Pruning, a regulatory tool of the vine vigour

In wine regions like Burgundy, quality wines are produced in steep slopes where the low-fertility soils make the control of the vines vigour easier and produce grapes of extraordinary quality. On the other hand, in flat lands where the weathered and carried ground of the slopes has accumulated, the soil usually has a high fertility. These lands are generally used for forage growing, but not for producing grapes used for making great wines. This is a proven fact over time by Burgundy producers.

From the 1990s, when wines from Priorat started to be mentioned by the international press, we had a doubt concerning the use of traditional varieties like Garnatxa and Carinyena in the new terrace fields. Could we keep the same quality and personality of the wines from old vines with these grapes from young vines? It was not sure, because we already observed that we obtained compact and big grapes from these young vines, grapes which would be difficult to ripe and produce wines with structure and a wide aromatic range, like the ones from old vines. The wide experience in Burgundy of not using fertile soils to produce high-quality wine was reflected here, in Priorat, by using vigorous varieties on terrace soils which were more fertile than the lands of the slopes where their fertility had been swept by the rain. This was not the right path, we needed to find a way to grow those varieties and make them produce quality wines from young vines.

Walking through the young vines, during harvest period, we observed loose grape bunches hanging from the thin shoots, and those grapes stood out for their uniform ripening, whereas in the case of shoots with a larger diameter, the grape bunches were compact, with big and crowded grapes. We took the decision of measuring this relationship by carrying out a small statistical study. The result, between the shoot diameter and the grapes morphology, was very significant:

The shoots with a diameter between 8 and 10 mm produced, at a very high rate, grape bunches with loose grapes and a medium-small size, whereas the shoots with a diameter exceeding 12 mm produced mainly big and compact grapes.

With this evaluation, a new path opened to us: find the way to make the vine produce small-diameter shoots.

For some years, we racked our brains to get thin shoots… We tried everything, or almost everything, to achieve this. We went to Piedmont to talk with the vine grower Roberto Voerzio, very known for his high-quality wines, and discuss about this issue. He just wanted to reduce the vigour of his vineyard by planting a vine in the middle of the vines that he had already planted in order to reduce the plantation framework, and thus reduce the vigour and increase the quality of his wines.
When returning from the trip, we thought carefully about the issue, and we concluded that, if we reduced the distance between the vines, we would also reduce the root volume, and as a result, the water absorption capacity too. And, at the same time, we would also reduce the leaf surface and the breathability, which means the water consumption capacity. This leads us to a compensation in the vine, which would remain in balance.

This reflexion told us that reducing the distance between the vines would not lead to a reduction of their vigour. It would not help us to achieve the proposed objective.

Later, in a plot with very vigorous and compact grape bunches, we tried not pruning, but we carried out a “preprune”, which means that we cut all the shoots 10 cm from the base, and we kept them all. Result: a big shoots production, with a big production of small, loose and non-compact grape bunches.
This result was spectacular. We had obtained a definitive proof: the increase of the number of shoots of a vine leads to a production of small and loose grape bunches.

The problem was that we needed more space to keep the shoots, and the space was limited. We had two plots where we wanted to apply this concept. In one plot there were very vigorous vines with thick shoots, and which were planted with a length of 1.20 m, so we could only keep 12 shoots. Here we carried out a reorganisation, and we transformed the simple trellis in a double cordon trellis. We guided the shoots towards two production lines, that is to say, two parallel arms, so the production line had twice its previous length (2 x 1.20 m = 2.40 m), and we could keep 24 shoots.

In the other plot, the vines were planted with a length of 0.80 m, which meant that we could only keep 8 spurs and they were not enough for the vigour that they had. Thinking carefully about it, we came up with the formula (2 x pi x r). We had the idea of building a trellis where we would put a circle on each vine, with a 60-cm diameter, so the length of this circle was 2 m, and we had enough space to keep even 20 spurs.

At that point, we understood that, in order to get non-compact grape bunches with rather small grapes, we had to keep more shoots, but how many shoots per vine? Pruning cannot be more or less. Everyone knows that, in the most vigorous vines, we have to keep more shoots, but how many?

In vineyards with different fertility areas, how could we know exactly how many? We wanted shoots with a maximum diameter of 8-10 mm.
During winter pruning, we chose shoots which had the morphology that interested us: a diameter of between 8 and 10 mm and a length of 1.20 m. This would be the “ideal” shoot. We picked many from different varieties. We weighed them, and it gave us an average of 50 g.

We can define vigour as the vine capacity to produce wood (shoots). And the amount of vigour of a vine is the weight in grams of the shoots produced during a growing cycle.
This capacity to produce wood is split or “wasted” while the shoots grow. When the vigour is “wasted”, no more shoots grow. Therefore, we understand that the control of the size of the shoots is in the pruning, and consequently, in the type of grapes.

Seen this way, we can weigh all the pruned shoots of a vine, and it gives us its vigour.

If we divide the vigour by 50 g, which is the weight that we want for a shoot to have, we will get the number of shoots that we have to keep in that vine when we carry out the green pruning. And the shoots will grow until all the vigour is used.

An example of a vine with a vigour of 500: “The weight of all its shoots is 500 grams”

• If we keep 10 shoots in the pruning (500/10), each one of the 10 shoots will weigh 50 g, and they will have an approximate length of 1.20 m. The diameter will be approximately between 8-10 mm.
• If we only keep 5 shoots in the pruning (500/5), each one of the 5 shoots will weigh 100 g, and they will have a length of more than 2 m. The diameter will be much thicker. We will say that this vine is very vigorous.
• If we keep 20 shoots in the pruning (500/20), each one of the 20 shoots will weigh 25 g, and they will have an approximate length of 0.60 m, and a very thin diameter. We will say that this vine has little vigour.

Therefore, as we can classify it as very vigorous or with little vigour, if it is the same vine, we only carry out a different pruning!

Former farmers already knew that, in a vigorous vine, they could keep more shoots than in a thinner one. All this process made us think that vigour is a property of the plant, and that pruning is a way to act on it.

Intelligent pruning is when we keep the adequate number of shoots to split their vigour, so that their length is not excessive to avoid having to cut off and make their thickness rather thin.
On this basis, we can state that no vine has an excessive vigour if we carry out proper pruning.

We encourage you to have a look at the attached presentation named PowerPoint 1st part, in which we explain all the experiences that we have carried out, and in which we can see the morphology differences between vines with thick and thin shoots.

Josep Lluís Pérez i Montse Ovejero