Grafting of the vine

Viticulture Climate resilience Pruning and training of the vine

Changing grape variety without uprooting then replanting, coppicing aging Vines, grafting directly onto the rootstock in place... Overgrafting is an interesting option for renewing a vineyard. It requires a lot of technical skill and involves good preparation of plant material and careful maintenance of the overgrafted vines.

Characterization of the technique

Strictly speaking, overgrafting consists of changing the grape variety of a vineyard without uprooting or replanting, but by grafting a different variety onto existing vine stocks. It can be based on various techniques:

The "bud grafting", also called "eye grafting": a bud is taken from a rootstock cane, then this eye is inserted into the trunk of the vine to be overgrafted. Two variants of eye grafting: Chip-bud and T-bud.
The "cleft grafting", one of the simplest techniques to perform. Overgrafting allows to modify the grape variety of a vine within one year and thus lose only one harvest. In this respect, it has a serious advantage over restructuring by uprooting and replanting which causes the loss of at least three harvests (more if a long soil rest is desired). Other advantages: the trellising is preserved and since the root system is maintained, the first years of production do not necessarily produce "young" wine. This rapid change of grape variety is a way to be more responsive to market demand and to quickly test the potential of a variety on its terroirs. Today, some winegrowers also use overgrafting to coppice tired vines or those affected by wood diseases, or to graft directly onto a rootstock in place, or for massal selection.

Bud grafting

Preparatory work

Be aware, the method remains costly and requires significant preparation and maintenance work. For overgrafting to succeed, it is necessary to harvest good rootstock canes and keep them well until the grafting operation. One can select rootstock canes in winter, at pruning time, before the vine bleeds in any case. It is better to avoid canes with cottony buds as they dry out faster. And choose fairly straight canes, about 1m long and of uniform diameter. Then, these rootstock canes must be stored in bundles, protected from cold, wind and light: ideally in bags, in a cold room. This can be for several months. Regarding the vine, the recommended preparatory work includes:

normal pruning or pre-pruning (if overgrafting is canceled, the year's harvest is preserved),
bark stripping of the stocks to create a flat surface (only the old bark),
removal of any stakes,
mechanical weeding under the row,
raising the trellis wires,
shoot thinning.

It is recommended to place the base of the rootstock canes in a water container two days before the overgrafting work, to slightly swell the buds.

Before harvesting the scions, it is better to check that the rootstock canes are green and that the buds to be used as scions are fresh. Some can be cut to check that they are green and not cottony. The eye removal can be done with a special blade - grafting knife - or with a tool developed by Worldwide Vineyards. The scion is cut at a bevel.

It is then advised to place the harvested scions on a damp cloth during overgrafting, so they do not dry out.

Chip-bud overgrafting

A grafting knife is used to make the incision.

Chip-bud overgrafting can be performed over a fairly wide period, ideally 2 to 3 weeks after budburst, until the end of June. For a Chip-bud graft, an incision matching the shape of the scion is made in the debarked trunk of the vine to be overgrafted, using a special knife (grafting knife). This method requires a lot of practice, especially to position the grafting knife correctly. Care must be taken to make the notch on the flattest possible area of the trunk.

Then, the scion is inserted into the notch, pushed with the back of the grafting knife blade.

Finally, the graft must be protected with a flexible tape, such as vinyl, leaving the bud exposed. The binding must be applied immediately after grafting to prevent the tissues at the incision from oxidizing or drying out.

T-bud overgrafting

T-bud overgrafting or T-grafting can only be done when the bark of the trunk peels easily, that is around flowering, when the sap rises strongly. In this variant, the incision in the trunk takes the shape of a T: first a vertical cut a few centimeters high is made, then two perpendicular notches are made by lifting the bark.

The scion is inserted into the incision, covered with the two bark flaps, then pressed with the base of the grafting knife blade. Finally, the binding must be applied immediately, covering the incision well.

Whether in the case of Chip-bud or T-bud, after binding, a small saw cut must be made below the graft. This drains the sap rising in the trunk and thus prevents congestion of the scion.

Maintenance work

After placing the scions, major maintenance work begins. "The labor requirement is one full-time person for three months, for a 4500-stock operation," estimates Marc Birebent, manager of Worldwide Vineyards. The days following the overgrafting work are devoted to decapitating the overgrafted stocks: this eliminates competition from the old vegetation for the growth of the scion. However, it is important to keep a bearing shoot that serves as a sap drawer and maintains the vitality of the vine. This sap drawer is pruned to one leaf twelve days after decapitation and is kept until the scion has produced a shoot over one meter long. At flowering, the overgrafted vines must be well watered. Shoot thinning must be done carefully so that the graft takes well. This task is very demanding because the necessary passes are often numerous. It is imperative not to chemically weed the plot from one month before overgrafting until the end of summer. Finally, phytosanitary-wise, only downy mildew can pose a problem. It must therefore be monitored, as in any other vineyard.

Cleft grafting

Cleft grafting can be performed on stocks whose rootstock is alive because only the aerial part is diseased or dead. This technique has the advantage of being performed over a more flexible period and potentially less busy, at the end of winter or early spring. However, it is advisable to carry out this technique when the risk of sudden cold snaps is lower because the stopping of sap rise after grafting is the main cause of failures observed for this technique.

In the case of declining vines, this technique can be used but action must be taken from the first year of symptom declaration of reddening foliage. It is then necessary to collect wood for regrafting from a different plot, planted with non-declining clones.

