If the tree owner knows a tree or branch is diseased or dead or rotten, and if the branch or tree falls on the neighbor’s property, the tree owner is usually held responsible for the damage caused by the branch. The classic example is when the branch falls on the neighbor’s garage roof. The tree owner (or his insurance company) is generally responsible for the cleanup and repair costs.
The reason behind this general principle is the idea that people are responsible for the consequences of their own carelessness. A tree owner who sees that he has a dead branch on one of his trees and that the branch is hanging over his neighbor’s land knows that the branch might fall and damage his neighbor’s property – or, worse yet, hurt the neighbor who might be standing underneath the branch at just the wrong moment. The law generally regards a tree owner who doesn’t take steps to fix the problem as being careless.
Tree owners might argue they didn’t know the branch was rotten. In this situation, courts generally inquire what the signs were that the tree was healthy or diseased. Tree owners are supposed to recognize obvious signs of disease, such as a branch without leaves in the middle of summer when the other tree branches do have leaves. Other signs of tree disease may be more difficult to recognize and may be more difficult to decide.
Example: It’s the middle of summer and Fred’s tree is all green and leafy except for one branch that doesn’t have any leaves on it, and that branch is in a place where it could come down on a sandbox where the neighbor’s kids play in the neighbor’s back yard. In most jurisdictions, Fred has a duty to get that branch pruned away. He knows, or he ought to know, that a branch without leaves is probably a dead branch and that dead branches can break, and that if this one breaks it’s going to come down on the sandbox and possibly hurt someone.