Drainage at planting: when the problem lies beneath the tree

A tree can suffer from too much water even when the surface of the garden looks dry. In compacted soils, water enters the planting hole and is held above an impermeable layer or next to the rootball.

Drainage needs to be understood before planting. Putting gravel in the bottom or installing a pipe without knowing where the water goes does not necessarily solve the problem.

Diagnosis starts from the soil profile, the levels and the behaviour of the water.

When a dense soil is dug out and backfilled with looser material, water enters the prepared zone easily.

If the surrounding ground has low permeability, it leaves slowly. The planting hole behaves like a buried container.

The tree is left with saturated roots, while the surface may dry quickly. Extra watering makes the lack of oxygen worse.

This effect also occurs when there is a compacted layer below the excavation.

Standing water after rain, an anaerobic smell, greyish colours and rusty mottling indicate periodic saturation.

The soil may show a shiny texture, an absence of fine roots and an abrupt transition between layers.

These signs should be looked for at several points. A plot may drain well overall and have an old trench or an area of fill exactly where the tree is to go.

A simple test is to fill a hole with water and see how long the water takes to disappear.

The result depends on the initial moisture, the size of the hole and the time of year. It is not a universal measurement, but it lets you compare positions and identify abnormally slow values.

The test should reach the relevant depth and not just the surface layer.

When there are doubts or important structures, fuller tests may be needed.

Gravel creates large pores, but it does not provide an outlet.

Water can stay in the layer or at the interface between materials. Passing from a fine soil to a coarse material does not automatically speed up the movement.

Without level, fall and a discharge point, the solution only changes the zone where the water collects.

Gravel can be part of a designed drainage system, but it should not be applied as an automatic response.

A drain needs to receive water and carry it to a lawful, working destination.

Pipework, filter material, geotextile, gradient and access for maintenance are defined together.

If the outlet is higher than the planting hole or becomes blocked, the system does not work.

Care is also taken not to create a drain that removes water needed by other areas or transfers the problem to neighbouring properties.

In some places, the best solution is to raise the planting area slightly and lead the water across the surface.

This strategy increases the aerated volume, but it needs to be blended gently into the ground. The tree is not placed on a narrow mound that dries out quickly.

Ground modelling can be combined with subsurface drainage when there is a safe outlet.

Widening the prepared volume and connecting it to the surrounding ground improves infiltration and root spread.

Decompaction must not destabilise foundations or cause settlement under paving. The depth and the method depend on the function of the soil.

Working saturated clay can make the structure worse, so the timing of the work matters.

Some trees tolerate moist soils or temporary waterlogging. Few accept permanent saturation without oxygen.

Choosing a more tolerant species can be part of the solution, but it should not hide a leak, a wrong level or a badly built planting hole.

When the problem is extreme and cannot be corrected, the position of the tree should be reconsidered.

The schedule is adjusted to the rate of drainage. A heavy soil may need longer intervals, but each watering should wet the whole rootball.

Moisture is checked through small observation points next to the roots. The aim is to see whether water is available and whether the soil has regained air.

Wilting does not automatically mean dryness. A suffocated root also stops supplying water to the crown.

Chambers, outlets and drains need inspection. Sediment, roots and leaves can reduce their performance.

After heavy rain, the area is observed and a note is made of how long it stays saturated.

Drainage is not an invisible element that can be considered solved for good. It is a part of the garden that needs access and maintenance.

• Waterlogging: why a plant can suffer with water available
• How to assess the soil
• Planting hole size

1. European Arboricultural Standards. European Tree Planting Standard.
2. European Soil Data Centre — principles of soil drainage, structure and saturation.
3. Technical literature on soil physics and landscape drainage.