You and your neighbour can create a 'boundary agreement' to record: the boundary between 2 properties. who's responsible for maintaining a hedge, wall, tree or fence between 2 properties.
The Boundary Line Agreement (BL AGR) provides a procedure for County review whenever a point or line determining the boundary between two or more parcels of real property cannot be identified from the existing public record, monuments, and landmarks or is in dispute. RCW 58.04.
In Washington State, boundary trees are considered common property of the adjoining landowners. Any decisions regarding the removal or substantial trimming of these trees must be agreed upon by both parties. Unauthorized tree work can lead to legal action and potential compensation claims.