This commonly thought about question is determined ultimately in the Courts.
The Law tends to decide on the level of duty of care regarding trees and safety, in court room situations when an incident has happened.
The Law expects that all people responsible for trees should inspect the trees regularly (annually) and after severe weather instances. It is judged to be reasonable to do this, as you are familiar with the trees you have and can readily access them.
There is an expectation that you should contact a professional if you observe something different than the last time you inspected them. Something which requires specialist knowledge. The Courts also deem it good practice to appoint a professional to routinely inspect the trees every 5 years.
Examples of things to look for when you inspect the tree may be:
- Fungi growing around the base (these can be particularly notable in Autumn although not exclusively)
- The tree has less leaves or doesn’t seem as healthy as it was
- The tree has started to lean
It is a good idea to keep a record when you inspected them and what you found. Draw up a plan of the area and draw a symbol to denote each tree, which you can then number in sequence. The burden of proof lies with the tree owner/manager to prove they have not been negligent.