Current Extent and Potential Distribution of Yellow Flag Iris in

1. INTRODUCTIONYellow flag iris (Iris pseudacorus L.) is a semi-aquatic herbaceous perennial plantwith bright yellow flowers. It was first reported as naturalised by Kirk (1878) in theHutt Valley, referring to it as ‘another welcome addition’ to the New Zealand flora.While it provides a dazzling display when flowering from September to December, ithas proven to be a problematic species in New Zealand and many other countriesoutside its natural range (Williams and Champion 2008).Yellow flag iris is an unwanted organism under the Biosecurity Act (1993) and is aNational Plant Pest Accord species. The Waikato Regional Council Pest ManagementStrategy (2008-2013) lists yellow flag iris as a ‘containment pest plant’, and landoccupiers are responsible for the total control of yellow flag iris on their property. Its’pest status results from its ability to form dense mats from seed and rhizomes, thatdisplace indigenous species and can overtake areas of low-lying pasture. It typicallyoccurs on the margins of lakes, rivers, and drains, but can also occur in freshwater andbrackish wetlands.In March 2010, the Department of Conservation Waikato Area Office undertook asurvey along the banks of the Waikato River, from Huntly to Te Onetea Stream, toassess the extent of yellow flag iris and the threat it poses to the WhangamarinoWetland. The survey confirmed that yellow flag iris was well established in this area,and was present along most of the river margins and associated alluvial floodplains.Concern that yellow flag iris may establish in Whangamarino Wetland led to a furthersurvey, in November 2010, of Lake Waikare, which is immediately upstream of theinternationally significant wetland. Yellow flag iris was found scattered along most ofthe lake margin. This level of infestation was not expected, but confirmed that it posesa serious threat to the Whangamarino Wetland and raised the question of howwidespread yellow flag iris was in other parts of the Lower Waikato River catchment.Wildland Consultants was commissioned to report on the extent of yellow flag iris inparts of the Lower Waikato River catchment, using information already gathered bythe Department of Conservation and from a helicopter survey of the Waikato Riverbetween Ngaruawahia and Tuakau. In addition, areas of potential habitat were to beidentified, management implications discussed, and recommendations provided onpriority areas for control.2. DISTRIBUTION, FORM, AND ECOLOGY2.1 Geographical distributionYellow flag iris is indigenous to all European countries (except Iceland), Caucasus,Western Asia, and North Africa (Sutherland 1990). It is adventive in North America,New Zealand, Australia, and South Africa. The NIWA FBIS database(https://fbis.niwa.co.nz/fbis) has records of yellow flag iris occurring throughout NewZealand, from Northland to Stewart Island.© 20111Contract Report No. 2714