Plant ID please
Can anyone identify this plant for me?
Leaves approx 10cm wide. It has the occasional greyish/pink flower spike, but other than that does nothing. It's taking over my garden, and efforts to dig it up have failed, as it has roots that go under my pond.
Any thoughts on identification or advice on eradicating it (preferably without resorting to weedkiller so near to my pond) much appreciated.