How can you get canna bulbs to produce more bulbs?

Thats not how nature works. You just need to be patient and let nature do it's magic.

https://www.gardeningknowhow.com/ornamental/bulbs/canna/canna-lily-care.htm

https://www.almanac.com/plant/cannas

When they are ready, here is how.

https://www.gardeningknowhow.com/ornamental/bulbs/canna/transplanting-canna-lilies.htm

Hi Terry,

Try feeding bulb once it has bloomed and started to fade.

Dig up--divide--and replant---put a good compost ( you can buy it bagged ) in the holes where you plant the bulbs--sometimes they get hungry--like all things--and need fresh soil, nutrients and fertilizer.

just make sure they have lots of sun and lots of water but not so they rot. do not cut them off they need to die off naturally so growth process is completed nutients are pulled back into bulbs. don't put too low into ground should rest near surface and give them some Miracle grow water soluble fertilizer 2x spring then just before they begin to die back seperate the clumps of tubers so you have more & more

Bulb plants need a lot of bone meal or calcium to increase their size and volume. You can find bone meal at the home improvement store. Follow the instructions for applying it.

More caannas:

You will have to dig the bulbs up in the fall before the ground freezes, preferably soon after the first frost,

Then separate the bulbs,

Remove. The excess dirt

Dry them

Wrap individually in newspaper

Place in breathable container

Store in a cool, not cold nor hot dry place over the winter, dry basements work well. Do NOT let them freeze.

In the spring plant into the ground at the same depth you removed them from, just after the frost free date for your area

Fertiliize and water.

Watch them grow but keep an eye out for insects.

I planted my first five or six and in the fall when I dug them up I had so many bubs, I had to give most away.

If they were a food crop they seem like they would likely feed a lot of people.