Grafted trees: what it means and where the graft union is

A grafted tree brings together two compatible plants. The rootstock forms the root and part of the trunk; the grafted cultivar supplies the characteristics to be preserved, such as leaf colour, flowering, a weeping habit or a globe form.

The join between the two parts is the graft union. It may be near the base or several metres up. Identifying it helps you understand how the tree was produced, the form it will have and which shoots belong to the rootstock.

Many ornamental cultivars cannot be reproduced true to type from seed. The resulting plants would be genetically different and could lose the form, colour or flowering that distinguishes the selection.

Grafting makes it possible to propagate that cultivar while keeping its essential characteristics. A bud or shoot is joined to a rootstock of the same species or of a compatible species. Once the tissues have healed, they carry water, nutrients and sugars across the union.

The technique is also used to place a crown at a set height. In several globe or weeping trees, the nursery forms the trunk first and grafts the cultivar at the top. The height of the crown is therefore tied to the graft union.

The rootstock influences the root, stability, vigour and, in some combinations, adaptation to particular conditions. The cultivar determines the visible part above the union.

This does not mean that each characteristic belongs to one of the parts in isolation. The final response results from the interaction between the two, the soil and the climate.

When shoots appear below the graft union, they belong to the rootstock. They may differ in leaf, colour and vigour. If left to grow, they compete with the cultivar and can alter the form of the tree.

The union may show a change in diameter, a healing line, a change in bark texture or a slight change of direction.

On young trees it is usually obvious. As the trunk thickens, the union can become less conspicuous. Some combinations keep a visible difference in girth for many years without this being, in itself, a defect.

On top-grafted crowns, identification is simple: the cultivar’s branches start from a very clearly defined zone of the stem. With low grafts, the union has to be distinguished from the root collar.

A good union should be fully healed, with no open cracks, extensive necrosis or signs of separation. The crown should show vigour in keeping with the trunk and should not depend on too narrow a connection.

Moderate differences in growth can be normal. Marked changes, progressive deformation or a break at the union warrant assessment.

Some incompatibilities only appear after several years. For that reason, the track record of the combination and the observation of mature specimens are important when selecting grafted cultivars.

The tree should be planted by the position of the root collar, where the trunk passes into the structural roots. The graft union may be close to that zone, but it is not the reference for how deep to set the plant.

Covering the base to hide a visible union can leave trunk tissue in contact with wet soil, reduce aeration and encourage unwanted roots.

Before backfilling, the root collar should be located, even when soil has built up on top of the rootball.

The height of the graft has a direct influence on form. A globe crown grafted at two metres starts at that level and grows mainly in width and volume. The trunk below it will not gain several more metres, as it would on a tree with a free-growing leader.

This characteristic helps to predict circulation, views and the relationship with walls. Two trees of the same cultivar, grafted at different heights, give different results.

Where relevant, the specification should state the clear stem height, the graft height and the expected crown diameter.

Rootstock shoots should be removed close to their origin, without leaving stubs. Inspection is especially important in the first few years and after stress or hard cuts, when the base may shoot more vigorously.

Pruning of the cultivar follows the architecture and the season of the species. The crown should not be cut back to the union, nor should the graft union be used as a reduction point.

Grafted trees are often cultivars. The name should include the genus, species and cultivar name, not just a common name or a colour.

There may also be trade names associated with a registered cultivar. In the project documentation, it is advisable to keep the botanical name and add the trade name when it is needed to identify the product.

Understanding grafting makes it possible to select form better, avoid planting mistakes and interpret differences that would be invisible on a technical sheet limited to height and girth size.

• Species, cultivar and horticultural form
• Root collar and planting depth
• Natural, globe, columnar and pleached crowns

1. Hartmann, H. T. et al. Plant Propagation: Principles and Practices.
2. Royal Horticultural Society — technical information on grafting and cultivars.
3. European Arboricultural Standards. European Tree Planting Standard.