How to Prune yellow iris?
Remove any dying foliage in autumn, old flower stems can be cut down after flowering

yellow iris · Iris pseudacorus
Also known as: daggers; flagon; Jacob's sword; water flag; water skegs; yellow flag; yellow fleur-de-lis
Grow in wet, deep, acid, humus-rich soil in full sun or partial shade. It thrives at the margins of large ponds or streams; has the potential to become a nuisance if not managed well so plant with care
Remove any dying foliage in autumn, old flower stems can be cut down after flowering