MONDAY, DECEMBER 25, 2023. BY STAN GRANT, VITICULTURIST.
There was a time when grape growers gave little thought to grapevine foliage beyond keeping it out of the way of machinery and controlling pests and diseases. Accordingly, hedging the apical ends of shoots and pesticide sprays were the only in-season practices commonly applied to canopies. During the same period, vines were bilateral cordon trained onto simple one, two, or three-wire vertical trellises or trellises with single cross-arms in a T configuration. Such trellises allowed vine foliage to sprawl.
Then, in the 1980s, the canopy management concept emerged. This revolutionary approach to caring for grapevine foliage addressed specific viticultural concerns and its impact has been profound. Canopy management has contributed to marked improvements in winegrape quality at all price points and decreased fungal disease. Just as important, attentive canopy management has improved viticultural efficiency with regard to the crop produced per unit leaf area (crop load) and the crop produced per unit applied resource. Such improvements are possible because vine productivity is contingent on the effective photosynthetic capacity of canopies over the course of a growing season.
In the spirit of continuous improvement, it pays to periodically revisit viticultural practices. Here we do so for canopy management based on a couple of questions. Can our approach to canopy management be more systematic and perhaps, more broadly applicable than in the past? Further, can we set and achieve canopy management goals that make us more consistently successful in our pursuit of efficient production of economically viable yields of high-quality winegrapes over the long term? In this article, we will endeavor to answer these questions as we revisit canopy management. But before we do, we will briefly consider some canopy fundamentals.
About Grapevine Canopies
The definition of a canopy is a cover made of branches. For cultivated grapevines, canopies consist of the current season’s vegetative branches (shoots) upon a foundation of annually replaced woody branches (canes or spurs) connected to permanent woody branches (arms or cordons).
Shoots are the primary canopy subunits of concern. They are multifunctional assemblages of organs. Paramount among these organs are leaves, the grapevine solar panels, in which sunlight-driven photosynthesis produces chemical energy, reducing power, and carbohydrates from water and carbon dioxide to drive vine growth and productivity (Figure 1). Shoots also support tendrils for clinging and climbing towards sunlight, and buds for shoot regeneration, buds and fruit borne on clusters for seed dispersal.
Shoots consist of three zones – basal, mid-shoot, and apical – that lengthen as shoots elongate early in the growing season. When sufficiently elongated, fruit zones reside within the basal sections of shoots.
Shoots undergo four developmental stages: rapid growth, maturation, senescence, and dormancy (Figure 2). Their emergence and initial elongation rely upon nutrient reserves stored in woody vine tissues and root activity, while environmental conditions (atmosphere and soil) influence their rate of growth. After 5 or 6 leaves unfold, shoots have adequate photosynthetic capacity to become self-sufficient to sustain their further growth and development. With further elongation, shoots begin to export carbohydrates to other parts of the vine, including roots.
In young leaves, the rate of photosynthesis rapidly increases before slowly declining about 40 days after they unfolded. Consequently, the zone of maximum photosynthesis moves progressively upwards from the shoot base. At the same time, canopies develop exterior leaves that shade some interior leaves, decreasing their photosynthetic contributions to vine growth and development. Lateral shoots that sometimes emerge from buds in the latter part of the growing season, like young leaves in general, have comparatively high rates of photosynthesis.
Shoot growth and canopy development slows after fruit set and under moderate water stress it stops. These events coincide with a redirection of internal vine resources away from the apical zones of shoots and into developing berries. Varieties, clones, and rootstocks influence shoot growth habits, which can affect canopy attributes (Table 1). Importantly, grapevine canopy attributes influence overall photosynthetic capacity, which in turn impacts vine growth above and below ground; fruit yield, maturation, and quality when ripe; cane ripening and winter hardiness; and the number of stored nutrient reserves in woody tissues at the end of the growing season. Additionally, canopy attributes may affect production efficiency (Table 2).
About Grapevine Canopy Management
Simply stated, canopy management is controlling the number, position, and health of shoots and the leaves and clusters they bear. Canopy management ought to strive to optimize leaf function, maximize production efficiency, enhance fruit and wood quality, minimize disease, and promote a long and productive vineyard life. To achieve these goals, canopy management focuses on three things: achieving growth balance, promoting exposure, and maintaining healthy leaves.
Growth balance refers to two aspects of seasonal vine biomass production. First, the number of exposed, healthy leaves relative to roots, affects soil resource acquisition and allocation in support of canopy and root functions. Second is the number of exposed, healthy leaves relative to fruit, which affects the rate of ripening. Fostering leaf exposure and health, therefore, are basic to achieving growth balance. Additionally, the exposure of clusters to dappled sunlight and moving air is an important concern for berry quality and disease control.
