Compost Iris . Most iris clumps become crowded and should be divided every three to four years. Each piece should have both roots and leaves. Indomitable Irises! Colorado Yard Care from coloradoyardcare.com Divide every three to four years for prolific flowering. Thrives on the edge of a pond or soil that is constantly moist. These include iris pseudacorus with yellow flowers in may and june, iris sibirica with blue flowers in june and july,.