Rashes? I don't know what you mean from a plant point of view - a rash is a skin eruption or irritation, that's the only one I know.

reeds sorry.

Well you're obviously a person of few words! Presumably you meant to say rushes or reeds then. Where are they growing and what are they? In water or in the ground, which plant?

Welcome, Merryfisher!
No, they'll just come back from the roots. Is this in a pond, or in moist ground? And are they reeds, rushes, horsetails, or cattails?

That's a fine old word, Merryfisher. He means rushes or, more accurately, sedges I think. Rushes and sedges indicate a wet, waterlogged ground that is soor (sour/acid) and undernurrished. To get rid of them permanently the ground really needs draining and cultivating.

Ah - I've never heard rushes called rashes before, a new one on me, just thought it was a typo.

sorry for the confusion,it's the local name for them up here in Orkney. they are growing in a large duck pond and starting to spread in from the edges.

where i live a it is quite comman for rushes to be called rashes have you tryed roundup on them

Yes i tried some on a patch that had dried out no difference though.

Unfortunately, Round-Up would be like dropping a nuclear bomb on the pond, and the surrounding environment. Likely, the only safe treatment is to dig up the roots on a regular basis, maybe re-contour the pond so it has steeper margins, and/or is deeper.