Pruning depresses vigour

A vine’s vigour is measured by the weight of wood it produces in a year. Heavily pruned vines will grow less shoots in the following summer than lightly pruned vines and so less leaves. This reduces the vine's total photosynthetic capacity and so reduces its vigour. This effect is particularly important in young vines, which should be pruned lightly to allow them to establish themselves. Flower removal is also a good idea. In older vines, lightly pruned vines are devigourated by increasing fruit production.

The notion of ‘Charge’

One of the aims of winter pruning is to produce an ideal balance between fruit and leaf area. This balance depends upon the yield and quality desired, which is determined by the returns on the sale of the wine. As a rough guide, the ideal balance between fruit and leaf occurs on a shoot with moderate yield that is about pencil thick, 12 – 15 nodes long, with an internodal length of 60 mm. These weigh about 30 – 40 g in winter.

In order to calculate how many buds to leave on a vine at winter pruning (the charge), the vinegrower can:

• Count how many ideal shoots were produced in the previous growing season. When counting, small shoots may count as ½ and larger shoots as 2 or even 3
• Remove most of the canes from the vine, weigh them, and divide the weight by 30 – 40.

The charge is increased significantly in young (< 8 year old) vines and by 5 – 15% in mature vines to compensate for buds that won’t break due to winter injury.

The choice of wood retained at pruning

The wood retained at pruning should be in a good state of health. Look out for spotting due to Botrytis (grey rot), powdery mildew and Phomopsis and poorly ripened wood.
Canes with deformities such as double buds and ‘twinning’ may be infected with viruses, and so should be eliminated if possible.

Choice of buds

The buds on canes formed in the previous year are the most fruitful. If a vine is pruned severely, old buds on the trunk will break, but the embryonic flowers within these will have degenerated, and so they will produce little fruit.

Large pruning wounds damage the vine

Wounds over 30 mm in diameter will never heal properly, but will die back and may affect the sap flow in the trunk. They will also deepen due to frost cracking and may allow the entry of parasitic fungi such as Eutypa. If large pruning cuts have to be made, it is a good idea to leave a short stump that can be cut back the following winter. On the other hand, canes must be cut back to the old wood, or the surviving basal buds will turn into watershoots.

The end-point principle

Vines will grow more vigorously at their extremities, so the buds at the ends of canes will tend to break first and produce the most vigorous shoots.The longer the cane, the greater the difference in vigour between shoots at the end and those in the middle, leading to uneven canopies.