Crop control is a vineyard practice that includes the physical adjustment of the vine as needed to balance the amount of fruit with the growth of foliage. This practice adjusts the crop level so there will be sufficient growth of shoots with leaves on each vine, and each shoot will be capable of maturing the fruit retained.
Growers often experience the adverse effects of allowing vines to overproduce and lose vigor. This leads to improper maturation of the crop and causes the vine to enter the winter in a weakened condition. Growers then realize that an effective crop control program for each cultivar must be developed to keep the vineyard productive and healthy over an extended period. As more cold-tender cultivars are planted and the need for high quality increases, growers more than ever must use other methods in addition to balanced pruning to control the crop in a given year.
It has been shown that cropping of young vines can reduce the size of the root system and overall vine vigor. Unless vines are vigorous, fruit should not be retained during the first and second growing season. Remove flowers and fruit early in the season, when shoots are about 12-inches long. If the vines exhibit high vigor, a small amount of fruit can be left during the second season, but it should be only one or two clusters per plant.
Leaving a small amount of fruit helps the grower check for off-type vines early in the life of the vineyard so they can be removed and replaced. Additionally, the presence of fruit on vigorous vines helps slow vine growth in the fall and promotes shoot maturation.
Since the first three seasons are used to establish the vine form, there is usually a considerable amount of fruitful wood retained during the second and third seasons. This can lead to a crop potential that will exceed the capacity of the vine to support the fruit load in the third year.
About one half of a full crop of fruit can be retained during the third season if vine vigor is sufficient. Heavy fruiting of young vines will result in small vine size and reduced yields, and vines may need several years to recover and regain vigor. It is best to be patient and crop vines only after they are well established.
With the tremendous vineyard expansion in the Midwest during this decade, production is estimated to increase exponentially in the coming years. Winery owners and managers will be more selective and demand high-quality grapes. Winemakers unanimously agree that quality grapes is the No. 1 criterion.
The recognition of the important role of viticulture in the production of premium wines has led to considerable interest in viticulture practices that can improve wine quality. Among these practices, canopy management has made one of the biggest impacts in world viticulture in the last 20 years.
Therefore, to ensure the production of high-quality grapes, growers in the Midwest should embrace canopy management practices. These practices are usually conducted in fine-wine regions to produce premium fruit and wine. However, they are becoming a routine practice for vineyards seeking maximum wine quality.
In order for the wine industry in the Midwest to continue to be successful, it is critical that high-quality grapes be produced on a consistent basis. Some of the ways to achieve that goal are described here.
According to Dr. Richard Smart, author of Sunlight into Wine: “Canopy management is the practice which results in the modification of position or amount of leaves, shoot, and fruit in space to achieve a desired arrangement.”
Vine canopy is the shoot system, which includes the stem, the leaves, and fruit clusters. In the viticulture world, canopy is described by its length, height, width, leaf area, number of leaf layers, and shoot density. Shoot density refers to the number of shoots per foot of row or foot of canopy. Table 12 describes the characteristics of an ideal canopy.
| Table 12. Ideal Vine Canopy. | |
| Canopy Characteristic | Optimum Values |
|---|---|
| Shoot density | 4 to 6 shoots per foot of canopy |
| Number of leaf layers | 1 to 1.5 |
| Number of nodes per shoot | 12 to 15 |
| Canopy gaps | 40% to 50% |
| Cluster exposure | 50% to 75% |
| Ratio of leaf area to fruit weight (sq. inches per oz.) | 44 to 53 |
| Ratio of leaf area to fruit weight (cm2 per gram) | 8 to 12 |
| Vine size (pruning weight in lbs. per ft. of canopy) | 0.3 to 0.4 |
| Ratio of fruit produced (lbs.) for each pound of prunings removed. | 5 to 12 |
Canopy management (CM) has several viticulture advantages, such as maximizing sunlight interception, which means minimizing shading, and very importantly, maintaining a balance between shoot growth and fruit production (Figure 28.) The benefits of CM include:
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| FIGURE 28. Canopy management enhances sunlight exposure of leaves, clusters, and basal buds which will bear fruit the following year. |
There are five major steps or practices that growers should follow and apply to their vineyards. Some grape cultivars require all five steps; others require fewer; and certain cultivars require a repeat of some of the five steps.
