I'm assuming you mean the bearded variety. Dead head them and leave them to get on with the business of growing larger for next year. After a few years the plants can be split.
Mine grow in my pond I do take off the dead flowers - but only because I'm saving them to use as a natural dye. Then they are just left - and each year they come back flowering more than the year before My nan always used to cut out the stalks and then tie the leaves up together with twine - she said it protected the plant from unexpected cold spells and frost.
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