Do I have to trim branches from my tree that extend past the fence?

Question

My neighbor has hired a tree trimming service to trim their tree that extends over our shared fence. I have never asked them to do so, nor expressed any resentment that it grows over the fence. It doesn't appear to be in danger of falling and causing damage. She has mentioned a few times that I should trim my tree that extends over the fence into her tree, saying that she doesn't want her tree to be diseased from mine. My tree is healthy and not diseased, so I am a bit confused.

From what I have found, I am only required to trim it if it is hazardous or potentially hazardous. Or if the roots are encroaching onto their property and causing damage (fence, septic, foundation). I can't find anything saying that one tree growing into another would spread disease from nothing, though I could just not be looking up the correct thing. I have never had any intention of trimming it, as it doesn't seem necessary.

She flagged me down as I was mowing my front yard to tell me she was paying for trimming my tree, which anything past the property line is perfectly within her rights.

Is there reasonable cause for concern that one tree's branches growing into another trees branches would spread disease if they are both healthy?

Answer 1

If two trees are close enough to touch branches then chances are good that their root systems are already in contact and they are exchanging nutrients and chemical signals. Whether trees are in competition or cooperation is still up for debate.

If a pest or disease is one tree then it can move to another tree without it ever touching by flying or wind action. It could be an issue if two trees are growing together which could possibly cause poor branch structure.