Crown grafting to propagate or rejuvenate a tree: how and why?

Crown grafting: what is it?

Simple and clear description

The crown grafting is a grafting technique that involves inserting several scions (segments of branch bearing buds) into the trunk or a main branch of a tree, under the bark. This method allows replacing part or all of the tree’s above-ground system while keeping the same rootstock. It is particularly used on medium to large trees, where the diameter of the trunk or branches is large enough to accommodate several scions.

The principle rests on perfect contact between the cambium (the living layer beneath the bark responsible for the tree’s growth) of the graft and that of the rootstock, which enables their fusion and sap flow.

Why is it called “crown” grafting?

Crown grafting is called so because of the circular arrangement of the scions around the trunk or a branch. Scions are inserted at different points around the circumference of the wood, thus forming a “crown”. This technique is particularly useful for maximising the chances of success, as each scion has its own connection point with the rootstock, increasing the likelihood that at least some scions survive and thrive.

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Its importance in arboriculture and gardening

Crown grafting is essential in arboriculture and gardening for several reasons:

1. Varietal renewal: it allows changing the variety of a tree while preserving its root system. For example, an apple tree can be grafted to produce a new variety of apples better suited to your needs.
2. Tree rejuvenation: old trees can be revitalised by grafting young and vigorous branches.
3. Saving damaged trees: if a tree’s above-ground system has been damaged (by a storm, excessive pruning or disease), crown grafting can breathe new life into the tree.
4. Production optimisation: it is often used in commercial orchards to improve the quality or quantity of fruit without waiting for the growth of a new tree.

Crown grafting of an apple tree

The basics of crown grafting

Prerequisites

Crown grafting is a versatile technique that can be used on many types of trees, including:

• Fruit trees : apple trees, pear trees, cherry trees, plum trees, walnut trees and citrus trees. It is particularly valued for these species because it allows improving or altering fruit production.
• Ornamental trees : maples, lilacs, or other trees can also benefit from crown grafting to introduce branches from a different variety or to revitalise an ageing tree.
• Forest trees : in some cases, this graft is used on forest species such as oak or birch, although this is less common.

Ideal period for performing this graft

• Spring : the optimal period is just after the sap rise, when the tree begins to wake from its winter dormancy. This generally corresponds to March–April in temperate climates.
• Sap rise : it is essential, as the sap rise makes the bark easier to lift, facilitating the insertion of the grafts. It also ensures a good flow of nutrients to the grafts, increasing their chances of taking.

To avoid: grafting too early, in winter, or too late, in midsummer, as these periods do not favour healing and the growth of the grafts.

Necessary conditions

To maximise your chances of success, several conditions must be met:

• Tree state :
  • The rootstock must be healthy, with no signs of disease or parasites.
  • It must be robust enough to withstand the grafting process, with a trunk or branch diameter of at least 5 cm (ideal for accommodating several grafts).
• Required tools :
  • Grafting knife : well sharpened for clean cuts.
  • Pruning saw : for preparing the rootstock.
  • Grafting tape or raffia : to hold the grafts in place.
  • Grafting wax : to seal wounds and protect against potential infections.
• Environment :
  • Work in clean conditions to avoid any contamination.
  • Ensure the tree is not subjected to water stress before or after grafting (water if necessary).
  • If possible, choose a wind-free day with moderate humidity to prevent the grafts from drying out.

The tree should be mature enough to have a trunk or branches with a minimum diameter of 5 cm

Anatomy and terminology

• The rootstock
  This is the lower part of the tree that includes the roots and a portion of the trunk. Its role is to provide:
  • The basic structure of the tree.
  • A well-developed root system capable of feeding the scions.
  • Tolerance to soil conditions, diseases or parasites (depending on the rootstock variety chosen).
• The scion
  This is a section of branch taken from a donor tree, containing viable buds. Its role is to:
  • Develop the tree’s above-ground portion.
  • Produce leaves, flowers and, in the case of fruit trees, fruits.
  • Provide the characteristics of the variety to be grown (taste of the fruit, cold tolerance, aesthetics of the flowers, etc.).

Below the rootstock, and above the scions, grafted

Importance of cambium and its alignment

The cambium is a thin layer of living cells located between the bark and the wood. It plays a major role in graft success because:

• This is where the graft union between the rootstock and the scion forms. The cambium is responsible for the tree’s radial growth and the transport of nutrients.
• Good alignment of the cambiums ensures sap flow between the rootstock and the scion, thereby ensuring the survival and growth of the scion.

Perfect alignment: the key to success!
When inserting the grafts, it is essential to:

• Place the cambium of the scion in direct contact with that of the rootstock.
• Ensure both cambiums remain aligned after securing, even if the rootstock’s bark is thick.

Detailed steps for crown grafting

1. Rootstock preparation

• Selection of a healthy rootstock : choose a tree in good health, free from signs of disease or parasitic infestation, and with a trunk or main branch diameter of at least 5 cm.
• Cutting the main branch : use a pruning saw to cut the branch cleanly, leaving a neat, horizontal section. Then smooth the cut with a sharp knife to avoid irregularities.

2. Selection and preparation of scions

• Criteria for an optimal scion : take one-year-old, healthy branches with well-formed buds. Scions should be collected during dormancy (winter) and stored in a cool and humid place until use.
• Preparation : cut the scions to a length of 10 to 15 cm (2–3 buds) and trim their base to a bevel to provide a smooth and elongated surface.

3. Insertion of scions into the rootstock

• Crown incision : with a grafting knife (or grafting tool), lift the bark from the rootstock by 2–3 cm, creating vertical slits spaced around the circumference.
• Placement of the scions : insert the scions into the slits, ensuring their cambium is well aligned with that of the rootstock. Orient them slightly outward for harmonious distribution.

4. Fixing and protection

• Bindings : firmly secure the grafts with grafting tape, raffia or an elastic band, without over-tightening to avoid damaging the tissues.
• Protection : apply a grafting wax or a special wax to all exposed areas (including the main cut and the base of the scions) to prevent desiccation and protect against infections.

Post-graft care and follow-up

1. Immediate care after grafting

• Monitor the seal : check that the grafts are well secured in place and that the sealant or wax protects the wounds effectively. Repair any cracks or detachment.
• Watering and nutrients : ensure a regular supply of water, especially during dry periods, to maintain good hydration. A moderate soil amendment may be helpful, but avoid excess nitrogen fertilisers, which could stimulate unwanted growth.

2. Evolution of the graft

• Early signs of take : wait 3 to 4 weeks to observe the first buds opening on the grafts. A light growth is a good indicator of success.
• Managing the shoots : remove shoots and buds arising from the rootstock, as they can compete with the grafts and hinder their development.

3. Common problems and solutions

• Failure to take
  • Possible causes : poor alignment of the cambium layers, poorly prepared scions, unfavourable weather conditions (late frost, excessive heat).
  • Solution : attempt grafting again at a more suitable time.
• Infections or parasites
  • Prevention : use a quality sealant and regularly monitor for the appearance of fungi or insects.
  • Treatment : apply specific treatments (fungicidal or insecticidal) if necessary, and remove the affected parts.