Current Extent and Potential Distribution of Yellow Flag Iris in

DraftDCURRENT EXTENT AND POTENTIALDISTRIBUTION OF YELLOW FLAGIRIS IN PARTS OF THE LOWERWAIKATO RIVER CATCHMENTSEPTEMBER 2011Contract Report No. 2714Prepared for:DEPARTMENT OF CONSERVATION5 NORTHWAY STREETTE RAPAHAMILTONandWAIKATO REGIONAL COUNCILP.O. BOX 4010HAMILTONHAMILTON OFFICE: ROOM 2.08, LEVEL 2, 169 LONDON STREET, HAMILTON CENTREPh 07-838-2428; Fax 07-838-2815HEAD OFFICE: 99 SALA STREET, P.O. BOX 7137, TE NGAE, ROTORUAPh 07-343-9017; Fax 07-343-9018, email ecology@wildlands.co.nz, www.wildlands.co.nz

EXECUTIVE SUMMARYYellow flag iris is a perennial semi-aquatic plant that occurs in temporarily wet habitats. Ithas a wide tolerance range and is reported as a pest in a range of habitat types. It is commonin agricultural landscapes, where it invades flood-prone pasture and drainage/irrigationcanals. It also occurs in many natural habitats, including freshwater and brackish wetlands,lake, pond and river margins, and alluvial floodplains. It develops a thick rhizome mat whichdisplaces other plant species and traps sediment, elevating ground heights and blockingwaterways.In New Zealand, yellow flag iris is an unwanted organism under the Biosecurity Act (1993)and is listed as a ‘containment pest plant’ in the Waikato Regional Council Pest ManagementStrategy (2008-2013). In 2010 concern was raised by the Department of Conservation thatyellow flag iris appeared to be expanding rapidly in the Lower Waikato River catchment,posing a significant threat to the internationally significant Whangamarino Wetland.In April 2011 a helicopter survey was undertaken of the Waikato River betweenNgaruawahia and Tuakau. Aerial photographs at a scale of 1:2,500 were used during theaerial survey to delineate the extent and density of yellow flag iris and to identify potentialhabitat. Density of yellow flag iris was assigned to one of four categories: high (>40% cover),medium (10-40% cover), low (

Area and density of yellow flag iris along the WaikatoRiver between Ngaruawahia and TuakauLow density56.8 haMedium density143.2 haPotential habitat576 haHigh density57.9 haManutahi Island is immediately downstream of Rangiriri, which contains some of the highestdensity populations of yellow flag iris on the Lower Waikato River. Further downstream,yellow flag iris become less common however there are considerable areas of potentialhabitat, particularly between Mercer and Tuakau. There are some extensive areas ofkahikatea forest on the alluvial floodplains and river islands along this stretch and the area isconsidered a good representative example of a lowland river system that supports nationallyuncommon ecosystems and associated rare plant and animal species.Yellow flag iris was also present at low densities at a number of other sites hydrologicallylinked to the Lower Waikato River. These include Lake Waikare, Te Onetea Stream,Whangamarino Wetland, Opuatia Wetland, Opuatia Stream, Lake Whangape, WhangapeStream, and Weavers Lake. A large area of potential habitat (273.2 ha) was identified at theinternationally significant Whangamarino Wetland.The management of yellow flag iris requires immediate action in the Lower Waikato Rivercatchment, to eradicate it from areas where densities are still very low, to protect areas ofhigh conservation value (i.e. Whangamarino Wetland, Opuatia Wetland, Lake Waikare andLake Whangape), and to prevent further spread into low-lying pasture and areas withinstopbanks that are managed by the Waikato Regional Council and are leased for grazing.It is recommended that high risk areas not investigated in this survey are assessed in the nearfuture. These include Lake Waahi, Pungapunga Wetland, and the Waikato River downstreamof Tuakau to Port Waikato.© 2011Contract Report No. 2714

