Legal Question in Real Estate Law in California

You don't have to trim them, but your neighbor has a right to prune the branches back to the property line.

I agree with Mr. Roach. You have no duty to trim the trees, but your neighbor has the right to do so if he chooses and can do so without harming the tree.

Overhanging branches (and invasive roots) are a trespass. Because the harm done and thus the money damages recoverable is usually slight, courts make an exception to the usual rule that the victim of a trespass must not resort to self-help, and may trim the limbs (or roots) back to the property line without a court order. However, the right to self-help comes with two limitations: the person doing the trimming is not entitled to demand payment from the tree owner; and the trimming must be done "in a non-negligent manner," i.e., without serious injury to the tree or creation of a hazard.

The right to self-help does not preclude the neighbor from electing to go to court and sue for an injunction ordering you to trim the branches - so, in a sense, the answer to your question, as asked, is that you "must" trim them, because the neighbor has the legal power to compel you to do so, but that's seldom what happens because the self-help rule exists and is a more sensible and economical approach.