I did a search here on canna (search box is found just below at list of messages) and found several replies.

I have copied a portion of a reply here that another member (nurmey) contributed .

Many cannas do not produce viable seeds. After the flowers fade, a pod will develop with 1 to several black, hard seeds the size of a pea in it. Let the pods turn brown and start to crack before you harvest the seeds.

If yours is a hybrid (and it probably is) it may or may not produce seeds at all and if it does, they will not come true.

Do you know if yours is a hybrid, or the name?

Sue

Here is a link that might be useful: Search results for canna