MONDAY, MAY 20, 2024. BY STAN GRANT, VITICULTURIST.

Grapevine fruit yield is a destination. It is the culmination of several developmental processes during a two-year journey. The developmental path leading to fruit yield is complex (Figure 1).

Figure 1. Grapevine fruit yield is a complex journey involving multiple pathways and many components. (Source: Progressive Viticulture, LLC©).

We can influence some stages of the fruit yield development journey, while other stages we cannot. Our influences are limited to the effects of vineyard design and vineyard management inputs and practices. In this article, we will simplify the complexity of fruit yield by focusing on these two vineyard factors. In addition, we will discuss how we can promote efficient and sustainable fruit production through them.

Grapevine Growth Capacity: The Foundation of Fruit Yield

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. Past stresses and management actions or inactions may diminish vine growth potential. Accordingly, the growth potential of any given vine is the net result of these three factors – genetics, site, and history.

Capacity is the viticultural term for a vine’s potential for growth, including fruit growth and yield. It is a function of root system density and wood mass of the above-ground vine (Figure 2). Capacity reflects a vine’s potential to acquire, synthesize, and store resources.

Figure 2. Grapevine growth and production capacity is a function of wood mass and root system size and density. These photos show two head-trained vines with different capacities residing in the same vineyard. (Photo Source: Progressive Viticulture, LLC©)

Growth capacity is not the same as growth vigor, which is the rate of growth and not the potential for growth. Capacity is like wall voltage, while vigor is like the current in a cord running to an appliance. Capacity and vigor, however, are related in that high-capacity vines can easily produce high-vigor shoots, but low-capacity vines cannot. Correspondingly, so-called invigorating rootstocks are actually rootstocks that promote high growth capacity.

Fruit Yield and the Limits of Vine Growth Capacity

Grapevine training and pruning can impact the quantity of permanent vine wood and thereby, growth capacity. Such an impact is manifest in the reduction of fruit yield when canes replace cordons and spurs (Figure 3).

Figure 3. When canes replace cordons, the reduction in wood mass and vine capacity is reflected in reduced yields. (Data for combined average yields for two trials involving quadrilateral cordon-trained Sauvignon blanc vineyards. Canes replaced cordons during late winter of 1989 and new cordons were reestablished in subsequent years).

With increasing fruit zone height above the vineyard floor, canopy leaf area, and trunk length simultaneously increase. These increases provide greater photosynthetic capacity and nutrient storage, respectively. That being so, increased yields are the net effect of higher fruit zones. Nonetheless, yield increases are not proportionate to changes in fruit zone height (Figure 4). Moreover, the effects of increasing fruit zone height, while initially significant, diminish towards the limits of grapevine growth capacity.

Figure 4. Fruit yields increase with heightened fruit zones within the limits of grapevine capacity.

Horizontal canopy division doubles the amount of cordon and potentially, leaf area and photosynthetic capability per vine. While canopy division does, in fact, substantially increase fruit yields, it does not double them due to the limits of grapevine growth capacity (Figure 5).

Figure 5. Doubling the number of cordons and spurs did not double fruit yield due to the limits of vine capacity. (Data for combined average yields for 3 trials involving Chardonnay. All vines supported 7 feet of cordon in a bilateral configuration ).

Narrowing vine row spacing increases the number of vines, buds, and shoots per acre and consequently, the yield per acre. This effect is most apparent in cooler sites (e.g., Oakville) where grapevine growth capacity is lower than in warmer sites (e.g., Kearney) (Figure 6). In cooler sites, yield changes appear nearly proportionate to changes in row spacing. In warmer sites, competition for sunlight and soil resources among larger-capacity vines presumably limits the effects of narrowing row spacing on yields.

Figure 6. Vine densities and vineyard yields increase as rows are spaced more closely. This effect is especially beneficial for low-capacity grapevines.

Relative yield per vine is disproportionately large for closely spaced, low-capacity grapevines (e.g., Seyval blanc) where their canopies can fully develop and fill their allotted space (Figure 7A). For both low and high-capacity vines, yield per vine increases with increasing spacing, but at a rate that steadily decreases in proportion to the intensifying strain of additional shoots on capacity. Differences in the influences of growth capacity are more apparent for yield per acre, for which yield per acre for low-capacity vines declines and yield per acre for high-capacity vines appears stable within the range of these data (Figure 7B).

Figure 7. On a per-vine basis, yields of both low and high-capacity grapevines slowly increase as the space between vines increases. Yields of low-capacity vines especially benefit from close vine spacing in the row (A). On a per-acre basis, low-capacity vines fail to fully use increasing space and yields decrease accordingly, while yields of high capacity remain stable within the range of vines spacings studied (B).

To summarize, grapevine growth capacity is apparent in yield responses to changes in the spacing-training-trellising-pruning system (i.e., vineyard design). In practice, these responses are less than proportionate to the changes in vineyard design factors due to the limits of vine growth capacity. Advantageous use of grapevine growth capacity for fruit production commonly involves cordon training and spur pruning, high fruit zones, canopy division, and to the extent feasible, close row spacing, and for low-capacity vines, close vine spacing.

As mentioned earlier in this article, the combination of scion and rootstock influences grapevine growth capacity and as a result, fruit yield (Figure 8). Hence, plant material growth capacity ought to affect vineyard design decisions. Still, there are other factors that may be more important in the selection of a rootstock than its influence on vine growth capacity, with resistance to soil-borne pests and diseases usually being paramount.

Figure 8. Amador County Zinfandel fruit yields reflect the influence of rootstock vine capacity.

This article is based on a presentation delivered at the 2021 Mid Valley Agricultural Services Grape Grower Meeting. The author dedicates this article to Maxwell Norton, UC Cooperative Extension Farm Advisor Emeritus, his partner in many a viticultural research project in pursuit of an understanding of winegrape yield

IN PURSUIT OF YIELD: GRAPEVINE CAPACITY, BALANCE, & CROP LOAD – PART ONE

Grapevine Growth Capacity: The Foundation of Fruit Yield

Fruit Yield and the Limits of Vine Growth Capacity

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