A weeping or “bleeding” tree can be alarming, especially when sticky fluid runs down the bark. This leakage is a common symptom indicating that the tree’s internal pressures are pushing liquid out through a wound. Causes range widely, from a normal seasonal process to a sign of serious disease or pest infestation. Correctly identifying the substance and the source of the leak is the first step toward understanding the tree’s overall health.
Identifying the Substance: Sap, Slime, or Something Else?
Not every sticky substance dripping from a tree is true sap; the nature of the fluid is a powerful diagnostic clue. True sap is the tree’s internal fluid, typically clear and sticky, flowing from a fresh wound or pruning cut. This flow is generally contained to the immediate area and is most noticeable during the spring growing season.
The sticky residue coating leaves, sidewalks, or cars beneath a tree is often honeydew, which is not a tree product. Honeydew is the sugary excrement of sap-sucking insects like aphids or scale insects feeding on the leaves above. This thin, water-soluble liquid can lead to the growth of sooty mold, which appears as a black coating on surfaces.
Another distinct exudate is gummosis, an amber-colored, resinous gum that hardens into a gelatinous blob. This is a common defense mechanism, particularly in stone fruit trees like peaches and cherries, and is a symptom of stress, injury, or canker infection. If the liquid is dark, frothy, and emits a foul, fermented, or vinegary odor, it is likely slime flux, indicating a bacterial infection within the wood.
Benign and Mechanical Causes of Sap Flow
Many instances of sap leakage are temporary and do not pose a threat to the tree’s long-term health. Seasonal flow is a natural occurrence for certain “bleeder” species, such as maples, birches, and walnuts. This phenomenon is driven by cycles of freezing nights and warm days in late winter and early spring, building high internal pressure that forces sap out of any opening.
Pruning wounds are another frequent, temporary source of sap flow. Sap weeping from a fresh cut is simply an exposed vascular system reacting to injury, and the tree will naturally compartmentalize and seal the wound. Similarly, mechanical injury from construction equipment, lawnmower strikes, or severe wind damage can cause a temporary rush of sap to the damaged area.
Sapsuckers, a type of woodpecker, create a unique pattern of sap leakage. Their feeding leaves small, shallow holes, approximately one-quarter inch in diameter, arranged in precise horizontal and vertical rows. While the resulting sap flow attracts insects the bird then eats, the damage is usually minor unless the holes completely encircle and girdle the trunk of a young tree.
Serious Causes: Pest Activity and Disease
While minor sap flow is often harmless, certain characteristics signal a serious underlying problem requiring intervention. Wood-boring insects, such as various beetles, cause damage that results in sap mixed with a powdery material called frass. Frass, a combination of sawdust and insect excrement, is a distinct visual cue often found near exit holes or collected in bark crevices. The holes are usually randomly distributed, unlike sapsucker damage, and their shape and size can help identify the specific borer species.
Fungal or bacterial infections can also manifest as sap leakage, signaling a systemic issue. Cankers are localized areas of dead, sunken bark from which sap or gum may ooze. This weeping is the tree’s attempt to isolate the infected tissue by pushing out the pathogen and sealing the wound. Slime flux, or wetwood, is a bacterial infection of the inner wood where sap fermentation creates gases like methane, building significant internal pressure. This pressure forces the foul-smelling, sometimes frothy liquid out of wounds and cracks, leaving a dark stain down the bark that slows the tree’s ability to heal.
Actionable Steps and When to Call an Arborist
For benign causes like seasonal flow or fresh pruning cuts, the best course of action is to do nothing, allowing the tree to use its natural defense system. Avoid applying pruning paints or wound sealants, as these trap moisture and can encourage decay or infection. Preventative steps, such as establishing a wide mulch ring around the trunk, can prevent mechanical injuries from lawn equipment that often serve as entry points for pathogens.
For species known to “bleed,” like maple and birch, proper pruning should be performed when the tree is fully dormant, typically in mid-summer or late fall, to avoid the peak sap flow of late winter and early spring. If the leakage is identified as slime flux, there is no curative treatment; management focuses on improving the tree’s overall health with proper watering and soil care. Attempting to install drain tubes to relieve pressure is discouraged, as it can spread the bacteria within the trunk.
A certified arborist should be consulted when the sap leakage is accompanied by signs of tree decline. These warning signs include the presence of frass indicating borer activity, significant branch dieback in the canopy, or cracks and splits in the trunk. If the tree is losing a large volume of sap over an extended period, or if the foul-smelling slime flux is extensive, a professional assessment is necessary to determine the structural integrity and long-term prognosis.