"Cosmos" My Cosmos has just started to flower

Sometimes they are very 'day length' sensitive. It's just unfortunate and there is nothing that can be done. Just enjoy some lovely late flowers and hope the autumn is mild to encourage more! Don't forget to deadhead them regularly.

Over the years I've found that perhaps 20% of cosmos don't flower at all if they're planted out in rich soil when small.

I now get around this by keeping them in 3 inch pots and unfed until the first bud appears.

I then plant them out and they grow very rapidly and flower profusely until the first hard frosts.

Thanks for your info both, just hope some more buds burst into flower so that I can save some seeds before any frosts.
:o))

It's probably a bit late for the seeds to develop now. You may be better off buying fresh seed next year, they aren't expensive. Also, if you save seed from your late flowering ones you risk the progeny being late flowerers too as this may be an inherited trait. If you do get some seed to save make sure it is dry before storing it, preferably in paper envelopes and then in an airtight container in the fridge.