Sometimes Cosmos grows and grows and refuses to flower. It's something to do with day/night length. Make sure that you use seed sold by a reputable British seed company as this will be bred to flower in our short summers. However, pinching out won't do any harm at all. Also, feed with a tomato food to encourage flowering, not with a general feed.

Mine usually grow pretty tall too,so I stake them,before they get to full height..they usually flower well into late Autumn,,still upright..

I grew cosmos from seed last year and did not read the packet with any great attention. I planted them out in the garden and was amazed they grew and grew till about five feet in height. When I then read the packet it gave the height as 48 inches....I always thought cosmos was a 36 inches high plant at most...now I know dirrerently.

I discovered a couple of years ago that there are short varieties and tall ones. I grow the shorter ones now.