If your Canna has produced viable seeds inside the pods, they will be the size of a pea, hard as a rock, and black. The bumpy pods will turn brown and crack open when the seeds are ripe.

Most Cannas are hybrids and many do not produce seeds. The plants that do produce seeds do not come true. However, I've grown many Cannas from seed and I like them just as well.

If your cannas have been growing all summer, the tuber will have double or tripled in size. It is easy as pie to divide the tuber. They multiply so fast that it only takes a couple years to fill you entire yard if you want to. lol

I planted a tuber with 2 or 3 eyes and got 3 flowers. So if I dig this up and divide it next year I will doulble that?

At least double but you may get more than that depending on the growing year. I've had some years that I could divide a bulb several times over but this year my cannas are puny. You won't know until you dig it up. Good luck!

I've got some pictures of the seed pod, the dried out seed pod and the seed of canna on one of my website pages. Take a look if you have time.

Here is a link that might be useful: Country Roads Gardens - seed identification

Do the seeds need to be cold-stratified before planting next spring? Here in my corner of Southern Illinois, we can get away with leaving the tubers in the ground if we cover the beds with 8 or 10 inches of pine needles. Therefore, I don't dig them up every year to divide them. Usually, we just dig them about every 3 or 4 years. I wouldn't have a place to store them all winter if they were out of the ground. Also, how early in the spring should I start the seeds indoors? I am assuming they shouldn't be planted directly in the beds--is that right?

No, they do not need cold-stratified since they are tropicals. Having said that, I have successfully Winter Sowed the several years in a row. lol

My inside method for Canna seeds is to nick the seed coat using a rasp. I then soak them in water in a warm location until I can see a root nub starting to form. (Sometimes a week or so - change the water) I carefully remove the seed without touching the root nub and plant in medium. It only takes another few days to a week after planting the pre-sprouted seed to see the leaves. Depending on your inside lighting space, I wouldn't start them more than 6-8 weeks before they can go outside because they grow fast!