Hi Jeff. Sorry, my first post so no background for you to check on. As previously mentioned, no chemical available to Joe Bloggs that can spot grass against any other plant - robots are our future!

The only thing I can think to try of is ground cover of some sort to stop the grass getting light to grow.

Woodchips are easy to distribute but need to be thick and will probably not last long enough or be barrier enough to kill the grass. If you have the money more than the physical ability, I'd go for this and reapply regularly. It wouldn't be perfect but it would be better and look ok.

500 micron plastic will probably kill what's below but won't stop it creeping round the edges and would be hellishly hard to cut round the existing plants! Might also reduce water reaching wanted plant.

Maybe plant a ground covering plant to potentially weaken the grass (by denying light) such as Lysimachia nummularia or an ajuga (planted below plants so they grow up or down towards the light) and planting bulbs will also help you remove some of the grass with more exciting aspiration to do so.

Just some suggestions which may be shouted down but maybe of use or inspiration.

Good luck

DM