Ground rods and vegetation

Outside of my garage where I do my coiling is a row of rhododendron bushes.
About a foot beside one of them is where I have driven my single 8 foot
ground rod.  This rod has been there for a little over a year now and I run
my coil on average perhaps 5 minutes per month.  I always soak the soil
prior to running the coil.   And this bush, and only this bush is just about
dead.  Has anyone else observed dying vegetation near ground systems?

Regards, Gary Lau
Waltham, MA USA