Been doing a bit of research as I have this plant in a pot from last year. I was planning to put it in the ground but I'm a little reluctant after reading that it is very invasive and an absolute nightmare to get rid of once it has established its self.
Is it really as much of a thug as I'm begining to think it is?
I have had one for some years - growing in a wild flower meadow, in rough grass, on heavy clay soil. It has clumped up a bit, but in that situation I have not noticed any seedlings.
If it's happy it will spread. I put some over by our unlined pond in a wild, damp corner and it loved the conditions and got a bit thuggish so now I just dig up and compost the plants I don't want.
I have it beside my pond where its feet stay wet. I have had it for several years and it has been very well behaved for me and I have not had any seedlings.