Sunscald on trunks: how to recognise it and reduce the risk

Sunscald occurs when an area of the trunk undergoes heating and temperature swings beyond what the tissues can withstand.

Newly transplanted trees can be more exposed because they have changed orientation, lost protective branches or been placed next to paving and façades that reflect heat.

The damage does not always show on the day. The bark may discolour, sink and crack weeks or months later.

Radiation heats one side of the trunk. Rapid changes in temperature damage living cells beneath the bark.

In winter, the sun can warm tissues that then cool sharply at the end of the day. In summer, the surface can reach very high temperatures next to hard materials.

Lack of water increases vulnerability because the tree is less able to regulate temperature and repair tissue.

In the nursery, one face of the trunk may be shaded by the tree’s own crown or by neighbouring trees.

When it is moved, that side ends up facing west or towards a reflective façade.

Lifting also reduces the roots, making the tree more sensitive to heat and drought.

Marking the orientation helps to keep a similar position or to identify the face that needs protection.

Trees with thin, pale bark are generally more sensitive. Young specimens also have less protection.

Thickness is not the only factor. Previous exposure, water and the time of transplanting all have an influence.

Birches, maples, magnolias and various ornamental trees deserve attention, but the risk should be assessed on the specimen in question.

The area may begin by showing a change of colour and a loss of sheen.

Later it becomes sunken, the bark splits and the margins become visible.

The damage often appears on an exposed side, but the orientation depends on the site and the season.

Shoots may appear below the lesion, along with dieback of branches connected to the affected tissues.

Impacts, ties, cankers, growth cracks and frost damage can look similar.

The history matters: when it was planted, which side gets the sun, whether it has lost branches and whether there has been mechanical contact.

Signs of fungi or progression beyond the exposed area may call for a plant health diagnosis.

No conclusion should be drawn from a photograph of bark alone.

Depending on the tree, Diaplant uses reused coffee sacks, cotton mesh, coir or specific materials.

The protection reduces abrasion and exposure. It should be breathable, well secured and inspected.

Closed plastic wraps can build up heat and moisture and are not a suitable solution.

On at-risk sites, the material can stay in place for a controlled period, provided it does not constrict the trunk or hold water.

Temporary shade can also be created, or design elements used to reduce radiation.

The protection needs to allow the trunk to be inspected.

Paints and products should be assessed on their composition and on technical advice, not applied automatically.

The best prevention starts with position.

Keeping the nursery orientation where possible reduces sudden exposure. Next to west-facing façades, the actual temperature should be taken into account.

Dark paving and stone increase heat around the base. Companion planting and soil cover moderate the microclimate.

Removing many branches exposes trunk and main limbs that grew in shade.

Crown lifting is done gradually. After transplanting, heavy pruning that combines loss of leaf, water and protection is avoided.

When a branch has to be removed, the risk of exposure is part of the decision.

A well-hydrated tree responds better to heat, although watering will not prevent every injury.

The rootball is kept moist without waterlogging. Scorched leaves and damaged bark can share the same origin in water stress.

Water should reach the roots and not keep the trunk protection permanently wet.

Bark that is still attached is not removed, and the wound is not cleaned aggressively.

The tree is assessed, protected against further damage, and water and exposure are corrected.

The capacity to compartmentalise depends on the extent of the damage and on vigour. Some lesions close slowly; others affect stability or the vascular tissue.

Monitoring should watch for progression, secondary pests and the response of the crown.

Recording orientation, transporting at the right time of year, protecting the bark and avoiding severe pruning all reduce the risk.

Inspection after planting makes it possible to correct ties and exposures before the damage grows.

Sunscald is not simply a problem of strong sun. It is an abrupt change between the environment where the tree grew and the place where it has been planted.

• How a tree is prepared before dispatch
• Transplant pruning
• Post-transplant stress

1. Arboricultural literature on sunscald and trunk protection.
2. European Arboricultural Standards. European Tree Planting Standard.
3. Diaplant’s trunk protection procedures.