Iris pseudacorus, Yellow flag iris. Although fairly naturalized over much of North America, Yellow flag iris is of European origin and is hardy in zones 4-9. Showy yellow colored flowers grow off a slender stalk just slightly taller than the sword-like foliage. May be grown in up to 6” of standing water making it suitable to use in water gardens. A profuse seeder, Yellow water iris should be planted with care in containers and best not to be planted in natural environments where it can quickly spread. To control the spread of this non-native species, remove the flowers as soon as blooming is over to prevent seeding. Bloom time May - June.