Answer: German or bearded iris foliage should be promptly removed any time it becomes discolored. Many gardeners routinely trim them off quite short in mid to late summer, to encourage the regrowth of fresh healthy foliage. This would also be the time to divide. Always cut the foliage back when you divide.
Siberian and Japanese iris would be pruned in late fall or early spring after frost has browned the foliage. Remove the old discolored leaves but be careful not to damage new foliage growing up from the ground.
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