Does fire caused by ash disposal count as accidental?

Disposal of ash causing a fire would be negligence and would not be accidental.  Accidental means it was unforeseeable.  Negligence refers to foreseeable harm. Negligence is based on due care which is that degree of care that a reasonable person would exercise under the same or similar circumstances to prevent foreseeable harm. 

In order to establish negligence, the landlord would have to prove that you owed a duty of care, breached the duty of care; that breach of the duty of due care was the actual and proximate cause of the fire. Negligence means that you owed a duty of care with your disposal of the ash.  You breached the duty of care by disposing of the ash on the hedge.  Actual cause means but for your disposal of the ash on the hedge would the fire have occurred?  If the answer is no, actual cause has been established.  Proximate cause means are there any unforeseeable intervening events which occurred after you disposed of the ash which caused the fire?  If the answer is no, proximate cause has been established and the landlord has established your liability for the damage to the hedge caused by the fire.

It was foreseeable that ash disposed of on a hedge instead of in an appropriate receptacle could start a fire.  There were no unforeseeable events that occurred after disposing of the ash on the hedge that caused the fire and would relieve you of liability because it was foreseeable that live embers could start a fire.