If its grass, it'll just spread, even if you bury it completely, especially if its couch grass - you need to dig out - every year.

Omg!

Yes agree, the way forward is to dig it out, if it’s a problem for you then do little by little each day, and stick with it, you will win the battle but only if you put the effort in, don’t let it defeat you, covering it with fancybark etc does not really work, but good old fashioned roll your sleeves up ehthic will do the trick, once achieved then never be complacent, tickle that area over each week and it will keep nice for you.

if you really don't want to resort to herbicide then you will have to lift your plants and carefully tease out all the rhizomes. even 1cm of it [if it has a growth node] will regrow.

I have to tackle a 30ftx8ft border congested with it. The front border that I cleared last year has the odd bit emerging and I am painting glyphosate on the blades if it is in another plant. I avoid herbicides as much as possible but there are times when I do use it.

Charles Dowding and John Walker are no dig gurus I have a article by John from his book organic earth friendly guide that shows him interlocking cardboard on a lawn putting 4 inch of soil on top and guaranteeing that no grass would grow through so I'm thinking although this is not suitable for mixed borders I thought that soil might work

in normal lawn grass that would work but not with couch grass I am afraid. It is just going to be down to spade work.

OK I see what you mean ...mine is a combination of 3 or 4 different grasses and none of them are lawn I've been here 18 years and never had such an outbreak before being on clay inhibits not all but lots of weeds but where all the outbreaks have occurred is where I have dug moving something or getting rid of something then left it over winter gone away for several months to find turmoil coming back in April

I got a euphorbia pasturi John Philips if anyone wants one they do need a little room but has become my favorite more than mellifera which is less hardy