CONTENTSEXECUTIVE SUMMARY1. INTRODUCTION 12. DISTRIBUTION, FORM, AND ECOLOGY 12.1 Geographical distribution 12.2 Form 22.3 Habitat and tolerances 22.4 Reproduction 23. IMPACTS 43.1 Pest status 43.2 Economic impacts 43.3 Ecological impacts 54. CURRENT EXTENT AND POTENTIAL DISTRIBUTION 54.1 Methods 54.2 Results 64.2.1 Waikato River from Ngaruawahia to Ohinewai 74.2.2 Waikato River from Ohinewai to Orton, Lake Whangape andOpuatia Wetland 74.2.3 Waikato River from Orton to Tuakau 174.2.4 Lake Waikare and Whangamarino Wetland 175. MANAGEMENT OF YELLOW FLAG IRIS 235.1 Control methods 235.2 Cost of control 235.3 Priority areas for control 246. DISCUSSION 25ACKNOWLEDGMENTS 25REFERENCES 25© 2011Contract Report No. 2714

PROJECT TEAMPaula Reeves - Aerial survey, report author.Kerry Jones (Department of Conservation) - GIS.Federico Mazzieri - GIS.Reviewed and approved for release by:_______________________W.B. ShawDirector/Principal EcologistWildland Consultants LtdWildland Consultants Ltd 2011This report has been produced by Wildland Consultants Ltd for the Department ofConservation and Waikato Regional Council. All copyright in this report is the property ofWildland Consultants Ltd and any unauthorised publication, reproduction, or adaptation ofthis report is a breach of that copyright.© 2011Contract Report No. 2714

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

2.2 FormYellow flag iris is an erect hairless perennial, 40-150 cm tall (Plate 1). It has thickrhizomes, up to 4 cm in diameter, with roots of 10-30 cm (Sutherland 1990). Leavesare glaucous (i.e. bluish grey or green), sword-shaped, and up to 90 cm long. Flowersare 8-10 cm in diameter, yellow, with a typical iris form. Long flowering stalks canproduce several flowers per stem. The fruit is a glossy green capsule resembling shortgreen bananas when mature (Plate 2). Seeds are brown, corky, variable in size, andusually disc-shaped.2.3 Habitat and tolerancesYellow flag iris occurs over a wide variety of environmental conditions. Below is asummary of habitats and environmental conditions it can tolerate (from Sutherland1990; Ramey 2001):Mainly grows at sites with a continuously high soil water content, occurring inwater depths up to 25 cm;Occurs on peat (in fens) as well as submerged organic and inorganic soils at theedges of ponds, lakes and rivers, and within fertile wetlands;Persists in the landward zones of saltmarshes and in dune slacks, and has beenfound surrounded by estuarine water of 24 % salinity;Can occur in waterlogged lower hill-slopes and valleys, and within wetdepressions in terrestrial habitats;Tolerates considerable soil acidity, occurring in soils of pH 3.6-7.7;Has a high nitrogen requirement;Can withstand periods of anoxia (low soil oxygen);Prefers part shade or full sun exposure.Rhizome can withstand considerable drought (at least three months).2.4 ReproductionIn New Zealand, yellow flag iris flowers from September to December. The flowersare insect-pollinated, and produce seed in the first summer following establishment(Tu 2003; Williams and Champion 2008). There appears to be wide variation in thenumber of seeds produced per capsule, ranging from 29 to 120 seeds (Simon 2008,Sutherland 1990). Seed capsules collected along the Waikato River in 2011 had anaverage of 80 seeds per pod (Craig Purvis, Department of Conservation, pers.comm.). Seeds are buoyant due to a hard seed coat and internal gas space, and canfloat for at least seven months (Coops and Van Der Velde 1995). In British studies,germination was not light-dependent, and optimal germination occurred at highertemperatures (20-30ºC) and following scarification (Sutherland 1990). Seeds do notgerminate underwater (Coops and Van Der Velde 1995).© 20112Contract Report No. 2714

Plate 1: Yellow flag iris plants on the edge of an eroding bank on the Waikato River.Thick rhizomes can be seen protruding from the bank. Photograph: Philip Mabin.Plate 2: Fruit capsule of yellow flag iris, containing smooth brown seeds.Photograph: Philip Mabin.© 20113Contract Report No. 2714

