The following question was submitted to John Roska, an attorney/writer whose weekly newspaper column, "The Law Q&A," ran in the Champaign News Gazette.
Who owns a tree that straddles a boundary line? Part of the trunk is on my property, and part is on my neighbor’s. Can I cut it down, or at least trim the branches?
You both own it. And since you share ownership, you can’t cut it down without your neighbor’s permission.
You can, however, trim branches that hang over your yard. And maybe branches on your neighbor’s side threatening your side.
An old Illinois case said it’s a truth universally acknowledged “that trees growing upon a boundary line are the joint property of the adjoining owners." Later cases specified that adjoining property owners own such trees as “tenants in common.” That’s a type of shared ownership, along with joint tenancy, and tenancy by the entirety.
One case hinted that caring for a neighbor’s tree “can give one party a protectable interest even if the subject tree is not actually on a boundary line.”
The most recent case to consider jointly owned trees, from 1988, said it all depends on where the trunk enters the ground. If any part of that trunk crosses the border, it’s a jointly owned tree that one neighbor can stop the other from removing.
In that case, it didn’t matter that most of an 80 foot elm was in one yard, or that 15 inches up the trunk was entirely in that other yard. The 5 inches of the trunk that crossed the border at ground level was enough to give that side veto power over the other owner who wanted the tree to go.
So, jointly owning a tree means removal must be a joint project. Slicing off the part that crosses the property line isn’t allowed, either, if that would harm the tree.
Otherwise, it’s OK to remove “encroachments” onto your property. You own the airspace over your ground—up to an uncertain limit—so trimming overhanging branches is certainly OK. That’s true even if the trunk is completely in your neighbor’s yard.
Not trimming those overhanging branches might be contributory negligence, if they cause you damage.
One Illinois case says that you’re “privileged to enter upon a neighbor's land to abate a condition thereon which constitutes a private nuisance.” But, even if that suggests you could ignore a neighbor’s protest, and enter their yard to trim branches hanging over your yard, acting without permission is likely to backfire.
Encroaching roots are more complicated. Until they actually cause damage, you can’t do much. But once they do, you can sue for compensation, and probably to make your neighbor remove them.
But, you’re not necessarily liable if digging in your yard incidentally damages the roots of a neighbor’s tree. In one case, digging a foundation for a home didn’t make someone liable when that killed their neighbor’s tree.
Finally, you can be liable for damages caused by your trees, even if they’re entirely on your property. That imposes a duty on you to take reasonable steps to prevent your tree from falling on their roof.