Any time, 134abromley, as they've finished flowering. However, if you want to save the seeds to grow more, then wait a while until they're ripe.

Many thanks for your reply. I assume you mean save the seeds if I wish to grow these flowers elsewhere in the garden as the label says this is a hardy perennial which presumably re-flowers each year. I have read that it reseeds itself - does this mean that it will get even bigger than it is now. The stated height was approx. 4ft but edged nearer 6ft and if it is going to reseed itself will it get even bigger in width and possible height, if so I will probably have to replant it to a more suitable position. As no doubt you can tell I am not an experienced gardener. Any comment would be greatly appreciated.

Yes, I meant exactly that, 134. I collected some seeds last year for a friend.

This plant is a hardy perennial, and it will die back to its base - you can cut back the dead stems at any time now ready for the plant to regrow next year. I have grown these for several years, and find that they're not long-lived plants. I haven't found that they seed themselves, which means that you would find baby plants around the parent in the spring. (That's a shame, as they're lovely!) Watch out for babies, though, as you just might find some, which you could dig up and pot up for a while until they're big enough to plant elsewhere - or to give to a friend! :-))

I'm surprised that yours got so tall! Mine have been around the 4' mark. It won't necessarily grow to that height next year, nor will it bush out, so you won't have to move it.

Did you know that there are purple and white-flowered versions?