Pretty but destructive: Yellow flag iris
There’s no doubt that yellow flag iris is a pretty, eye catching plant.
Unfortunately, it’s as destructive as it is pretty.
Yellow flag iris is prevalent in gardens throughout the Waikato.
This native to Europe, Asia and North America was introduced to New Zealand as an ornamental garden plant.
It has also been deliberately planted around waterways and wetlands in many areas.
The plant’s seeds and rhizomes are spread by water movement and machinery to infest new areas.
Yellow flag iris is tolerant of many climatic extremes and grows happily in any open, sunny swampy ground, fresh or brackish water margins, lakes, salt marsh, and wet sandy areas – even in paddocks near waterways or wetlands.
The rhizomes form dense floating mats, displacing native plants and potentially causing flooding and water level changes in swamps. Poisonous seeds may also affect native birdlife.
Identifying features include yellow flowers up to 120 millimetres across, typically in an iris like form, flowering in spring and early summer. Leaves are broad (20-30mm wide), flat and sword shaped.
Hamilton Lake has historically been invaded by yellow flag which still comes up in small amounts and is vigorously controlled.
Because seeds float in water, yellow flag has spread seed down the banks of the Waikato River.
There have been a number of small infestations from Hamilton to Ngaruawahia.
Also large infestations have been reported from Ngaruawahia to Port Waikato.
One plant can produce up to 1000 seeds, creating a large seed bank and making eradication difficult.
Yellow flag iris is easier to identify in spring and early summer due to the flowers.
Control in spring means plants are controlled before they set seed.
Once eradicated, re-plant the cleared space with native rushes and sedges (including Juncus, Leptocarpus, Baumea and Carex species) or other non-weedy iris species.
Waikato Weed Watch is supplied by Waikato Regional Council.