KK wrote:
No - this has nothing to do with illegal substances - but with patches of
'bad' grass in my lawn - coarse ugly stuff, bright green & grows very
quickly.

Do I have to dig it out ?

Most upright-growing British coarse grasses and sedges will die off in
a lawn which is cut a couple of times a week; but if they don't, yes,
they need to be dug out. In my case the pest was Yorkshire fog, which
is a glaucous colour, so that can't be the one you have; but I think I
got rid of it easily enough. If it's any consolation, it probably won't
come back once you've done the job.

If the coarse species take hold, it may be a sign of poor drainage or
low fertility: they're stronger competitors at root level. I think the
smoothness of the lawn is a factor, too: if it's rather bumpy, in the
hollows mowing may leave enough of the green top on to help them
survive.

--
Mike.