I have a driveway hedge of James Stirling (approx 20 yrs old) that has deteriorated in the past 6 mths. It was trimmed up in March, other than that we don't touch it. I believe the middle tree is dead and I can see the 2 on the left deteriorating and drying out rapidly. What has happended here? Can I save the trees on the left and prevent this happening to the 2 on the right?