Viticulture
The Annual Cycle of the Grapevine in the Douro
The viticultural year begins at budburst, around the middle of March. This usually signifies the end of the harsh winter weather, during which time the vines have lain dormant, and the beginning of the milder spring. The damp climatic conditions prevalent at this time of year mean that delicate young shoots are immediately at risk from attacks of powdery mildew. However, proper vineyard management will help ensure that they grow longer and thicker during the next few weeks. They still remain very susceptible to damage from late frosts.
By the middle of May the shoots have reached around a metre in length and flowering starts. The flowers, which will in time become bunches of grapes, are usually found at the third and fourth internodes from the base of the shoot. The whole flowering process, from the first cap fall to fruit set, can take as little as a week or as much as a month, depending on the weather.
After flowering and fruit set (when the ovary of each fertilised flower begins to swell and develop into a grape) the vine continues with its vegetative growth. There is however a change of emphasis from the growing tips to the developing bunch, which becomes the primary nutrient sink.
From around the 20th July veraison can be expected. This occurs when the bunches of red grape varieties begin to change colour from green to purple, and the berries soften in consistency. It also marks a shift between berry growth by cell division and berry growth by accumulation of water. At the same time, sugars begin to accumulate in the berries and the natural acids start to break down. In short, veraison is the start of the ripening period.
Towards the beginning of August, the normal low levels of rainfall causes the shoot growth to cease. This is usually accompanied by hedging the vines to allow better light penetration into the canopy, which is important for fruit maturity. Ripening continues during daylight hours weather permitting. Warm temperatures accelerate this process, although it should be noted that during periods of high wind or very high temperatures ripening is suspended. By the middle of September the grapes are usually ripe enough to pick, with high sugar levels having been achieved due to a combination of good ripening together with the concentration effect of mild dehydration. All the fruit is picked by hand.
Shortly after harvest, around the second half of October, the vines begin to translocate carbohydrates from the leaves to the roots in preparation for dormancy, during which time the foliage turns a brilliant red colour. When all the useful nutrients have been recovered, the leaves turn brown, die, and the vine enters into dormancy. During this period pruning is carried out, which is necessary to maintain the balance between vegetative growth and fruit production in the following season. The pruning level depends upon the variety and vigour of the vines, but a typical crop load would be achieved by leaving around 12 buds. The later the pruning is carried out, the later the bud burst, so areas with a high risk of frost are pruned as late as possible - possibly even into February.
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