The observation of a tree appearing to “bleed” is a phenomenon known to science as exudation, where a liquid is expelled from the trunk or branches. This liquid is not blood, but rather a complex fluid that has been forced out through a wound or natural opening in the bark. The process can signal a healthy, expected biological function, or it can be a visible sign of serious physical injury or internal distress. Understanding the difference requires looking closely at what the tree is producing and why it is being pushed out.
The Composition of Tree Discharge
The liquid exuded from a tree is classified into three main types, each with a distinct chemical structure and purpose. Sap is the most common discharge, a watery solution that transports sugars, nutrients, and hormones throughout the tree’s vascular system. This fluid travels through the xylem and phloem tissues.
Resin is a thick, sticky substance composed primarily of aromatic hydrocarbons, mainly produced by conifers like pines and firs. Its function is defensive, as it hardens rapidly upon exposure to air, creating a physical barrier to seal wounds and deter pests. The third type is gum, or gummosis, a gelatinous material made of complex polysaccharides often seen in stone fruit trees such as cherry, peach, or plum. Gummosis is usually a pathological response, indicating the tree is attempting to wall off a site of infection or stress within the bark.
Seasonal and Natural Sap Flow
A tree’s “bleeding” is sometimes a normal event driven by internal pressure changes, unrelated to injury or disease. This phenomenon is most commonly observed in early spring in deciduous trees like maple, birch, and walnut.
For species like birch, the flow is explained by root pressure, where high soil moisture causes mineral ions to accumulate in the roots. This accumulation draws water into the root xylem via osmosis, pressurizing the system. The influx of water causes the watery xylem sap to exude from any opening, such as a pruning cut or a stump.
Maple trees rely on a freeze-thaw cycle in their branches to generate the pressure that causes sap to flow. This natural, seasonal flow is harmless and stops once the tree begins to leaf out and transpiration increases.
Bleeding Triggered by Injury or Disease
When exudation occurs outside of the normal spring flow, it signals a specific reaction to external or internal threats. The most immediate cause is physical damage, such as a poorly timed pruning cut, lawnmower injury, or animal rubbing. The resulting flow of sap or resin is the tree’s natural compartmentalization process, attempting to seal the wound and prevent the entry of decay organisms.
Insect Infestation and Defense
A more concerning trigger is insect infestation, particularly by boring insects like bark beetles. Trees respond to a borer attack by rapidly producing a large amount of resin or sap to “pitch out” the insect, flushing it from the tunnel. This defensive resin flow is visible as small, sticky globs or pitch tubes on the bark surface and indicates an active struggle against a pest. The color and texture of the discharge can offer clues, with clear or amber pitch suggesting a protective reaction.
Pathological Issues and Gummosis
Pathological issues like cankers can also cause localized bleeding as the tree attempts to isolate an infection. Cankers are dead areas of bark and underlying tissue caused by fungi or bacteria. The tree creates a barrier zone, and the resulting pressure can force out a thick, gelatinous gum called gummosis from the edges of the infected area.
Bacterial Wetwood (Slime Flux)
Another common disease is Bacterial Wetwood, also known as Slime Flux, where bacteria ferment inside the wood, building up internal gas pressure. This pressure forces a gray-brown, often alcoholic-smelling, foamy liquid out through cracks or wounds. A dark, milky, or foul-smelling ooze often indicates this type of bacterial or fungal issue.
Determining If Your Tree Needs Help
Assessing the nature of the exudate is the first step in determining if your tree requires intervention. If the flow is clear, watery, and occurs on a deciduous tree in early spring, it is a normal, temporary pressure phenomenon. If the discharge is thick, dark, milky, or has a sour or alcoholic smell, it suggests a disease like gummosis or bacterial wetwood.
Examine the area for accompanying signs of distress, such as sawdust-like material (frass) near the base of the tree, which indicates borer activity. The location of the bleeding is also important; minor weeping from a small, healing branch wound is less alarming than a persistent stream from the main trunk or a large canker.
For minor wounds, allowing the tree to heal naturally is best, as wound dressings are generally not recommended and can impede the natural sealing process. If the flow is excessive, foul-smelling, or accompanied by widespread canopy dieback or clear evidence of pests, consulting a certified arborist is the most appropriate action.