For plots affected by wood diseases, it is necessary to ensure that the rootstock does not show necrosis.

Cleft regrafting requires some skill to acquire, but if the technique is well explained, a novice can apply it and achieve results close to an experienced practitioner from the first grafting campaign. The main advantage of in-place regrafting is to benefit from an adult and functional root system, allowing fruiting from the second year. Success rates are decent, around 60-80% but some sometimes approach 90%.

Implementation of cleft grafting

Cleft regrafting requires few specific tools:

Wheelbarrow
Saw
Small axe
Mallet
Flat screwdriver or small blade
Machine to prepare scions

In November, the autumn before regrafting: identify stocks to be regrafted and mark them.
In February: harvest scions from healthy stocks. The wood harvested must be of good diameter to have sufficient reserves. They will be stored in a cool place away from light, with the lower end immersed in a few centimeters of water, or wrapped in a cold room.
In March, 2 to 3 days before grafting: prepare a basin around the vine stock and cut (with a saw a few centimeters above the ground) just below the graft point, to "let the rootstock bleed" to avoid congesting the upcoming scion. Loosen the soil around the stock to facilitate earthing up of the graft which will take place during grafting. If necrosis is observed, cut lower until a healthy section is reached.
On the day of grafting: prepare the scions. Cut wood into 2-bud sections. The lower cut, a beveled V shape 2 to 3 cm high, is made using a graft cutting machine. This operation can also be done by hand but the gesture is quite technical. The prepared scions are kept during the grafting work in an airtight box, wrapped in a damp cloth. Preparing scions by day or half-day of grafting prevents them from drying out.
Split the rootstock vertically using a small axe and mallet. The split area is then wrapped and tightened with hemp or jute string (biodegradable). Slightly scratch the bark to clearly see the orientation and location of vessels.
Open the split using a flat screwdriver or small blade and insert a scion on one side of the graft. The cambiums – peripheral zones of living wood where the year's vessels are present, located just under the bark – of the rootstock and the scion must coincide on the outer part of the vine stock. The lower bud of the scion is oriented outward from the trunk, the hormones of the bud facilitating the union with the wood just below. Once inserted, the scion is compressed at the split by tapping it with a small hammer. Hence the interest of previously wrapping the split with string, making the operation easier. At this stage, a double graft can be made (if the trunk diameter allows) with a scion inserted at each end of the split, but since scions never have exactly the same thickness, tissue junction may be less good on one of the two grafts. The best side to graft is often distinguished by more abundant bleeding on that side. Care must be taken that the bark of the scions is in perfect continuity with that of the trunk.
Install the stake, tie it to the support wire. Make a mound around the graft. The soil must completely cover the scion to protect it from cold and maintain humidity favorable to tissue union. The stock can also be surrounded by a sleeve filled with sand to cover the graft. If the soil is worked, the loosened inter-row soil can be used.
Late May - early June, remove the mounds. Usually, they are already cracked, but the scion bud must be allowed to emerge from the soil without being too deformed.
As growth progresses, select the best shoot and tie it. Remove the other buds. As much as possible, keep the shoot located in the sap flow which is normally the lower one, located on the outside. The scion is fragile the first years: be careful with soil work. The installation can be secured by installing a double stake. Since the vine's root system is already in place, watering the grafted plants is generally not necessary.

Costs of overgrafting

The cost of overgrafting performed as a service varies depending on the number of stocks to be overgrafted and can range from €1.35 to €2.05 excluding tax per stock. Preparation and maintenance work are the responsibility of the winegrower. The main service providers are Worldwide Vineyards, Vitivista, and l’Epibiote.

Comparison of overgrafting techniques


Technique	Cleft grafting	Bud grafting
Timing	End of winter / early spring	3 to 4 days around flowering, when the bark peels easily.
Advantages	Can be performed over a longer period. Return to production within 2 years.	Success often > 95%. Return to production within 2 years.
Disadvantages	Variable success, often around 70%.	Performed over a short period. Technical skill to acquire. Meticulous maintenance during the season: maintenance of a sap drawer and progressive pruning back.

Overgraft or replant?

Since overgrafting causes the loss of only one harvest compared to at least three for replanting, it may seem more economical. The viticultural service of the Chamber of Agriculture estimated the cost of uprooting and replanting at about €22,000/ha and the cost of overgrafting at between €12,000 and €16,500 /ha depending on whether it is done by the grower himself or as a service (in whole or in part). Even taking into account uprooting subsidies, overgrafting is on average less expensive according to this estimate. However, these calculations do not include preparation and maintenance costs. Be careful, it is difficult to generalize this average to all cases: the total cost of overgrafting varies greatly depending on the condition of the vines to be overgrafted (age, diseases, management...), the quality of the scions, weather conditions, etc.

Administrative formalities

Administrative formalities are similar to those for uprooting and replanting. An overgrafting operation must be subject to:

a declaration of intent at least one month before the start of work
a declaration of completion of work
authorization from the INAO in appellation areas

Changing grape variety by overgrafting may fall within the framework of a restructuring aid application.

Sources

This article was written based on the PDF document "Vine overgrafting (by eye graft)" published by the Chamber of Agriculture of Var.
This article was written based on the PDF document "The technique of (re)grafting in place by cleft" published within the framework of the National Vineyard Decline Plan.
French Institute of Vine and Wine Occitanie, Vine overgrafting

Appendices

S'applique aux cultures suivantes

Vine