Vegetative reproduction is by rhizomes. Rhizomes spread radially, producing largeclonal populations which fragment as they develop (Sutherland 1990). Rhizomes canbe washed downstream during flooding, where they may re-sprout to form newcolonies.3. IMPACTS3.1 Pest statusYellow flag iris is an unwanted organism under the Biosecurity Act (1993) and is aNational Plant Pest Accord species, preventing its’ sale within New Zealand. TheWaikato Regional Council Pest Management Strategy (2008-2013) lists yellow flagiris as a ‘containment pest plant’, and land occupiers are responsible for the totalcontrol of yellow flag iris on their property.In the North Island, yellow flag iris is listed as a regional pest plant in Northland(‘eradication pest plant’), Auckland (‘surveillance pest plant’), and the Bay of Plenty(‘containment pest plant’). In the South Island it is listed as a ‘progressive controlplant’ in the Tasman-Nelson and West Coast regions.3.2 Economic impactsFlood-prone pasture is ideal habitat for yellow flag iris due to it’s preference fornitrogen-rich soils with a high water content (see Section 2.3). In the Lower WaikatoRiver catchment, yellow flag iris has invaded Waikato Regional Council land that isleased for grazing and has the potential to invade most of the license areas betweenthe Waikato River and adjacent stopbanks.While yellow flag iris is usually ignored by most grazing animals, cattle and sheepwill graze it if feed is short (Sutherland 1980). It is reported as being poisonous due toglycosides contained within all parts of the plant with rhizomes (underground stems)being the most toxic (Cooper and Johnson 1984). Symptoms of yellow flag irispoisoning include abdominal pain, gastroenteritis, nausea, vomiting, diarrhoea,spasms, staggering, and paralysis (Tu 2003). Yellow flag iris also contains a resin thatcan cause skin irritations in humans (Williams and Champion 2008).In addition to impacts on pasture quality and quantity, yellow flag iris is known toconstrict drainage channels. Studies in Montana have shown that yellow flag iris canreduce stream width by up to 25 cm per year by trapping sediment, creating a newbank and then dominating the new substrate with its seedlings, creating still moresediment retention (Tyron 2006, unpublished study cited in King County NoxiousWeed Control Program 2007). In northwestern Montana it was reported to haveinvaded 2100 kilometers of irrigation canals (Jacono 2001).Dense stands on the margins of waterbodies can restrict access for fishing and otherrecreational activities. Hamilton City Council has spent considerable resourceseradicating yellow flag iris from Lake Rotoroa, a very popular recreational area(Kemble Pudney, Hamilton City Council, pers. comm.).© 20114Contract Report No. 2714

3.3 Ecological impactsThe ability of yellow flag iris to tolerate a wide range of conditions makes it a threatto many natural habitats that are periodically wet. These include riparian floodplains,lake shorelines, swamps, marshes, fens, and brackish wetlands.In North America, yellow flag iris has invaded and dominated a range of vegetationtypes (Tu 2003). Along the Mercer River in California, it completely displaced Typhaand other indigenous marsh plants within 20 years of establishment (Raven andThomas 1970), although it is possible that other factors also contributed (Thomas1980). Along the riparian floodplains associated with the Lower Waikato River,yellow flag iris appears to displace some of the few indigenous plant communitiesthat still exist, including raupo (Typha orientalis) and Carex sedgeland (author’s pers.obs.).Once established, yellow flag iris develops a thick rhizome mat that can suppressgermination of other plant seedlings and also elevate local topography by trappingsediment and thereby creating a drier habitat (Thomas 1980). This can allow it tospread into previously unsuitable habitat but also enable other species to invade,altering successional trajectories. Along the Potomac River near Washington D.C.,yellow flag iris has contributed to the conversion of riparian marshes to less wetforests dominated by ashes (Fraxinus spp.) rather than willows (Salix spp.) (Thomas1980).While a reasonable range of invertebrates feed on the stems, roots, leaves, fruits andseeds of the plant it has not been recorded in the diet of any bird species (Sutherland1980). In North America wildlife has declined in areas where yellow flag iris hasdisplaced key food species such as Peltandra virginica (Thomas 1980; Cox 1999). Noresearch has been conducted in New Zealand to determine whether it provides anyhabitat value for indigenous fauna.4. CURRENT EXTENT AND POTENTIAL DISTRIBUTION4.1 MethodsThe current extent and potential distribution of yellow flag iris along the LowerWaikato River was mapped during a 2.5 hour helicopter survey from Ngaruawahia toTuakau on 6 April 2011. Aerial photographs at a scale of 1:2,500 were used duringthe low-flying survey, to delineate the extent and density of yellow flag iris and toidentify potential habitat. Density of yellow flag iris was assigned to one of fourcategories, as listed in Table 1.Approximately 500 photographs were taken during the helicopter survey. These wereused, along with Google Map and a report from a ground-based survey conductedbetween Huntly and Te Onetea Stream (Department of Conservation 2010), to refinemaps of yellow flag iris extent and potential distribution.© 20115Contract Report No. 2714

