Root collar and planting depth: a mistake that stays hidden

The root collar is the zone where the trunk gives way to the structural roots. At planting, it should sit close to the ground surface.

When a tree is set too deep, the problem may not be obvious in the first year. The crown keeps its leaves, but the base gets less oxygen, the trunk tissues stay in contact with damp soil and the roots can develop in the wrong position.

The depth is confirmed before the crane is released or the planting hole is backfilled.

During production, field operations can build up soil around the trunk. The top of the rootball does not always correspond to the root collar.

Before setting the level, the team carefully draws the surface soil away from the base until it finds the start of the main roots.

This work should not break up the rootball. Only as much is removed as is needed to identify the transition.

In container-grown trees, the same problem occurs when the plant has been set too deep in the growing medium.

The graft union is the join between rootstock and cultivar. It may be near the base or several metres above it.

It should not be used as a reference for depth. A high graft makes the distinction obvious; with a low graft, you need to look at the roots.

Burying the union to hide it can leave the trunk covered and change the behaviour of the cultivar.

Soil over the base reduces gas exchange and keeps moisture against the bark. Some species form adventitious roots above the original root collar.

These roots can circle around the trunk, compete with the main structure and create problems later on.

The tree may also show reduced growth, chlorosis, progressive dieback or instability, without the cause being recognised straight away.

In clay or waterlogged soils, the risk increases.

The opposite excess is also a problem. Part of the rootball is left exposed to the air, where it heats up and dries out.

A slightly raised level can be appropriate on ground prone to settlement or slow drainage, but it needs to be blended into gentle mounding.

A wall of rootball is never left exposed above the surrounding soil.

A mature tree is very heavy. If the bottom of the planting hole has a thick layer of loose soil, the tree sinks after planting.

The base is prepared to support the rootball without losing level. The soil at the sides may settle with watering; the tree should not do so to any significant extent.

The level is checked after the first watering and on the following visits.

While the crane is taking part of the weight, material can be removed or added under the rootball and the level confirmed.

The team looks at the tree from several positions and takes into account the finished level of the paving and the mulch.

Only once the depth has been approved are the backfilling and staking completed.

A recently planted tree can be lifted and repositioned. The operation disturbs the roots again, but it can prevent a permanent problem.

The more time passes, the more roots leave the rootball and the more complex the correction becomes. In some cases, the controlled removal of soil over the root collar is enough; in others, the whole position needs to be reviewed.

The diagnosis should avoid cutting roots or digging without knowing the structure.

Organic mulch should not form a mound against the trunk. A clear zone is kept around the base.

Gravel, soil and lawn can also hide the root collar over the years. Maintenance should keep the transition visible.

Topping up mulch does not mean raising the height continually.

A base with no visible flare, like a post going into the ground, can indicate deep planting. It is not proof on its own, because the shape varies between species, but it justifies a check.

Circling roots, damp bark or the absence of structural roots at the surface strengthen the suspicion.

The right depth is a simple decision on planting day and hard to put right afterwards. Locating the root collar should be part of every operation, whatever the size of the tree.

• Planting hole size
• Technical planting sequence
• Grafted trees

1. European Arboricultural Standards. European Tree Planting Standard.
2. Arboricultural literature on planting depth and the root collar.