what did I do wrong?

[Aliarse]

I sprayed some ground elder with round-up - its in a flower bed but rapidly advancing through my lawn. The ground elder has survived but the grass bordering this bed has died for quite some distance into the lawn area.

What on earth caused this to happen and how do I put it right?

Round Up kills grass as well as other weeds. Some of the spray has drifted onto your lawn.

All you can do is re-seed the lawn and wait for new grass to grow.

Ground elder is evil stuff .

Best way to get it out is to dig it up, but that's also the most intensive way to do it. Or you could try a weed burner and keep burning it back every tiem it reappears.

If it's not a particularly visible part of the garden root out what you can and cover with a bit of old carpet or THICK black plastic for 6 months. Should help suppress it a bit.

For the bit in the lawn you could try Verdone - it targets broad leaved weeds only so would leave your grass unscathed. Ground elder is a bugger though. Digging it out is fine if you've got reasonably loose soil - I had it in clay and you just could not win as the roots snapped off everywhere (and then promptly re grew). In the end I decided that it was self contained enough and I'd live with it - life's too short for some battles.

I'm currently fighting bindweed. Current battle plan is to douse in over-strength round up (or paint with gel) whenever I see it looking green and healthy. Time will tell...

As has already been said, choose your battles! Take out the plants/shrubs you care about, from the affected area and replant or put into pots. If you don't you will always be tinkering. Then dig up the ground elder, assuming you have good garden fork etc. If you are into this, it is really satisfying if you know what I mean. The shoots are long and if you get good at it, come up whole like a root vegetable (result)! Pick out any broken bits as this devil will regrow from that. This needs to be repeated again, you will win with persistance.