Table 1:Categories used to map the density of yellow flag iris in the lower WaikatoRiver catchment.Density ofYellow Flag Iris% Cover CommentHigh > 40% Contains large swards of yellow flag iris.Medium 10 - 40% Contains medium to large patches of yellow flag iris.Low < 10% Contains small patches or clusters of young plants.Scattered IndividualplantsIsolated patches too small to be mapped. Locations onFigures 1-4 are indicative only.The current extent of yellow flag iris in other parts of the lower Waikato Rivercatchment was based on verbal information from DOC staff who had conductedground surveys at Te Onetea Stream and along the margins of Lake Waikare.Location of yellow flag iris in other areas was based on recent surveys by WildlandConsultants Ltd at Opuatia Wetland (Wildland Consultants 2010 and 2011), LakeWhangape (report in preparation), Waikato Regional Council-administered landwithin the lower Waikato River catchment (Wildland Consultants 2010), and theauthor’s personal observations of yellow flag iris at Whangamarino Wetland(February 2011) and Weavers Lake (June 2011).Potential distribution of yellow flag iris in other parts of the Lower Waikato Rivercatchment was determined for the Whangamarino Wetland only. This was the onlyarea for which a recent vegetation map (Wildland Consultants 2008) sufficientinformation to delineate potential habitat was available.ARC GIS 9.3 was used to digitise the extent and potential distribution of yellow flagiris and to calculate the areas of yellow flag iris of different densities.4.2 ResultsAn overview of the area currently occupied by yellow flag iris at different densities isprovided in Table 2, with a total of 258 ha mapped in the lower Waikato Rivercatchment (including the Whangamarino Wetland). Approximately 50 km of riverbank and island shoreline contained isolated patches of yellow flag iris and576 hectares of potential habitat was identified along the Lower Waikato River. Twohundred and seventy-three (273.2) hectares of potential habitat were identified in theWhangamarino Wetland. These figures are likely to be conservative as the densityand potential habitat of yellow flag iris is difficult to assess accurately from ahelicopter due to other taller species (e.g. alder and willow) obscuring large areas ofhabitat.Table 2:Area of yellow flag iris at different densities within parts of the LowerWaikato River Catchment.Density of Yellow Flag IrisArea (ha)High 57.9Medium 143.2Low 56.8Total 257.9© 20116Contract Report No. 2714

