Short answer: nearly everywhere you may cut back branches and roots that cross onto your property, but only to the boundary line and at your own expense. Whether you can also make the tree’s owner pay depends on your state and on whether the tree is doing real harm, not just dropping leaves. Two questions hide inside "the neighbor’s tree is over my yard": what you may cut, and who pays. They have different answers.
What you may cut: the self-help rule (almost universal)
The oldest and most widely followed rule is "self-help": you can trim the intrusion yourself, to the line, and that is your remedy. Massachusetts stated it in Michalson v. Nutting, 275 Mass. 232 (1931):
"His right to cut off the intruding boughs and roots is well recognized.... His remedy is in his own hands."
The catch built into self-help: you cut only what crosses the line, you stay on your own property to do it, and you cannot cut so far that you kill or destabilize the tree, because that can make you liable to your neighbor. In the strict self-help states, trimming is the whole remedy: you cannot send the neighbor a bill or sue over leaves, shade, or fruit.
Who pays: three approaches to actual harm
Where states diverge is whether you can recover from the tree’s owner when the tree does more than drop leaves. Virginia’s Supreme Court laid out the modern line in Fancher v. Fagella, 274 Va. 549 (2007):
"encroaching trees and plants are not nuisances merely because they cast shade, drop leaves, flowers, or fruit, or just because they happen to encroach upon adjoining property either above or below the ground. However, encroaching trees and plants may be regarded as a nuisance when they cause actual harm or pose an imminent danger of actual harm to adjoining property."
Hawaii’s Whitesell v. Houlton, 2 Haw. App. 365 (1981), is the case that approach is named for. When a tree causes real (sensible) harm:
"the damaged or imminently endangered neighbor may require the owner of the tree to pay for the damages and to cut back the endangering branches or roots and, if such is not done within a reasonable time, the damaged or imminently endangered neighbor may cause the cut-back to be done at the tree owner’s expense."
And Fancher is careful to keep self-help alive alongside the new nuisance remedy:
"the adjoining landowner may, at his own expense, cut away the encroaching vegetation to the property line whether or not the encroaching vegetation constitutes a nuisance or is otherwise causing harm or possible harm to the adjoining property. Thus, the law of self-help remains intact . . . ."
So the map is: strict self-help states (trim it yourself, no lawsuit), and "actual harm" states (Hawaii, Virginia, and a growing number) where a tree that causes real damage becomes a nuisance you can sue over, while self-help still stays on the table.
The line that changes everything: whose trunk is it?
All of the above assumes the trunk is on your neighbor’s side. If the trunk straddles the boundary, ownership flips to shared. California Civil Code §834 is the model many states track: "Trees whose trunks stand partly on the land of two or more coterminous owners, belong to them in common." A jointly owned boundary tree generally cannot be removed or fatally cut by one owner alone.
Before you get the saw out
- Trim only to the line, from your own side, and don’t kill the tree. Over-cutting is the fastest way to turn your neighbor’s problem into your liability.
- Leaves and shade are almost never actionable. Real, documented damage (a cracked foundation, a lifted driveway, a broken sewer lateral) is what moves a tree from "annoyance" to "nuisance" in the states that allow recovery.
- Check your own state, and get the trunk question right first.
For root damage specifically, see who pays for sidewalk damage from tree roots, and look up your city’s verified rule at the city lookup.