The Annual Cycle of the Grapevine
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 start 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, although vineyard disinfection with sulphur allows them to grow rapidly longer and thicker during the next few weeks. They remain very susceptible to damage from late frosts, however.
By the middle of May the shoots have reached around a metre in length, with perhaps 12 internodes formed, 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. Low light levels, high winds and rain during this period are particular problems, all having major effects on vine yield. Root growth also peaks at about this time.
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 although there is also change of emphasis from the growing tips to the developing bunch, which becomes the primary nutrient sink. Foliar sprays of micronutrients may be applied if required.
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 start of August a reduction in available water causes 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, however, weather permitting. It is accelerated by warm temperatures, although it should be noted that during periods of high wind or very high temperatures ripening is suspended. This is because photosynthesis would entail prohibitive water loss through the stomata. By the middle of September the grapes are usually ripe enough to pick, with high sugar levels having been achieved by a combination of good ripening and 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 or yellow 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. Spur pruning with six two-bud spurs is the most common system, although often in older vineyards where the vines are not so well trained cane pruning (or Guyot) is used. This is also a good system to use to control the vegetative growth of vigorous varieties as a higher pruning load can be left.
The later pruning is carried out, the later budbust can be expected, so areas of high frost risk are pruned as late as possible – possibly even into late February. The winter is also a good time for any fertilisations or soil corrections which might be required as this gives the applied nutrients time to mobilise in the soil prior to the first flush of growth in the spring.