2.7.5.b Over-grafting

When is over-grafting appropriate?

Over-grafting may, for example, be appropriate when the grape variety in place is not optimal for the climate. With this technique, only one harvest is lost, unlike grubbing up followed by replanting (and perhaps also time for the soil to rest). Costs are reduced because the trellising can be left in place, and no new materials need to be ordered. This practice is also appropriate when the roots are well developed and the vineyard is healthy, with no symptoms of wood diseases. Nevertheless, it is also possible to over-graft old diseased vines in order to preserve their root systems.

What over-grafting techniques are there and when should they be implemented?

There are several over-grafting techniques, the major ones being "eye" or "bud" grafting:

• T-budding is carried out by peeling back the vine's bark, which corresponds to a significant sap increase, characteristic of the flowering period. This is the most accessible and efficient method of over-grafting. However, it can be carried out during a short period of time (about 15 days, during the flowering period). The execution rate is high and the risk of accidents is reduced. The positioning is done under the bark with an optimal contact potential both on the dorsal zone of the graft and on the facial bevel. The incision on the trunk is easily accessible to beginners, however, a high level of technical skill remains necessary for the graft to succeed. The T-Bud graft cannot be performed on small vines (rootstock with a diameter of less than 2 cm), but there is no age limit for the stock or for graft diameter.

• Contrary to T-Budding, Chip-Budding can be performed on all diameters of vines, the only requirement being to have trunks with a greater diameter greater than the grafts (about 12 mm). It is therefore possible to carry out this technique in the vineyard on vigorous one-year-old or moderately vigorous two-year-old rootstock. Chip-bud grafting has a wider intervention period than T-bud grafting and can start in early spring and end in early summer. The preservation of a sap-drawing branch increases the plant's chances of survival in case of failure.

Another method of over-grafting exists: cleft grafting can be carried out on young vines. This method must be done in spring when the sap is rising, and when the risk of frosts has decreased. A check or a long-lasting slowing down of the rising sap following the intervention is a major cause of failure. The trunk of the vine to be grafted is cut a few centimetres above the ground. A slit is made in the middle of the trunk with a sharp tool, which is held open with a small wedge. Depending on the diameter of the split trunk, one or two bevelled grafts are inserted into the slot at both ends, so that the bark of the grafts is in perfect continuity with that of the trunk. The bevel on the grafts must be clean and regular, so that the length of the cut graft is 5 to 6 times its diameter. The generative layers must be in perfect contact. The graft is then tied with a degradable tie, and covered with fine soil. The grafting success is confirmed a few weeks later (depending on weather conditions), by the budburst of the graft, and by the appearance of the first leaves out of the hilling. depending on the appearance of the mounds, the nature of the soil used and the weather conditions, it can be necessary to water them to maintain sufficient moisture around the graft. It will also be necessary to remove the soil around the plants during the growing season to avoid the graft growing roots at the new grafting point.

Excerpt from Vigne Vin Occitanie