Yellow flag iris (Iris pseudacorus)
Yellow flag iris (Iris pseudacorus) invades ditches, irrigation canals, marshes, stream and lake shorelines, and shallow ponds, displacing valuable plants like cattails, sedges, and rushes that are used by many birds for nesting. Established infestations can reduce the carrying capacity of water storage in temperate wetlands, and can also block irrigation canals and flood control ditches.
Yellow flag-iris reproduces quickly through seed dispersal and horizontal root systems, creating thickets in the water. Up to several hundred flowering plants may be connected under the water, and fragments can form new plants when they break off and drift downstream.
Yellow flag-iris is widely sold for garden wet areas and ponds, which makes it very hard to control. Plants are identifiable by showy yellow flowers with 3 sepals that curve backward and 3 petals pointing upwards. Leaves fold and clasp the stem at the base in a fan, with long sword-like leaves toward the outside of the plant. At maturity, plants can reach 1.5 metres in height.
WARNING: Yellow flag iris can sicken livestock if ingested, though it is generally avoided by grazing animals. Contact with the resins can cause skin irritation in people.
To remove yellow flag iris:
COMPLETELY DIG OUT ALL IRIS RHIZOMES (CORMS) AND REMOVE ALL FRAGMENTS. Even very tiny pieces will grow to form new plants. And even after burning the rhizomes can sprout and seeds can germinate. Because iris seeds float they are easily distributed by flowing water or higher winter water levels. Watch for and pull new plants beyond the site of the original infestation (again, removing all fragments). It is possible to control yellow flag iris to a degree without using herbicides, but to do so requires constant vigilance for years.
The most effective removal method is to cut the stems off and apply an appropriate herbicide to the stump. This may be useful in your contained garden pond but is not advisable in wild wetlands and streams.