A vertical split in a tree trunk is a longitudinal fissure running parallel to the wood grain, often extending through the bark and into the wood itself. This damage signals significant internal stress. A trunk split is a serious concern because it provides an entry point for wood-decaying fungi, insects, and disease pathogens. The fissure also compromises the tree’s structural integrity, potentially leading to failure during weather events.
Stress from Temperature Extremes
Rapid and extreme temperature shifts are a frequent cause of vertical trunk splits, often seen during the winter months. This phenomenon, commonly referred to as a frost crack, occurs due to the differential expansion and contraction rates between the inner and outer wood. When temperatures drop suddenly, the outer layers cool and contract much faster than the inner core, generating intense tensile stress that forces the wood to crack.
The sun’s intensity on clear winter days can also trigger sunscald, especially on the south or southwest side of the trunk. Direct sunlight warms the bark and underlying cambium layer, temporarily bringing those cells out of dormancy. When the temperature plummets after sunset, the rapidly cooling tissue ruptures and cracks vertically. This thermal stress is particularly damaging to young trees and those with thin bark.
Damage from Sudden Physical Force
Vertical splits can result from a single, acute application of mechanical or electrical force. A lightning strike, for instance, delivers an electrical current that heats the water and sap within the trunk to extreme temperatures. This heat instantly vaporizes the moisture into steam, causing an explosive expansion that blows the bark off and creates a deep vertical split down the trunk.
High winds during severe storms can generate leverage forces that exceed the wood’s tensile strength, resulting in a split. This is likely if the tree’s roots are compromised or if the soil is saturated, reducing anchoring ability. Less frequently, a direct physical impact from a vehicle or heavy falling debris can cause a sudden vertical split. These high-energy events create immediate structural defects that differ from gradual thermal damage.
Predisposing Internal Structural Issues
In many cases, a vertical split is the result of a pre-existing internal weakness rather than solely external forces. Included bark is a common internal defect that makes a tree susceptible to splitting. This occurs where two or more stems grow closely together, trapping bark within the narrow junction instead of fusing wood tissues.
The trapped bark prevents a strong wood connection, creating a weak point characterized by a narrow, V-shaped crotch. Under the load of wind, ice, or snow, the stems can be levered apart, causing the union to split cleanly. Internal decay or rot, often initiated by previous small wounds, also significantly reduces the wood’s density and strength. A tree with compromised structural wood is far more likely to split under external force than a healthy one.
Evaluating the Split and Tree Safety
When a split is observed, the first step is to determine its depth. A superficial crack limited to the bark is less concerning than a deep fissure that penetrates the wood and exposes the interior tissue. Superficial cracks may close and heal over the growing season, but deep splits permanently compromise the tree’s stability and represent a serious structural hazard.
Signs of immediate danger include a split extending through more than one-third of the trunk’s diameter, a noticeable lean, or a split that visibly opens and closes under load. For any deep or extensive split, a certified arborist should be consulted to assess the risk, especially if the tree is near a structure. Stabilization techniques like cabling or bracing can sometimes be implemented to mechanically support the damaged section and help the tree compartmentalize the wound.