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Coping with global warming: Summer pruning

Global warming causes an accelerated ripening of the grapes, and this is not always beneficial. An increased temperature during the growing season and the ripening of the grapes results in unbalanced alcoholic wines that are low on acids and color, and have different aromas than usual. Achieving the optimum ripeness of the grapes is becoming problematic in more and more wine regions. Summer pruning may be a method to adjust the ripening period of the grapes.

Harvesting ripe grapes is not that simple. There is more to it than just tasting whether they are sweet enough. During the ripening phase sugars are formed and the amount of acids in the grape decreases. To make a good wine, grapes with a good balance between sugar and acid levels are required. The sugars provide (sufficient) alcohol, and the acids ensure that the wine retains its freshness and does not become jammy and heavy. The moment when the sugar-acid balance is optimal is called the alcoholic maturity. However, wine is more than sugars and acids, also the phenols (e.g. tannins and anthocyanins) and aromas in the grape are important, and these too must mature. In the optimum case, this phenolic maturity coincides with the alcoholic maturity (see Figure 1).

READ ALSO: Anthocyanins are pigments with taste!

In very warm conditions, sugars are produced very quickly in the grape and the acids are broken down at breakneck speed. When this happens, the phenols are by no means matured at the time of alcoholic maturity. The winegrower then faces a difficult choice: harvest now, or wait until the grapes reach phenolic maturity? Climate warming causes classic wine regions such as Bordeaux, Burgundy, Champagne, Rioja and Chianti to have problems with the maturing of their traditional grape varieties. From a commercial point of view it is not convenient to plant other varieties. Traditions are not to be broken, and therefore solutions are being sought to get the traditional grape varieties optimally ripe despite the warmer weather.

Graph of the alcoholic and phenolic maturation of grapes expressed in acids, sugars and phenols over time
Figure 1. The optimum harvest time based on sugars, acids and phenolic maturity. Under optimal conditions, the correct sugar-acid balance (the alcoholic maturity) coincides with the phenolic maturity (left figure). In too hot conditions, a lot of sugars are formed quickly, the acids are broken down fast and the phenols are not yet sufficiently matured at the alcoholic ripeness (right figure).

Summer pruning

Summer pruning is one of the methods with which there is plenty of experimentation. By pruning the new shoots during the summer, the grapevine is forced to regrow its shoots. This results in a growth delay, and a shifting of the growth and ripening period of the clusters and berries. At the “Universidad de la Rioja” in Spain, they have the objective of delaying the ripening period of the famous Tempranillo grape and the (lesser known) Maturana Tinta grape by two months (!). The harvest should therefore not take place in the very hot August month, but in the cooler months October and November. As a result, the grapes ripen in a cooler period of the year and they are less likely to develop large amounts of sugar, the acids are preserved, and due to a longer ripening period more mature phenols are formed.

Delaying and prolonging the ripening phase

In three consecutive years (2015-2017), the vines were trimmed to two nodes per shoot during different growth stages (see Figure 2) to determine the optimum pruning moment. Pruning during stages G-H (until flowering) causes a growth delay of more than a month. Harvest therefore takes place in September until the beginning of October, instead of in August. When pruning takes place after flowering during growth stages I-K, the harvest time is in October-November. This means that the intended delay of the ripening period by two months can beachieved! Pruning at a later growth stage (L) causes too great a delay in the ripening time, which makes that the Tempranillo grapes no longer are able to completely mature.

The growth stages of the wine grape according to Baillod and Baggiolini (1993)
Figure 2. The growth stages of the wine grape according to Baillod and Baggiolini (1993)

It is striking that the sprouting of the shoots is better when pruning in growth stage I-K than in the earlier stages G and H, but that nevertheless – due to smaller clusters and berries – the yield in the later stages is still lower. In all cases, the summer pruning results in a smaller harvest yield compared to the non-pruned vines.

In contrast to the Tempranillo grape, pruning of the Maturana Tinta grape during growth stage K does not results in the formation of new shoots. In general, it was assumed that the buds from which new shoots can emerge only go dormant during the veraison (stage M). Apparently, for the Maturana Tinta this happens in an earlier stage, which makes that shoot regrowth does not occur. The optimum pruning moment therefore differs per grape variety.

A balanced wine

Despite a lower harvest yield, the forced vine regrowth due to the summer pruning does result in grapes with more acids. The amount of sugars is equal to that of the non-pruned vines when the pruning takes place during stage G-H, and is slightly lower when pruning during the later stages. In addition, there are also more anthocyanins in the grapes when the pruning takes place during growth stage G-I. The summer pruning in the Rioja therefore ensures that there is a better balance between the alcoholic and the phenolic ripeness of the grapes. Naturally this has a positive effect on the quality of the wine.

Burden on the vine

Grapevines are burdened extra by the summer pruning. It takes more energy to grow the shoots again and the question is whether they can handle this. Although the vines theoretically produce enough leaves (> 1.5 m2) to built up sufficient reserves, the yields in the second and third years of the study are much less than in the first year that the summer pruning was applied. This is mainly because the fruitfulness of the buds was less and not all shoots sprouted. The researchers therefore advocate pruning the shoots to three or four nodes instead of two in order to increase the yield. They also note that the long-term effect of the summer pruning on the vine still needs to be thoroughly investigated. Previous studies have shown that grapevines that underwent the forced vine regrowth were not adversely affected in the next year when the normal pruning treatment was again applied. However, also this effect may depend on the grape variety, the growing stage in which pruning has taken place or the specific weather conditions during the growing season. The technology is promising to deal with global warming, but it still contains a number of uncertainties that need to be examined carefully.

What can winegrowers in cool climates do with this knowledge?

In the northern growing regions (e.g. Belgium, the Netherlands, the UK, Canada) climate change is noticeable, but does not result in the extreme temperatures as seen in Spain or other countries closer to the equator. Even so, also in these regions some (early maturing) grape varieties have already been harvested with extremely high sugar levels, resulting in wines with 15% alcohol or more. Summer pruning would be a possibility for these grape varieties to delay the ripening and lower the sugar content. However, the maturing time does not have to be delayed by two months or more, as in Spain. Other techniques such as shortening the deciduous wall (apical pruning) or leaf thinning during the growing season are probably better options here to delay the ripening of the grapes by a few weeks.

Contact Koen Klemann, WineScience

Reference:
F. Martinez de Toda, J. García, P. Balda. Preliminary results on forcing vine regrowth to delay ripening to a cooler period. Vitis Vol 58 no 1 (2019). https://doi.org/10.5073/vitis.2019.58.17-22

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