Hi Tom.

I wonder where you are living. I'm in West Wales where rhododendrons
flourish so much that they're despised by many. One of the main reasons
for this, from what I can work out, is that NOTHING will grow beneath a
spreading rhododendron apart from the odd highly adaptable fern and a
bit of moss. I hear that even when an old rhododendron is removed the
ground is "good for nothing", all its normal qualities vanished. So, I
know this solution demands great patience, but just WAIT and when your
rhodies are big and umbrella-like, nothing, including grass, will grow
inside and under!

Ellie.