If your seeds are for Lychnis coronaria they will self seed but the seedlings are very easy to spot with their grey leaves and pull out easily. Personally I think its a plant well worth growing, if deadheaded it flowers for weeks.

If its Lychnis chalcedonica [maltese cross] it is also worth having and weeding excess seedlings out isn't a problem. both are lovely.

A lovely Perennial,and as long as you keep dead heading, it will flower till the frosts,if you do,and you won't then have to remove seedlings..very hardy,and easily split in spring..A good idea to remove yellowing leaves as it ages..not sure if it's mildew or not,but spoils the look of the plant..

Thanks,its chalcedonica, will give it a go

Hy Jenfren
Could i have a few seeds please ive never grown them and like the look of it
Have some sweet peas seeds I can swap you if you like
Please let me know
Irene