The following sections provide descriptions of the location, extent and density ofyellow flag iris in different parts of the Lower Waikato River Catchment.4.2.1 Waikato River from Ngaruawahia to OhinewaiThe current extent and potential distribution of yellow flag iris between Ngaruawahiaand Ohinewai is shown in Figure 1. A few small patches of yellow flag iris occurupstream of the SH1 bridge at Ngaruawahia on the true left bank. There is littlesuitable habitat upstream as the river is confined within a steep-sided channel.Downstream of Ngaruawahia, yellow flag iris is scattered along most of the true rightbank to Taupiri. No plants were observed on the opposite bank, despite large areas ofpotential habitat (Plate 3).Low to medium density infestations are common between Taupiri and Huntly(Plate 4), occurring on both river banks and some of the river islands.Two large, medium density infestations occur within the Huntly township, on alluvialfloodplains adjacent to the Waikato River. One of these occurs just upstream of theconfluence of the Waahi Stream and the Waikato River (Plate 5). It is not knownwhether yellow flag iris is present at Lake Waahi, but two patches (c.5 m 2 ) wereobserved in wet depressions next to the adjacent Weavers Lake. Both Lake Waahi andWeavers Lake contain habitat suitable for yellow flag iris.Some of the densest infestation of yellow flag iris occur along the Lower WaikatoRiver occur between Huntly and Ohinewai. Typical habitats include old river courses(Plate 6) and alluvial floodplains (Plate 7). Most of the true right bank betweenHuntly and Ohinewai contains isolated patches of yellow flag iris.Indigenous vegetation cover is extremely limited alongside the Waikato River and itsassociated floodplains between Ngaruawahia and Ohinewai. Most of the riverbanksand wetlands are dominated by exotics trees such as alder (Alnus glutinosa), crackwillow (Salix fragilis), and grey willow (Salix cinerea), with a mixture of exotic andindigenous herbaceous plants occurring in areas without a tree canopy. Reed sweetgrass (Glyceria maxima), Persicaria species and water purslane (Ludwigia palustris)are some of the more common herbaceous species. Indigenous species such ascabbage tree (Cordyline australis) and flax (Phormium tenax) are more common onthe river islands, although exotic vegetation remains the dominant component. Whilevegetation quality is low, there are significant areas of habitat for indigenous fauna.4.2.2 Waikato River from Ohinewai to Orton, Lake Whangape andOpuatia WetlandThe current extent and potential distribution of yellow flag iris between Ohinewai andOrton and at Lake Whangape and Opuatia Wetland is shown in Figure 2. Almost theentire length of both banks of the Waikato River between Ohinewai and Rangiriricontains yellow flag iris at varying densities (Plate 8). There are large areas ofsuitable habitat along this part of the river, including the largest area of alluvialfloodplain encountered in the survey (Plate 9). There are also numerous islands alongthis stretch of the river, all of which contain yellow flag iris. The largest island,Manutahi, which is administered by Waikato Regional Council, contains large areasof suitable habitat with high densities of yellow flag iris (Plate 10).© 20117Contract Report No. 2714

Plate 3: Potential habitat for yellow flag iris, along the true left bank of theWaikato River near Taupiri. Photograph: Craig Purvis.Plate 4: A medium density infestation of yellow flag iris on the true left bankof the Waikato River near Taupiri. Photograph: Philip Mabin.© 20118Contract Report No. 2714

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Plate 5: Part of a large alluvial floodplain immediately upstream of theconfluence of Waahi Stream and the Waikato River. Yellow flag iris occursat medium densities within this area. Photograph: Craig Purvis.Plate 6: An old river course on the true left bank, north of Huntly. Some of the highestdensity infestations occur in this type of habitat. Photograph: Craig Purvis.© 201111Contract Report No. 2714

Plate 7: Alluvial floodplain opposite Fisher Rd, south of Ohinewai.This area contained some of the highest density infestations seen duringthe helicopter survey. Photograph: Philip Mabin.Plate 8: Yellow flag iris along the banks of the Waikato River betweenOhinewai and Rangiriri. Photograph: Philip Mabin.© 201112Contract Report No. 2714

Plate 9: The largest area of alluvial floodplain seen during the helicopter survey. It islocated between the Waikato River and Te Ohaaki Road, and contains medium andhigh density infestations of yellow flag iris. Photograph: Craig Purvis.Plate 10: A high density infestation of yellow flag iris on Manutahi Island,near Rangiriri. Photograph: Philip Mabin.© 201113Contract Report No. 2714

