MONDAY, JANUARY 1, 2024. BY STAN GRANT, VITICULTURIST.
Continued from GRAPEVINE CANOPY MANAGEMENT REVISITED PART 1
Direct Canopy Management – Configuration
Direct canopy management is canopy management by intervention. It takes two forms. The first, which is addressed here, involves either fine-tuning normal indirect canopy management effects or remediation of indirect canopy management miscalculations. Direct canopy management, in effect, sets the final physical configuration of the foliage in a vineyard. Canopy configuration practices include shoot thinning, fruit zone leaf removal, and shoot tipping or hedging (Figure 5).
Figure 5. Dappled sunlight at full canopy following shoot thinning and basal leaf removal of Zinfandel. (Photo Source: Progressive Viticulture, LLC©)
Shooting thinning to preserve or at least, approximate the shoot density set during pruning is an example of an intervention to fine-tune indirect canopy management. At the same time, retaining shoots arising from dormant buds on old wood to replace missing spurs and retaining shoots near the bottom of upward creeping arms to replace excessively high spurs are remedial actions. These shoot number adjustments are best made while shoots are short for expeditiousness and to conserve internal vine resources for the remaining shoots.
Even with careful pruning and shoot thinning, fruit zones of varieties with large leaves, such as Chardonnay and Malbec, usually require some leaf removal on the morning sun side of canopies to fine-tune cluster exposure to light and air movement. In some instances, the removal of lateral shoots within fruit zones achieves a similar result. Leaf removal is a remedial task where fruit zones are shaded due to shoot crowding. For maximum benefit and minimum risk of reduced yield and sun-damaged fruit, remove leaves shortly after fruit set.
Figure 6. Hedge canopies late and lightly to ensure adequate ripening capacity. (Photo Source: Progressive Viticulture, LLC©)
Hedging to improve the uniformity of canopy dimensions across a vineyard is a fine-tuning operation. In contrast, hedging is remedial where applied to impose growth balance in vineyards with excessive canopy growth. When employed, hedge as late and no more severely than necessary to minimize the loss of photosynthetic and ripening capacity (Figure 6).
Direct Canopy Management – Configuration for Extremes in Growth Vigor
In vineyards where vine growth vigor is very low and canopy development is incomplete, dramatic remediations are required (Figure 7A). In addition, to shoot thinning, cluster thinning before or shortly after fruit set is required to further reduce competition for limited internal vine resources. Also, eliminate weeds in vine rows and cover crops in tractor rows to minimize competition with vines for root zone resources. Simultaneously, increase the supply of external resources through liberal irrigation, soil amending, and fertilization to enhance root activity and stimulate shoot growth.
The reverse situation, with very high growth vigor, and excessive canopy development, requires measures beyond hedging (Figure 7B). Create a competition for soil resources with robust cover crops. At the same time, to the extent possible, limit water and nitrogen applications. For chronic cases, consider removing alternate vines within the vine rows (i.e., retrofit vine spacing) and extending the cordons of the remaining vines.
Figure 7. Chronic low growth vigor (A) and excessive growth vigor (B) require dramatic remediation. (Photo Source: Progressive Viticulture, LLC©)
Direct Canopy Management – Maintenance
Beyond directing shoot growth and configuring foliage there is a final aspect of canopy management. It is maintaining healthy leaves so canopies may remain adequately functional to achieve our goal of efficient production of economically viable yields of high-quality wine grapes over the long term. Naturally, many risks to leaf health emerge in vineyard environments over the course of a growing season.
Cordon and trunk diseases can compromise canopy size, root diseases can induce canopy collapse, and severe powdery mildew infections can cause premature leaf loss. Several damaging insect pests, including leafhoppers, leaffolders, and spider mites, can reduce leaf function and effective area. Fortunately, pests and diseases of grapevine canopies are controllable.
Even mild water stress impacts canopies, slowing shoot elongation and leaf expansion. Shoot growth and canopy development completely stop under moderate stress (leaf water potential of -10 to -12 bars). In well-designed and well-managed vineyards, such water stress occurs after canopies are fully developed. In other vineyards, canopies are either under or over-developed at the onset of moderate water stress, resulting in inefficient growth imbalances.
Figure 8. Severe water stress (A) and heat stress (B) leaf damage reduce the effective leaf area of canopies. (Photo Source: Progressive Viticulture, LLC©)
When water stress becomes severe (leaf water potential ≤ – 15 bars), photosynthesis is very slow or comes to a stop. With intensifying severe water stress, leaves wilt, thereby decreasing canopy leaf area and sunlight absorption capacity. The effects of prolonged severe water stress include the production and release of damaging active oxygen species within leaf cells and associated premature leaf tissue yellowing and death, causing an irreversible loss of canopy (Figure 8A).
During periods of triple-digit afternoon temperatures, sections of leaf tissue directly exposed to the sun can be damaged and die if not fully hydrated (Figure 8B). High levels of ultraviolet radiation can intensify the harmful effects of both heat stress and severe water stress on leaves and canopies. Clearly, careful regulation of the vineyard water supply and the mineral nutrients involved in vine water relations, especially potassium, is a critical element of direct canopy management.
Figure 9. Magnesium deficiency (A) and iron deficiency (B) diminish canopy functions. (Photo Source: Progressive Viticulture, LLC©)
In addition to restricting or altering shoot growth, foliar symptoms of most mineral nutrient deficiencies include specific patterns of leaf yellowing or reddening due to their impacts on photosynthesis or associated metabolism (Figure 9). At the other extreme of the mineral nutrient supply, certain excesses can become toxic, killing leaf tissues and decreasing production capacity. Accordingly, maintaining a balanced supply of mineral nutrients is essential for canopy health.
Among mineral nutrients, nitrogen may be most connected with grapevine canopies. Leaf chlorophyll concentration and photosynthetic rate are positively associated with vine nitrogen status (Figure 10), while at the same time, the vineyard nitrogen requirement to ensure adequate yeast assimilable nitrogen (YAN) for fermentation increases with canopy size. Importantly, excess nitrogen leads to excessive canopy growth and low production efficiency.
Figure 10. Leaf chlorophyll and photosynthetic capacity depend upon adequate nitrogen. (Photo Source: Progressive Viticulture, LLC©)
Canopy Management Measures of Success
As with all aspects of grape growing, we need some assurance our canopy management actions are effective. Perhaps the most obvious indicator is regularly achieving balanced growth, moderate water stress, and fruit exposure to dappled sunlight with minimum inputs shortly after the fruit set. Other measures of success involve controlled costs through operational efficiency, including limited need for canopy remediation, and low bunch rot incidence for disease-prone varieties. Consistent fruit yields and positive winery feedback regarding grape quality may be the most consequential indications of successful canopy management. A shortfall in any one of the measures of success ought to serve as a call for vineyard management self-evaluation.
Conclusions
Canopy management is the tending of grapevine foliage and the production machinery therein. It is a pivotal viticultural endeavor that has both short-term (current season) and long-term (vineyard longevity) impacts. Consistently successful canopy management involves setting and pursuing specific goals, optimizing indirect canopy management through a well-conceived vineyard design, fine-tuning through carefully applied direct canopy management practices, and assessing performance and identifying areas for improvement.
A version of this article was originally published in the Mid Valley Agricultural Services February 2006 newsletter and was updated for this blog post.
Further Reading
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