Why is My Tree Weeping Sap? Causes, Diagnoses and What to Do in Dorset

Discovering liquid oozing, dripping or staining the bark of a tree in your Dorset garden is understandably alarming. But the causes range from entirely normal physiology to several distinct diseases — each with different implications and different management responses. This guide walks through the most common causes of tree sap weeping in Dorset, helps you understand what you are seeing, and explains what action — if any — is needed.

Normal Sap Flow — When Weeping is Not a Problem

Before examining the pathological causes, it is worth understanding that some sap flow from trees is entirely normal and harmless:

• Spring sap rise: In late winter and early spring, sap pressure in many deciduous trees rises rapidly as the tree prepares for bud burst. Small amounts of clear or pale amber sap from minor cracks, bark injuries or natural openings are common and harmless. Birch (Betula) and maple species are particularly prone to vigorous spring sap flow.
• Minor wound response: Small amounts of sap from a fresh pruning cut, a split from frost, or mechanical damage are a normal response and are part of the tree's wound-sealing process. Resins and latexes produced at wound sites in many conifers and some broadleaved species are protective responses.
• Insect frass mixed with sap: Some boring insects produce a mixture of sap and sawdust (frass) at entry/exit points in the bark. Small amounts of this at isolated points on the bark are common in Dorset, particularly on stressed oak and fruit trees, and do not necessarily indicate serious disease.

When Sap Weeping Indicates Disease or Damage — What to Look For

The following characteristics should prompt professional investigation:

• Dark colour: Sap that is dark brown, black, rust-coloured or grey-green rather than clear or amber
• Strong or unpleasant smell: Foamy, fermented, sour or putrid odour from the exudate
• Dead bark: Bark tissue that is dying, cracking or peeling away around the weeping point
• Quantity: Very copious flow — particularly if it is staining a significant area of the trunk and running down to the soil
• Location at the base: Exudate at or near the root collar (ground level) is particularly concerning
• Multiple weeping points or a continuous lesion: Rather than a single isolated drip point

The Main Pathological Causes of Tree Sap Weeping in Dorset

1. Bacterial Wetwood (Slime Flux)

What it is: Bacterial Wetwood — also called Slime Flux — is caused by a complex of anaerobic (oxygen-tolerant) bacteria that colonise the heartwood of susceptible trees. These bacteria ferment the sap, producing gas that builds up pressure and forces the resulting liquid outward through bark cracks, wounds and natural openings.

Identification: The characteristic signs are a foul-smelling, foamy or frothy liquid ranging in colour from light grey-brown to near-black. It often appears at the same location on the trunk year after year. The liquid is toxic to the bark where it runs, sometimes causing a vertical streak of dead or discoloured bark below the weeping point. The smell is the most distinctive feature — it resembles fermenting vegetation.

Species most affected in Dorset: Elms, oaks, willows, poplars, horse chestnuts and some ornamental maples. Very common in mature street trees across Bournemouth and Poole.

Management: Bacterial Wetwood cannot be eliminated from an infected tree. It is generally not directly fatal, and many trees live for decades with it. The main concern is the bark damage caused by the exudate where it contacts the tree's surface. Where the weeping point is accessible, drilling a drainage channel slightly downward through the bark at the base of the wetwood column (to redirect the liquid away from the bark surface) is sometimes recommended — but should only be done by a professional arborist.

2. Bleeding Canker (Horse Chestnut)

What it is: As described in our bleeding canker guide, this is caused by the bacterium Pseudomonas syringae pv. aesculi and is a serious disease affecting horse chestnut trees across Dorset.

Identification: Dark rust-to-black sticky liquid weeping from patches on the lower trunk. Unlike Bacterial Wetwood's foamy consistency, bleeding canker produces a thicker, stickier exudate. The bark beneath and around the weeping point is dying — this is the key distinguishing feature from simple sap flow. The dead bark area is typically reddish-brown to khaki when the outer bark is peeled back.

3. Phytophthora Collar Rot

What it is: Phytophthora species causing collar rot can produce bleeding-canker-like symptoms at the base of the trunk — a dark, wet-looking exudate from lesions at or near ground level.

Identification: The weeping is typically at the root collar (ground level or just above), often accompanied by bark death and orange-brown staining beneath the bark. Species at risk in Dorset include beech, horse chestnut, oak and many ornamentals.

4. Physical Damage and Abiotic Causes

Frost cracks, lightning strike, mechanical damage from lawn equipment and vandalism can all produce sap flow from fresh wounds. These are typically clear or amber, without the foul smell of bacterial wetwood, and are associated with a visible physical wound or crack in the bark rather than a discoloured lesion. Management involves keeping the wound clean and allowing natural wound closure — do not seal with any product.

5. Wood-Boring Insect Activity

Several wood-boring beetles produce small amounts of frass-mixed sap at entry holes. Goat moth larvae and hornet clearwing (Sesia apiformis) can produce visible sap weeping at the base of poplars and willows. In most established trees this represents a cosmetic and minor health issue rather than a structural threat — but in stressed or previously damaged trees, severe infestation warrants assessment.