The growing season also has a major impact on CM. Dry summers require fewer CM practices than wet summers. Growers have to follow the basics and fine-tune the steps according to cultivar, site, season, and their own experience. The five CM steps are listed here in chronological order of vine development throughout the growing season:
Suckering Trunks and Cordons: This consists of removal of suckers (unwanted shoots that grow on the trunk or cordon). One or two suckers are left at the base of the trunk only if a new trunk needs to be trained in second- and third-year vines, or if trunk replacement is deemed necessary due to injury—cold, disease, or mechanical injury. On the cordons, unfruitful shoots are removed first unless they are needed for spur renewal.
Once all shoot thinning is done, shoots should be spaced evenly along the cordon length and have a density of four to six shoots per foot of cordon or canopy. With 8-foot vine spacing, this corresponds to 32 to 48 shoots per vine on a single curtain/high cordon (HC) system, and 64 to 96 shoots per vine on a Geneva Double Curtain (GDC) training system. Note that with a divided canopy (e.g., GDC and Scott Henry), there are two feet of canopy for each foot of row.
When: This is best done early in the season when shoots are about 1 to 3 inches (trunk suckering) to 6 to 12 inches (cordon suckering) long. At this stage, shoots are easily rubbed off; fruit clusters are visible to distinguish between fruitful and unfruitful shoots; and less labor is involved. Suckering may require more than one pass. Growers with frost-prone vineyard sites should plan to thin shoots after the spring frost threat has passed.
The best scenario is when shoots grow vertically (parallel to the trunk), either up or down, depending on the training system. In the real world, however, shoots tend to grow sideways and attach to the cordon wire with their tendrils. This is why shoot positioning (Figure 29) is conducted to disallow lateral and horizontal shoot growth. Shoot positioning also allows the spread of shoots to promote an open canopy, improve spray penetration, and adhere to the shape of the given trellis system.
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| FIGURE 29. Shoot positioning: Canopy before (L) and after (R) combing shoots downward. Note fruit exposure after combing. | ||
Combing: This is the generic term for positioning shoots downward. Combing is conducted on high training systems such as High Cordon (HC) and Geneva Double Curtain (GDC). Shoots are combed in a vertical downward position.
Tucking: This is the generic term for positioning shoots upward and is used on low training systems such as Vertical Shoot Position (VSP). Shoots are held upright by using two or three pairs of moveable catch wires, 10 to 12 inches apart. Sometimes extra tying with tape is needed in order to keep the shoots upright.
Combing/Tucking: Both practices are required in vertically divided canopies such as Scott Henry and Smart Dyson training systems. The upward growth of both systems is tucked between catch wires, and the downward growth is combed.
When: Timing is critical for reducing the amount of shoot breakage and ease of positioning. Growers should realize that the most important aspect is to reduce shading as soon as possible after bloom as fruit bud development begins about that time and sunlight exposure is critical for bud fruitfulness.
Growers are sometimes tempted to take the risk and avoid cluster-thinning altogether for quick vineyard production. Others do cluster thin and have in mind the long-term benefit of this practice for the well-being and life span of the vineyard.
Cluster thinning is a MUST for some cultivars that have very fruitful primary buds and tend to produce three or more clusters per shoot. Examples include Seyval, Chancellor, Vidal, and Chambourcin.
Among these cultivars, some also have fruitful secondary and base buds, which in turn produce several clusters per shoot. Seyval and Chancellor are good examples. These two varieties still produce a normal crop after losing their primary buds to cold injury.
When: There are two periods—before bloom (pre-bloom) and after fruit set (post fruit set).
Pre-Bloom Thinning: This consists of the removal of flower clusters. This practice can be done at the same time as shoot thinning.
