Marestail (equestrum of some sort) is reputed to be one of the oldest plants on earth (pre historic) ... the reason being that nothing seems to kill it
I have been demented by it since I started the Japanese garden 12 years ago. It apparently thrives in poor soil, and the digger driver unleashed plenty of that when he dug my pond
I use a product called 'Kaspar' which I believe to be a commercial product better than (though similar to) Roundup, but it takes repeated applications every year
Unfortunately it can't be used where the darned stuff has infiltrated plant areas
The first pic shows the stae of my waterfall area before I first
(a) Sprayed
(b) laid double layers of newspaper
(c) covered thickly with composted bark chippings
I still have to spray every year but the second pic shows the considerable improvement
no problem at all 