The sudden appearance of a foamy or bubbly liquid on the bark of a tree is known as tree foaming. This phenomenon can range from a thin, fleeting suds to a thick, persistent ooze that weeps down the trunk. While the sight of a tree seemingly “foaming at the mouth” often suggests a severe problem, the underlying causes are varied and often represent a natural, if unusual, occurrence. Understanding the distinction between a benign, external reaction and an internal biological issue is the first step toward determining the tree’s health.
Slime Flux and Bacterial Wetwood
The most concerning cause of tree foaming is slime flux, the external symptom of an internal bacterial disease called bacterial wetwood. This condition begins when anaerobic bacteria, such as Enterobacterium or Klebsiella, enter the tree’s heartwood or sapwood through a wound, like a broken branch or a pruning cut. Once inside, these microorganisms infect the water-soaked wood, a condition referred to as wetwood.
The bacteria begin to ferment the tree’s sap, a process that produces gases like carbon dioxide and methane, along with various alcohols. This fermentation causes a significant buildup of internal pressure within the affected wood tissue. The pressure forces the resulting liquid, the flux, out through any available opening, often a crack, V-shaped branch crotch, or previous wound site.
This oozing flux is typically dark, wet, and slimy, and it often has a distinctive, unpleasant odor, which may be described as sour, rancid, or fermented. The foam itself is created when the pressurized liquid is released and aerates as it runs down the bark. Tree species commonly affected by this internal issue include elm, maple, oak, poplar, and willow. While bacterial wetwood is rarely immediately fatal to a mature tree, the condition is chronic and can contribute to a general decline in vigor over many years.
The flux is mildly toxic to surrounding plant life and prevents the tree from properly forming callus tissue to seal its wounds, leaving it vulnerable to secondary infections. The weeping liquid leaves a dark, characteristic stain on the bark that remains even after the moisture dries. Because the sap contains sugars and alcohols, the ooze attracts insects, which may further compound the issue by creating entry points for other pathogens.
Environmental and External Causes of Foaming
A far more common and less serious cause of foaming is a natural chemical reaction that occurs on the tree’s exterior during rainfall. This phenomenon is often mistaken for slime flux because it results in a visible, bubbly liquid running down the trunk. The foam is a natural soap, created by the flow of water over the bark, known as stemflow.
During dry periods, various substances accumulate on the bark’s surface, including organic acids, salts, air pollutants, and natural chemicals released by the tree. Among these are saponins, which are detergent-like compounds produced by some trees and plants. When rain flows down the trunk, this water mixes with the accumulated particulates and the saponins.
The resulting solution acts as a crude soap. The turbulence created as the water runs over the rough, furrowed bark agitates the liquid, causing it to foam. This type of foam is usually white, lacks a foul odor, and is short-lived, disappearing soon after the rain stops. Trees with deeply ridged bark, such as pines, oaks, and tulip poplars, are most likely to exhibit this benign foaming.
External chemical contamination can also cause foaming, especially in urban environments. Runoff from nearby sources, such as roof cleaners, household detergents, or fertilizer products, can mix with the stemflow water. Although this chemical runoff creates foam, it is an external contaminant and not a symptom of an internal tree health problem.
Assessing Severity and Mitigation Strategies
The first step in assessing tree foaming is determining the cause by observing the liquid’s characteristics. If the foam is white, odorless, appears only during or immediately after heavy rain, and is spread across the trunk, it is likely harmless, natural saponin-based foaming. This benign condition requires no intervention other than ensuring no persistent chemical contaminants are running onto the tree.
If the liquid is dark, slimy, continually oozing from a specific wound or crack, and has a rancid or fermented smell, the tree is suffering from bacterial wetwood. Harsh chemical treatments, such as fungicides or antibiotics, are ineffective and not recommended for this internal condition. Mitigation focuses on improving the tree’s overall health and sanitation to help it manage the chronic infection.
One recommended practice is to gently clean the affected bark area with mild soap and water to remove the flux stain. This discourages insects from feeding on the sugary ooze and attracting further pests. Ensuring the tree receives adequate water and proper nutrition can help boost its immune response and vigor. Removing any dead or diseased branches using proper pruning techniques can limit the spread of bacteria and improve the tree’s structure.
A certified arborist should be consulted if the slime flux is extensive, if the tree shows other signs of decline like wilting or branch dieback, or if the leakage is persistent and large. Arborists can perform a structural risk assessment to determine if internal decay has compromised the tree’s stability. Modern arboriculture favors improving the tree’s environmental conditions and allowing its natural compartmentalization process to wall off the infection, rather than attempting to drain the fluid.