Comprehensive canopy management to achieve the goals stated above involves two complementary approaches. The first, indirect and passive canopy management is the topic of the next section. The sections that follow address facets of direct and active canopy management.
Indirect Canopy Management
Here are a few vineyard realities. Every combination of grapevine variety and rootstock has a certain inherent potential for growth. And every vineyard site imposes limitations on the growth potential of a variety and rootstock. The design of the vineyard directs vine growth, as determined by genetics and environment, and partitions it into organs above and below ground. Simultaneously, the vineyard design will indirectly influence the rate of canopy growth, as well as the final canopy size and spatial dimensions. As such, vineyard design is the basis for indirect canopy management.
The basic objective of vineyard design is to achieve the basic canopy management goals (balanced growth, moderate water stress, and fruit exposure to dappled sunlight) before veraison during an average climate year with minimum inputs after the vines are fully established. Again, in addition to viticultural efficiency, these characteristics contribute to prompt ripening and favorable winegrape quality. Balanced growth is evident in 14 to 20 nodes per shoot, while moderate water stress is apparent as arrested shoot growth, senescent or absent tendrils, and aged, healthy leaves. Dappled fruit exposure occurs when less than 1.0 to 1.5 leaf layers with gaps cover the fruit zone.
The vineyard design process begins with an assessment of environmental factors – climate and soil – which, from a vineyard management perspective, are largely stable (Figure 3, Table 3). Such assessments are often most meaningful when undertaken after the vineyard root zone has been appropriately amended and deep cultivated. Based on the assessment and an approximation of the growth potential of the variety and rootstock, make the best estimate of the mature vine size and growth capacity. This, perhaps the difficult part of vineyard design, requires experience and sometimes, additional expertise drawn from one or more outside sources.
Next, select the variable vineyard design components to accommodate the estimated mature vine size and capacity; using them to direct and partition vine growth in ways that foster the pre-veraison balanced growth, moderate water stress, and favorable fruit zone exposure mentioned above. First, select the length of cordon per vine that favors shoots about 20 nodes long when the shoot density is about 5 to 6 shoots per foot cordon. In the Northern Interior of California, such cordon lengths commonly range between about 8 feet for low-capacity varieties like Pinot Noir to 12 feet of cordon for high-capacity varieties like Cabernet Sauvignon.
Then select a cordon configuration – unilateral, bilateral, or quadrilateral. For unilateral and bilateral cordon-trained vines, the length of the cordon per vine determines the spacing of vines within the vine row. For quadrilateral cordon-trained (horizontally divided) vines, dividing the cordon length per vine by 2 determines the vine spacing.
The space required for equipment when shoots are fully elongated (14 to 20 nodes) and draping into the vine rows determines row spacing. Rows may be narrower for vineyards with trellises providing vertical shoot positioning, which is discussed below, than for those that allow shoots to spread.
The next decision in the vineyard design process involves trellis foliage support. There are four basic types to choose from – no foliage support, passive foliage support, upright & vertical foliage support, and downward foliage support (Figure 4). A few varieties with naturally stiff and mostly upright stems, like Cabernet Sauvignon and Sauvignon Blanc, do not need foliage support unless grown in an area where high winds are common. The passive foliage support provided by T and Wye trellises works well for many high-growth capacity varieties with limber shoots because when sufficiently long, such shoots drape outwards with shoot tips down, and downward tip positioning invigorates shoot growth. Upright and vertical shoot positioning is appropriate where cool temperatures and limited sunshine hours restrict the rate of shoot growth and fruit exposure to sunlight. Downward shoot positioning places the fruit near the tops of canopies, rendering it exposed to direct sunlight – a situation not suited to California’s growing season sunshine and temperatures.
The final vineyard design and indirect canopy management choice involves cordon and canopy height. Leaf area per vine and fruit yield per vine increase with increasing cordon and canopy height. Moreover, higher fruit zones are subject to greater air movement than lower fruit zones. Further, putting canopies higher places them further from soil surfaces, and the radiation reflected from them, can be damaging during hot summer days. Given these factors, it is advantageous to position canopies as high as feasible in most California grape-growing areas.
Failure to use the vineyard design principles presented above can have significant consequences. Where cordons are too long and vine growth capacity is diluted into too many spurs and buds, shoot growth is severely invigorated, canopies fail to fully develop, leaf area is insufficient, vines are overcropped, fruit is overexposed, and berry and cane ripening is impaired. At the other end of the spectrum, where cordons are too short and vine growth capacity is concentrated into too few spurs and buds, shoot growth is unduly invigorated, canopy development is excessive, vines are under-cropped, fruit and interior leaves are shaded, and berry and cane wood quality is substandard. Obviously, indirect canopy management through vineyard design is foundational to sound viticulture.
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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