of the vine’s size. The size of a vine is determined by the extent of growth of roots, shoots and perennial wood. Because the growth of roots and other perennial wood cannot be conveniently measured, vine size is measured by weighing the one-year-old wood (canes) removed at pruning. Essentially, we balance the number of nodes retained against the weight of pruned canes: More nodes should be retained on a large vine than on a small vine, because the large vine has a greater capacity for both vegetative growth and crop production.
Pruning formulas for many varieties have been developed to calculate the number of nodes to be retained for a given pruning weight. A pruning formula of 20 + 20, for example, would require leaving 20 nodes for the first pound of canes removed, plus an additional 20 nodes for each additional pound above the first. A 3.2-pound vine would therefore retain 64 nodes if the 20 + 20 schedule were used at pruning. Weighing is done to the nearest tenth of a pound.
Nodes, specifically count nodes, are the units counted in the pruning formulas. Count nodes have clearly defined internodes in both directions on the cane. Once the appropriate pruning formula has been determined, the vine size is visually estimated and the number of nodes that should be retained on the pruned vine is calculated on the basis of that estimate. This requires some experience, but 5- to 6-foot canes average about 0.1 pound. The vine is then pruned, leaving 10 to 15 extra nodes as a margin of estimation error. The cane prunings are weighed with a hand-held scale and their weight is entered into the pruning formula to determine accurately the number of nodes to be retained. Nodes in excess of that number are then removed. Commercially, it is neither necessary nor practical to weigh cane prunings from every vine. In practice, most pruners acquire an ability to estimate the pruning weights and node retention closely. Thereafter, only an occasional vine is weighed to check estimates.
Pruning formulas allow for additional shoots to develop from non-count node locations (base buds). Generally, the native American and vinifera varieties do not produce many base shoots unless the vines have been pruned too severely. Many of the interspecific hybrid varieties, however, produce numerous, fruitful base shoots, even with moderate pruning. Balanced pruning of hybrid varieties has limited utility. Crop control with some hybrid varieties, notably Seyval, must be achieved through a combination of fairly severe pruning and shoot or fruit cluster thinning.
There are other, more arbitrary means of determining the number of nodes to retain at pruning. Node retention figures are sometimes based on the linear row space or the square area a vine occupies. For example, mature vines trained to conventional, non-divided canopy training systems should generally retain four to six nodes per linear foot of row. Expressing node retention on the basis of the linear measure of row or the square area of vineyard is convenient; however, it ignores individual variation in vine capacity and can lead to overcropping of small vines or undercropping of large vines. It is not as precise as balanced pruning, and is therefore not a recommended procedure where variation in vine size is great.
When to prune
Vines can be pruned any time between leaf fall and bud break the following spring. However, there is evidence that fall-pruned vines are more susceptible to winter injury than vines pruned in late winter or early spring. Delaying pruning until late winter makes it possible to evaluate bud injury and compensate by increasing the number of nodes retained. Spring pruning does not harm vines, even when sap bleeding is observed; however, swollen buds and young shoots are extremely susceptible to breakage. Therefore, the removal of unwanted wood from the trellis should be completed before bud swell. Experienced pruners require 30 to 40 hours to cane-prune an acre of vines. Somewhat less time is required for spur-pruned vines.
Double pruning of vines is sometimes practiced in areas where spring frosts are common. At the initial pruning in late winter or early spring, canes or spurs are retained with two to three times the desired number of nodes. Buds nearest the pruning cut develop shoots as much as seven days earlier than the basal buds of the same cane or spur. To correct shoot density, a second pruning cut is made after the threat of frost, but before appreciable shoot growth has occurred.