Answer: Mid-August is the recommended time for digging and dividing irises. At that time you can cut the foliage down to 4-5" tall fans. If you're not digging and dividing, you can simply allow the foliage to die down naturally and then pull it from the rhizomes in late fall or early winter. The bottom line is that it isn't necessary to trim the foliage down into little fans - it's just easier to handle the plants for digging and dividing when the foliage is short.
To deadhead your cannas, simply cut off the flowering stalk as near to the foliage as possible. Cannas will respond by producing a new flowering stalk.
Best wishes with your garden!
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