The advantage of this timing is that clusters are easy to see; thus, thinning can be done quickly. By removing flower clusters this early, several things happen—berry set is improved (more berries per cluster as a result of less competition with fewer clusters), and berries are bigger at harvest. Other advantages include increased yield, increased sugars and flavors of the fruit, improved vine size and hardiness.
Disadvantages of early thinning include tighter clusters (as a result of increased fruit set and larger berries); thus, bunch rot can be a problem. Seyval produces large and tight clusters and is susceptible to bunch rot. Therefore, thinning before bloom is not recommended for Seyval. However, this practice is beneficial to cultivars that have loose clusters and are not susceptible to bunch rot, such as Chambourcin and Chancellor.
Post Fruit Set Thinning: In this case, berry set is less than that of pre-bloom thinning. There are fewer berries per cluster; thus clusters are looser, and bunch rot incidence is reduced. This practice is more common and recommended for cultivars susceptible to bunch rot, such as Seyval. (See Figure 30.) With this method, yield, sugars, vine size, and hardiness are improved.
This method, however, is more time-consuming, hence more expensive—it is more difficult to see the fruit due to a more developed canopy. At this stage of shoot development, the vine canopy is about 75% formed.
The following rule of thumb for cluster thinning can be used as a general guideline:
Special Cases: In almost all commercial vineyards, there are always some vines that seem to fall behind in growth and production. They look like one- or two-year-old vines in a five-year-old vineyard.
In this case, these undersized vines should be cluster thinned heavily, sometimes completely. This will allow the vines to recover by diverting the carbohydrates to trunks and roots.
Follow the same procedure with vines that have not filled the trellis yet.
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| FIGURE 30. Post fruit set thinning is recommended for cultivars susceptible to bunch rot, such as Seyval. |
Leaves, and sometimes lateral shoots, are removed in the fruiting zone in order to accomplish two goals. First is to improve air movement and spray penetration, thus reducing bunch-rot infection. This is especially critical for cultivars susceptible to botrytis bunch rot such as Pinot noir, Pinot gris, Pinot blanc, Riesling, Vignoles, and Seyval.
Second is to improve sunlight exposure of fruit and basal buds. This results in better color for red wine varieties, and lower potassium and pH in the juice.
Leaf pulling is done on the shade side of the canopy, which is either the east side of a north-south row or the north side of an east-west row. One to three leaves are removed at the base of each shoot and around clusters. (See Figure 31.) Leaf pulling is either minimally done or completely avoided (depending on the canopy thickness) on the sun side of a canopy in order to avoid sun burning of fruit.
A disadvantage of leaf pulling is increased bird damage as result of exposed berries.
When: Leaf pulling is first performed after fruit set. One more cleanup pass may be necessary before veraison by removing old and yellow leaves. Leaf pulling should be avoided at or after veraison as this may lead to fruit sunburn.
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| FIGURE 31. Leaf pulling consists of removing one to three leaves at the base of each shoot and around clusters between fruit set and veraison but not after. Picture taken after veraison of Norton, but leaf pulling was conducted after fruit set. |
Shoot hedging consists of cutting shoots that grow beyond the allocated space in a given trellis system in order to control shoot length. It is called hedging for upward shoot training, such as on a VSP system (Figure 32), and skirting for downward shoot training, such as on a high cordon (HC) system.
Shoot hedging may be required for VSP systems and upper canopies of Scott Henry (SH) and Smart Dyson (SD) systems. High cordon (HC) and GDC systems do not usually require skirting unless shoot tips interfere with traffic in row middles. In general, a minimum of 12 to 15 leaves per shoot should be left after hedging in order to mature the fruit and wood.
When: Shoot hedging is performed when shoots grow beyond the trellis space and desired length—about 3.5 feet for VSP and about 5 feet for HC and GDC. The amount of summer rain will determine the number of hedging passes (typically one to three passes). However, do not hedge shoots after veraison, since this may result in delay of fruit maturity and reduces wood maturity and thus winter hardiness.
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| Figure 32. Shoot hedging of VSP-trained vines. Before (L) and after (R) hedging. Note fruit zone is exposed after hedging. | ||