Red flag raised about yellow flag iris
Yellow flag iris on banks of the Waikato River
Yellow flag iris (Iris pseudacorus) is a member of the family that the other iris’s don’t like to mention. Yellow flag is an emergent aquatic perennial that grows on the edges of swampy ground and in fresh or brackish water bodies. It has bright yellow flowers from October to December and flat sword shaped leaves that emerge from a fan at the base and grow up to 1-2 m.
Yellow flag will creep across the water, forming mats of rhizomes so thick that they can support the weight of a person. These thick mats can impede water flow and can trick the unsuspecting wanderer into walking out on (and into) water. Yellow flag spreads quickly with seeds dispersed by wind and water, and by rhizome fragments. It’s not just New Zealand where the red flag has been raised about yellow flag - it is troublesome in Australia and North America also.
Getting into to it early is best and small areas can be cleared by hand. First make sure you remove the seed heads, then dig the plants out taking care to get the underground rhizomes. The rhizomes will need to be bagged and taken to the landfill for deep burial. Yellow flag can’t stand herbicide, but take care using it around water and contact your regional council for advice.
If you like the visual effect of yellow flag around waterways, why not try growing Raupo (Typha orientalis) which has similarly shaped leaves but with a characteristic twist in the top.
For more information on weeds visit www.weedbusters.org.nz