Weed Watch – Yellow Flag Iris

Yellow flag irisYellow flag iris (Iris pseudacorus), is listed in the 2008 National Pest Plant Accord (NPPA) and is banned from sale, propagation and distribution in New Zealand. The plant is native to Europe, West Asia and parts of North America and was introduced to New Zealand as a garden plant. It was first found growing wild in Lower Hutt is in 1938 and has since spread to many other parts of New Zealand.

It has been deemed a pest plant because of its invasive nature. The rhizomes are usually anchored firmly in the banks of waterways and its densevegetation can exclude native plants and block waterways. This in turn traps silt increasing the potential for flooding. It may also degrade whitebait spawning areas. Yellow flag iris is also toxic. Its resin can cause skin irritation in humans, and stock will sicken if the plants are ingested.

The plant is named for its striking yellow flowers – up to 12cm across – that are produced in spring and early summer. It has the typical long, sword-like leaves of most iris species and it produces large green seed capsules packed with poisonous seeds. It favours the margins of urban streams, tidal rivers, ponds, lakes, salt marches and swamps; and it spreads via its prolific seeds and its thick, creeping rhizomes.

IrisThere is a large infestation of yellow flag iris at Harts Creek – easily visible from the walkway leading out to the bird hide. There are scattered populations and individuals all around the catchment along waterways (e.g. Selwyn River near the upper huts, Halswell canal, etc). To protect the most high value ecological areas scattered plants will be controlled where feasible (e.g. landowner permission) around the lake margins. Sightings can be reported to Environment Canterbury (biosecurity or biodiversity teams) or recorded on the ‘Nature Watch’ website.

Small areas of yellow flag iris can be controlled by digging out all parts of the plant (wearing gloves), taking care to recover all parts of the rhizomes and disposing of them in a landfill, or by burning them. If the plants have flowered, remove all seed pods before you start digging. Check the area regularly to control new seedlings.

The only way to successfully control large areas of yellow flag iris is to spray them and it is best to use professional spraying operators. As always, all herbicides should be used in accordance with manufacturers’ instructions and every care should be taken to ensure sprays do not come in contact with other plants, the soil or waterways.