Not till late spring/early summer next year,Deadhead..they are annuals..you will need to keep them somewhere frost free over winter..the seeds are usually sown in Feb/march time..

Thanks very much.I am really looking forward to growing them I visited Nymans this year and saw the flowers all in bloom ,the colours and the movement of their Cosmos had me sold ,and I thought I must have some of that.

One of my favourites too.Mine are still flowering,but slowing down now..first frost,and that will be it..but well worth growing,which I do every year..

What type of Cosmos? There is a half-hardy perennial called Chocolate Cosmos that is worth keeping in a greenhouse of buried in free draining soil below frost depth. If normal Cosmos then they die in the first frosts. They are annuals, so even if you kept them frost free they will grow tall and etiolated in winter and then die in summer.