By Siris
Hampshire,
United Kingdom 
Does anyone know what this plant is? It's a tall hardy perennial with pink double flowers, supposedly invasive, but only the one tall stem after 2 years.
31 Aug, 2020
Answers
Sorry I don't know where the photo went. Having looked up Saponaria officinalis Dancing Bet it certainly looks like it.
Thank you Seaburngirl.
31 Aug, 2020
It is bouncing all through my borders and into the grass. I am trying to eradicate it from 2 borders. It was slow to settle but once she gets going............
1 Sep, 2020
Thank you SBG, I'll bare your warning in mind, when I resite it.
1 Sep, 2020
I see you have seen my photo. its like that in the other border and luckily gets mown off when it encroaches into the grass. the thing is it is a lovely colour and flowers just before the asters so fills that flowering gap.
2 Sep, 2020
Mmm, I really want to keep it, she's so pretty, unfortunately doesn't show in front of Anemone Queen Charlotte, same colour, another spreader.
2 Sep, 2020
yes I have both of these but is separate beds luckily.
3 Sep, 2020
no photo siris but I wonder if it is Saponaria officinalis [double bouncing bet]
31 Aug, 2020