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They can be placed up to a depth of up to 6 inches.

Yes, Siberian iris foliage should be trimmed back for the winter, but only after the foliage has turned brown. Then trim back to about 2 inches in height.

Try to remove the flowers just as they begin to wilt or fade as doing this prevents them from growing seed pods. To deadhead an Iris bloom, either use your fingers or take a clean sharp pair of scissors and snip or pinch off the single bloom just behind the flowerhead.
Once all the blooms on a flower stem are gone and you don’t expect any more, you can then remove the flower stem from the plant.

Cut back the foliage after it has died back in the fall.

https://www.gardeningknowhow.com/ornamental/bulbs/iris/growing-siberian-iris.htm