Mine are about that tall as well. Depending on the variety, they can grow between 2' - 7'or 8' tall in Chicago. Have you seen the ones grown in the middle of Jackson Bd. just east of Michigan? I haven't down that way this year, but in the past, there normally were very tall cannas with striped leaves.

I have seen them in Grant Park and they were tall but I can't remember what month that was. My mom says the ones she gave me get 5-6 ft tall but I thought they would be reaching that height about now. We'll have to wait and see. Thanks.

I'm in Ohio. My canna's are all growing at different rates. I don't know what causes this. The ones on the west side of my garage only get sun after 2, so I understand why they are slower. I have 4 plants in one bed and they are all different size.
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Start in the middle of the pic. The big one behind the elephant ear is over 4 feet tall. Move to the right and they all get smaller. They all get the same sun, water, and compost. I have one to the left of the big one that isn't in the pic, and it is also smaller. It's almost like the elephant ear is feeding the canna. I don't believe it's a giant legume.