You need to wait until the seeds are fully developed, which means you should wait until the pods are brown and the seeds are black.

Is there a risk of waiting too long and not checking often enough? In other words, might the seed pods fall off and the seeds be lost?

Yep, there is a risk. But you can tie a paperbag or a nylon stocking around the pod to catch the seeds when they mature.

Take care,
Hanne

My daughter and I are newbies to canna growing. Wow were they beautiful this year all from the exchange here. All of them bloomed except 2. I believe the sun exposure being too little is why. Oh well, live and learn, however; we did enjoy the wyoming canna colorful foilage. We took some of the seed off too early as I have read. I will now wait till I see the color change to dark and brown and seeds black. Thanks for info...Karen

I think I'm going to harvest my seeds tonight. I didnt know you can grow them from seed. I have Cleopatra Cannas , will they flower next year if I plant them early enough ? The seeds look so small ! ~ Tom

Tom, I'm not so sure cleo sets seeds, and if it did the seeds would be 1/4" round- not really that small. Cannas will generally develop a pod, but there isn't necessarily seed inside the pod.

Lynn

huh - looked like tiny round pods on top of the plant - maybe i'm wrong - i'll go out and look .... could have sworn the last time i cracked one open, there were little tiny black things - or maybe, i dont have cleopatra!

Sometimes the plant will form green pods, and there will even be what looks like tiny seeds inside. Real canna seeds are, as Lynn says, about the size of a pencil eraser, one per pod I think. My Cleopatras didn't set seed. I am waiting on my journey's end canna, I read somewhere that these do set seed, but mine haven't yet.

Why do some of the same Cannas create seeds, while others don't? I have several stalks of the same type of Cannas, I think Purpea, while some of the stalks form pods with seeds, other stalks will form the pods without seeds. Also, does anyone know a good site with good pictures of Cannas? I found one site with some pictures, but mine do not look exactly like any of them. I have some that are only about 1-2 feet tall with peach blooms, some that are around 4 feet with yellow blooms, and some that are 7-9 feet tall with purple to dark red blooms ( the ones I think are the Purpurea).

The best known site for information and photos is karcheskycanna
If the seed is not the size of a BeBe it is not a viable seed. Cannas grown from seed are not the clone of it's pod parent and should not bear it's name. Only rhizomes are clones. At least this is what I have read. I'm sure there are people here that know more than I do about it.
Sky~~