Waterlogging: why a plant can suffer even with water available

A plant can wilt in waterlogged soil. The water is there, but it fills the pores where air should circulate. Without enough oxygen, the roots slow down, lose fine tissue and stop supplying the crown.

The symptoms can look like a lack of watering: drooping leaves, yellowing, leaf drop and branch dieback. Adding more water without checking the soil makes the problem worse.

Diagnosis needs to look at moisture, drainage and distribution.

Roots breathe. They use oxygen to release the energy needed for growth and uptake.

When the soil stays saturated, the diffusion of oxygen becomes very slow. Activity falls and the young roots die first.

The tree loses part of its absorbing surface. The crown receives less water and fewer nutrients, even though the ground is wet.

Long periods can favour microorganisms associated with rots, but the presence of an agent does not remove the environmental cause.

In both cases, the leaf does not receive enough water.

In drought, there is no water available in the soil or the rootball has lost contact. In waterlogging, the root cannot take water up because it is suffocated or damaged.

The appearance of the crown alone does not tell the two apart.

You need to open inspection points next to the roots and check whether the profile is dry, moist or saturated.

General yellowing, wilting with wet soil, premature leaf drop and short growth are common.

The base may show dark bark, an anaerobic smell and brown or soft roots.

The symptoms may start in one part of the crown if the saturation is localised.

An evergreen tree may lose old leaves and keep producing weak new growth before it shows obvious decline.

Compaction, an isolated planting hole, an impermeable layer, an irrigation leak, discharge from roofs and a low-lying position are recurring causes.

A controller may keep the zone running during rain. In gardens with several layers, the trees may receive water from their own drippers and from the lawn sprinklers as well.

The water table or a seasonal spring may also rise only in winter.

The cause should be identified before drains are installed.

One side of the rootball can be saturated while another stays dry.

Differences in soil, slope and emitters create uneven distribution. The tree shows symptoms and the team gets contradictory readings.

The inspection should be made at several points and depths.

The aim is to understand the profile, not to obtain a single answer.

Where there is saturation, watering is suspended or reduced in a controlled way.

Check whether other systems are still supplying water and whether there are leaks.

One part of the rootball should not be left to dry out completely in order to correct another. Monitoring determines when to resume.

The solution may include surface ground modelling, working drainage, decompaction or raising the level.

A pipe needs a fall, filter material and somewhere to discharge. Gravel with no outlet solves nothing.

In some cases, the problem comes from an extensive layer and calls for work beyond the tree.

Species from riverbanks or wet soils tolerate greater availability, but not necessarily standing water without oxygen.

The duration and season of the waterlogging matter. Some trees withstand winter flooding and suffer from warm saturation in summer.

Choosing a tolerant plant can reduce the risk, but it does not correct a leak or a planting hole built like a container.

Once oxygen has been restored, the root needs to produce new tissue.

The crown may go on losing leaves for some time. Fertiliser is not applied to force growth while the roots are weakened.

Branches that are definitely dead are dealt with at the right time. General pruning is avoided.

Recovery depends on the duration, the species and the amount of root lost.

Assessing the soil, testing infiltration, preparing drainage and separating irrigation zones reduces the risk.

The root collar sits at the surface, and the ground should not form a permanent basin next to the trunk.

Maintenance watches for puddles, water consumption and changes after the works.

Waterlogging is not simply ‘too much water’. It is a condition in which the root loses the air it needs to use the water that is there.

• Drainage at planting
• How to water a mature tree
• Post-transplant stress

1. Literature on root physiology, hypoxia and anoxia.
2. European Soil Data Centre.
3. European Arboricultural Standards. European Tree Planting Standard.