What makes a crack dangerous
Not every crack is equal. The ones that matter are deep, long, and running through a structural part of the tree.
Horizontal cracks that cross the grain rather than follow it are among the most serious signs. Wood carries load along its fibers. A fracture across those fibers cuts through the load path and often leads to sudden failure.
Vertical cracks that follow the grain are serious too, but they sometimes respond to cabling or bracing if the rest of the wood is sound. The catch: a vertical crack that runs through the center of the trunk usually means a major loss of strength, and a tree like that is rarely safe to keep near a house or a frequently used area.
The proportion of the trunk involved matters a lot. When more than about half the trunk cross-section is compromised, arborists treat the tree as structurally unsound. A crack affecting less than roughly 25% of the cross-section in sound wood is a different situation, and some of those trees can be retained with support hardware.
Included bark in a tight V-shaped fork is a related problem. The bark grows inward at the fork instead of forming a strong wood-to-wood bond, leaving a weak seam that splits under load. That seam is not always visible from the ground until the fork has already started to open.
Signs that need urgent attention
Call an arborist the same day if you see any of these:
- A crack that is visibly widening over hours or days, especially after a storm
- A new lean in the tree paired with a trunk crack, or soil cracking and heaving near the root flare
- Any horizontal crack in the main trunk
- A dark gap or visible opening at a major fork where bark is pulling apart
If the tree has developed a new lean and the ground is moving, stay out of the fall zone and treat it as an emergency. A tree actively uprooting or splitting does not wait for an appointment.
After storms or strong winds, walk the property and look at every trunk and large branch for dark lines, gaps, or bark separating. Cracks are a common precursor to storm failure and easy to miss until the light is right.
What is usually less urgent
Surface checks in the outer bark, often called weather checks, open and close with moisture changes. They do not penetrate deep into the wood and do not affect structure. Old wound seams where a branch was cut years ago sometimes look alarming but are stable if the wood around them is solid.
A crack is less concerning when it is shallow, affects a small part of the trunk, is not at a union or fork, shows no sign of spreading, and sits far from targets like the house, a patio, or a fence. That does not mean ignore it. It means the urgency is lower.
What to do next
Start with TreeNerd's free tree-check tool. Answer a few questions about the crack and get a quick risk read you can act on.
For anything that looks deep, is at a fork, involves a lean, or is near your house, book a certified arborist. ISA-certified arborists are trained to assess structural defects and can tell you whether cabling, removal, or monitoring is the right call. Photos help, but there is no substitute for someone standing in front of the tree.