I don't grow these myself by my understanding is you prune back in autumn after flowering has finished.

hello grannymac,
moongrowe is right, after they finish flowering and are looking a bit sad you can cut them down a bit, then in early Spring prune down to ground almost, as new growth will come along.That's what we did when we used to have them and they were great every year.The pink ones are lovely.

Personally I leave them, they are only 2 years old so are small.

Once they're fully established thery sucker everywhere and even weed killer struggles to kill them.

I hoed a patch to an inch under the ground twice last year and weed killed regrowth 3-4 times and some of it is still coming back.!

My daughter cut off all the old dead foliage yesterday. the new furry leaves are about 2" tall and will grow away fine. I suspect the white ones were out competed by the pink ones snoopdog.

The dead foliage harbers slugs and snail I find.

So in answer to your question cut them down. neatens them up.

I dont think its possible to kill them once they establish. They grow from the tiniest bit of root. We had them in three parts of the garden,and in spite of a lot of effort we still have them and they have spread under the fence into the front garden grrrrr.

I wouldnt complain if they were all over my garden as they are such lovely things! They are great as cut flowers too, and so reliable every year.

thank you moon growe for the sharp reply.