The yellow iris (Iris pseudacorus) is also known as yellow flag. It is an exceptionally beautiful flower, especially when freshly unfurled and seen close up.
It flowers from May through until July and is very common anywhere there is standing water, both saline and fresh. It can be found in abundance in ponds, marsh, reed beds, damp woods - as long as it is wet then yellow iris has a chance of surviving. Where it does occur, once established, can become abundant and although a nice addition to a garden pond it can take over very quickly if left unchecked.
Once the flowers are pollinated they develop large green seed heads which, when ripe, will split open, ejecting the seeds in to the water where they float away to form new plants which is why they can be so prolific.