Pruning Is the First Crop-Load Decision of the Season

Winter pruning is often viewed primarily as a way of controlling vine size and maintaining the structure of a vineyard. But for wine grape production, pruning is also the first major crop-load decision of the coming season.

Every bud retained has the potential to produce a shoot, and each fruitful shoot has the potential to carry one or more inflorescences. Consequently, the number and position of buds retained at pruning establishes a significant proportion of the vine’s yield potential before the growing season has even begun.

This becomes particularly important in premium wine production, where the objective is not necessarily to produce the maximum possible crop, but to produce a crop that is appropriate for the vine’s capacity and the desired wine style.

Two buds or three?

Consider a mature Pinot Gris vineyard trained to vertical shoot positioning (VSP) and spur pruned along a bilateral cordon.

Imagine a vine carrying 10 evenly spaced spurs on each side of the cordon, giving 20 spurs per vine.

If each spur is pruned to two buds, the vine retains approximately 40 buds.

If those same spurs are pruned to three buds, the retained bud number increases to approximately 60.

That is a 50% increase in retained buds without changing the number of spurs, the vine spacing or the trellis system.

The important point, however, is that a 50% increase in retained buds does not necessarily result in a 50% increase in yield.

Bud fertility varies according to cultivar, bud position, vine condition and season. Research has demonstrated that individual buds can differ substantially in their probability of producing inflorescences, meaning that simply counting buds does not provide a complete estimate of crop potential.

This is particularly relevant to Pinot Gris.

Research examining 26 Vitis vinifera cultivars in high-altitude vineyards in Santa Catarina, Brazil, found significant differences in bud fertility according to both cultivar and bud position. Pinot Gris had an average bud fertility of approximately 62% across the first ten bud positions examined. Importantly, the researchers classified Pinot Gris among the cultivars that should preferentially be managed using longer pruning because of its bud fertility pattern. (Ceres Journal)

This finding has an important implication for spur-pruned Pinot Gris.

A two-bud spur relies almost entirely on the first two basal buds to establish the crop. Extending the spur to three buds introduces another node into the crop-bearing structure. Depending on the cultivar and site, that additional bud may have a different fruitfulness potential from the basal buds.

Therefore, the decision between a two-bud and three-bud spur should not simply be based on tradition or what the vineyard has always done.

More buds can mean more crop potential

Increasing bud number generally increases the potential number of shoots and bunches, although the relationship is not linear.

Research has repeatedly demonstrated that increasing retained bud number can increase cluster number and yield, but excessive crop can come at the expense of fruit composition and vine balance. A classic study examining cane and bud number found that increasing buds per vine increased cluster number and total yield, while excessive crop was associated with reductions in cluster weight and sugar concentration. (ScienceDirect)

This is the fundamental crop-load trade-off.

If the vine has sufficient vegetative capacity, increasing retained buds may allow more of that capacity to be distributed across a greater number of shoots. Shoots may be less vigorous individually, the canopy may become more open and the fruiting zone may be better distributed.

But if the vine does not have sufficient capacity, simply retaining more buds can create a different problem. The vine may carry more fruit than its canopy can adequately support, potentially delaying ripening and reducing the concentration and consistency required for premium wine production.

The relationship between pruning weight and bud number.

This is where pruning weights become particularly valuable.

Rather than applying the same pruning prescription to every vine or every block, pruning weight provides an indication of the previous season’s vegetative growth.

For example, suppose a Pinot Gris block has an average pruning weight of 0.5 kg per vine. Another block of the same variety has an average pruning weight of 1.2 kg per vine.

Applying exactly the same retained bud number to both blocks may not be appropriate.

The first block may have limited vegetative capacity and require a conservative crop load. The second may have considerably greater growth capacity and potentially benefit from retaining more buds to distribute that vigour.

This is the fundamental principle behind balanced pruning: the vine’s previous growth should influence the crop load imposed on it in the following season.

The objective is not simply to leave a predetermined number of buds. It is to match retained reproductive potential with vine capacity.

VSP makes distribution particularly important.

In a VSP vineyard, the number of retained buds also influences shoot density and canopy architecture.

A vine with 40 retained buds and a vine with 60 retained buds may ultimately produce very different shoot densities. If most buds burst successfully, the additional shoots generated by the three-bud spur treatment can increase canopy density and potentially alter the fruit-zone environment.

This makes shoot thinning an important part of the crop-load equation.

Research on Pinot Gris grown in a warm climate demonstrated how pruning, shoot number and canopy management interact. Geller and Kurtural found that manipulating shoot density affected yield and canopy characteristics, with their work identifying approximately 35 shoots per metre of row as part of an effective crop-load management strategy under their specific vineyard conditions. Their study also found that mechanical hedging combined with moderate shoot thinning increased yield while maintaining fruit composition. (ResearchGate)

The important qualification is that these numbers should not be transferred directly to another vineyard. Row spacing, vine spacing, climate, soil, rootstock, cultivar, water availability and vine vigour all influence the appropriate shoot density.

The research demonstrates the principle rather than providing a universal target.

Pruning determines potential, but the vineyard determines reality.

A vine pruned to 60 buds does not necessarily produce 60 fruitful shoots.

Budburst, basal bud fertility, frost, disease, vine condition and spring weather all influence the final number of shoots and bunches.

This is why crop-load management should be considered a process rather than a single winter operation.

Pruning establishes potential. Budburst establishes the number of shoots. Shoot thinning determines the final shoot density. Inflorescence thinning can further refine crop load, while fruit thinning may be used later if necessary.

Each stage provides an opportunity to bring the vine closer to the desired balance.

The importance of measuring the result.

The most useful way to compare two-bud and three-bud pruning is not simply to observe the vines during winter. It is to measure what happens throughout the season.

A replicated trial within a Pinot Gris block could compare two-bud and three-bud spurs while keeping the number of spurs per vine constant.

The resulting measurements could include retained buds, budburst percentage, shoots per vine, bunches per shoot, bunch weight, yield per vine, pruning weight, fruit composition and ultimately the Ravaz Index.

The Ravaz Index provides a useful way of relating crop to vine growth because it compares fruit yield with dormant pruning weight.

A vine producing 5 kg of fruit with 0.5 kg of pruning weight has a very different crop-to-growth relationship from a vine producing 5 kg of fruit with 1.5 kg of pruning weight.

This is why yield alone is a poor measure of vine balance.

Premium winegrowing is about finding the balance.

There is no universal rule that says Pinot Gris should always be pruned to two buds or always to three.

The correct decision depends on the cultivar’s bud fertility, vine vigour, vineyard architecture, target yield, fruit-quality objectives and seasonal conditions.

For some vineyards, two-bud spurs may provide an appropriate crop load and canopy density. In a vigorous vineyard with excessive pruning weights, increasing the retained bud number may be a useful way of distributing vine vigour and increasing crop utilisation. In another vineyard, increasing bud number could simply increase crop beyond the vine’s capacity.

This is why pruning decisions should be based on data rather than habit.

The combination of pruning weight, retained bud number, shoot density, yield and fruit composition provides a much more complete picture of vine balance.

Ultimately, pruning is not simply about deciding how much wood to remove.

It is about deciding how much reproductive potential to leave behind.

For premium wine grapes, that decision made in winter can have consequences all the way through to harvest, fruit composition and ultimately the style and quality of the wine.