Planting hole size: why there is no universal measurement

The size of the planting hole depends on the rootball, the soil, the drainage and the species. A fixed measurement does not respond in the same way to cultivated farmland, compacted clay or construction fill.

At Diaplant our minimum reference is soil prepared to a width and depth of at least twice the dimensions of the rootball. The figure is adjusted to the site, the drainage, the size of the tree and the needs of the species. The larger the volume of soil that is loosened and decompacted, the better establishment and growth tend to be, provided the base remains stable and the water has a way out.

For a large tree of medium maturity, around 8 m³ — a volume of 2 × 2 × 2 m — can be an ideal reference where the site allows. It should be adapted to the ground and to the tree.

The opening in which the rootball sits has to guarantee the correct level. The root collar stays level with the finished ground, or slightly above it when the soil and the species justify that.

Digging to twice the depth of the rootball does not mean burying the tree deeper. That excavation is there to loosen and decompact the soil beneath and around the roots. The base is then rebuilt and firmed at the level needed to support the rootball without settlement.

Preparation and planting depth are separate decisions: one improves the volume available to the roots; the other keeps the root collar in the right position.

The rootball is the starting point. The area needs to allow the tree to be brought in, turned and backfilled without the sides ending up hard against it.

The diameter depends on the species and the size. The hole should be confirmed against the actual measurement, not just a figure estimated months earlier.

Anchoring systems also have to be allowed for; they may need space at the sides or a specific base.

In a deep soil that has already been cultivated and has good structure, roots meet less resistance outside the hole.

Preparation can concentrate on correcting local compaction, adjusting organic matter and making sure the rootball is properly placed.

This does not do away with the drainage assessment. An agricultural plot may have cultivation pans, saturated areas or variation from one parcel to the next.

On construction platforms, compaction can extend well below the surface. Spreading topsoil over the top does not deal with the dense layer.

A narrow hole creates an interface where water gets in and cannot get out, or where the roots stay confined.

In these cases, the larger the decompacted volume, the better the conditions, provided there is stability and drainage.

The work should extend sideways and connect the tree’s zone to the surrounding ground.

Excavations in clay can leave smooth, smeared sides. Roots find it hard to get through that surface.

The faces are broken up and left rough. It is not simply a matter of increasing the size; the connection has to be improved.

The base must bear the weight of the rootball. A deep layer of loose soil directly underneath can settle and bury the root collar after planting.

Before the tree is placed, we watch how long water stays in the opening.

If there is an impermeable layer, digging deeper can bring the roots even closer to the standing water.

A layer of gravel at the bottom with no outlet does not guarantee drainage. The solution may call for ground modelling, a working drain or a change of level.

Organic matter and other materials are incorporated in a balanced way. A very rich, very different mix inside the hole can create a compartment that the roots never leave.

The aim is a gradual transition to the existing soil.

When the ground is unsuitable over a very large volume, there should be a discussion about whether the species or the position is still right.

A wide area is generally more useful than an excessively deep excavation, because many roots develop in the upper layers, where there is oxygen.

The geometry adapts to walls, paving and services. In urban settings, it may be necessary to create continuous strips or volumes beneath paving.

The soil is placed around the rootball without heavy compaction. Deep watering helps it to settle and reduces air pockets.

The finished ground is shaped to carry water away without letting it collect by the trunk.

Preparation should be as generous as the site and the technique allow, without losing drainage or stability. The figure that matters is the volume of continuous soil the roots can actually explore.

• The root collar and planting depth
• Drainage at planting
• How to assess the soil

1. European Arboricultural Standards. European Tree Planting Standard.
2. Principles of soil science and tree establishment.
3. Diaplant’s technical references and planting practice.