The neighbor has a legal right to trim branches from your plants that intrude into his property as long as the plants are not fatally damaged. He has no legal right to trim further than the property line unless he has that right via a document such as a covenant.
If your neighbor would not listen to reasoning, you will need a court order.
In WA, a person found to have committed timber trespass can be ordered to pay up to three times the value of the damages, attorney's fees for the winner, and other costs.
Before going to court, you will have to consider that, unless one of you is moving soon, you two will be neighbors for some years. You should not let the neighbor push you around. On the other hand, you likely do not want him so antagonized that he is looking for revenge.
You may want to consider mediation. Many cities and counties in WA offer mediation to neighbors to resolve disputes. The mediation may be free or very low cost. (The mediation groups get some funding from the government.)
You can check at http://www.resolutionwa.org/ for a mediation group near you.
If a lawsuit is what you want, you should review the specific facts with your attorney.
I agree with Mr. Nguyen and if he crossing the line it is trespass and cutting trees, or limbs it is timber trespass RCW 64.12.030 however damages will be a subject of contention.
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