There were large infestations of yellow flag iris at the Te Onetea spillway, althoughthese have been reduced to scattered clumps by aerial spraying (Plate 11).In the 2 km stretch between the Horahora Bridge and the confluence of LakeWhangape with the Waikato River, there are a high number of low to medium densityinfestations. Water from the Waikato River backs up the Whangape Stream duringfloods, accounting for the occurrence of yellow flag iris on the banks of theWhangape Stream and Lake Whangape (Plate 12). At one location in LakeWhangape, yellow flag iris appears to have been planted.A recent survey of Lake Rotongaro and Rotongaro-iti (Wildland Consultants 2011a)did not find any yellow flag iris. A floodgate where the Rotongaro Canal joins theWhangape Stream prevents water from the Waikato River backing up the canal andunless yellow flag iris is deliberately introduced into the catchment, it is unlikely toestablish at these lakes.The extent and density of yellow flag iris decreases along the Waikato River betweenthe outlets of the Whangape Stream and the Opuatia Stream. However as floodwatersfrom the Waikato River back up the Opuatia Stream, yellow flag iris is found withinparts of the Opuatia Wetland. It is probably a recent arrival as it was not recorded in acomprehensive species list for Opuatia Wetland updated in 2006 by de Lange (2006).Grey willow and crack willow occupy most of the suitable habitat at Opuatia Wetlandfor yellow flag iris, but aerial spraying of willow in 2007 on the margins of an area ofpeat bog managed by Waikato Regional Council may have facilitated thedevelopment of a medium density infestation (1.62 ha) at the northern end, close tothe causeway (Plate 13). Yellow flag iris appears to be spreading southward from thislocation and is present in surprisingly dense vegetation (Plate 14), (WildlandConsultants 2011b).From the Opuatia Stream confluence with the Waikato River to Orton, habitat suitablefor yellow flag iris is considerably reduced with banks frequently steep along thissection. The worst infestations along this stretch of the Waikato River occur near theTe Kauwhata water intake and at Karihoa Island.Indigenous vegetation cover alongside the Waikato River and its’ associatedfloodplains between Ohinewai and Orton is very similar to the previous section of theWaikato River. However, kahikatea (Dacrycarpus dacrydiodes), cabbage tree, andCarex sedgeland are more abundant amongst the willow and alder forests on alluvialfloodplains. This type of habitat is very suitable for yellow flag iris and, as a result,large areas of potential habitat have been identified along this stretch.© 201114Contract Report No. 2714

Plate 11: Te Onetea spillway, where aerial spraying has reduced yellow flag iristo low densities. Photograph: Philip Mabin.Plate 12: Yellow flag iris appears to have arrived relatively recently on thenorth-eastern shoreline at Lake Whangape. Note that the plants are quite young asthey haven’t yet formed radial clumps and are located along the ‘high tide’ mark.Photograph: Paula Reeves.© 201115Contract Report No. 2714

Plate 13: A medium density area of yellow flag iris amongst reed sweetgrass atthe Opuatia Wetland. Aerial spraying of this site occurred in 2007.Photograph: Paula Reeves.Plate 14: Yellow flag iris amongst dense Baumea sedgeland at theOpuatia Wetland. Photograph: Paula Reeves.© 201116Contract Report No. 2714

4.2.3 Waikato River from Orton to TuakauThe current extent and potential distribution of yellow flag iris between Orton andTuakau is shown in Figure 3. Immediately north of Orton is the privately-ownedPunga Punga Wetland (384 ha) which lies adjacent to the Waikato River on the trueleft bank (Plate 15). This area is a key ecological site for pest control in private tenurein the Waikato Region with a priority ranking of 13 of 109 sites (WildlandConsultants 1999). This area is mostly grey willow forest, but also contains areas ofkahikatea forest, manuka shrubland, raupo reedland, and Cyperus ustulatus sedgeland.Only the edge of the wetland was viewed during the helicopter survey. Yellow flagiris was scattered along the edges of some of the kahikatea stands, with several smallto medium-sized areas where it occurred at low densities (Plate 16). It is likely to bepresent throughout other parts of the wetland and there are large areas of potentialhabitat.Between the Punga Punga Wetland and Mercer, yellow flag iris mainly occurs at lowdensities along both banks of the Waikato River, although medium density areasoccur in some backwaters (Plate 17). Large areas of potential habitat have beenidentified along this stretch.As the Waikato River heads west away from Mercer, the potential habitat for yellowflag iris increases substantially. Grazed kahikatea forest is common on floodplainsalong this part of the river and high density areas of yellow flag iris are often presentadjacent to them (Plate 18). There are also a number of large river islands which havealso been identified as potential habitat along this stretch. The islands currentlycontain only scattered clumps or low density areas of yellow flag iris (Plate 19).The Waikato River and the islands between Rangiriri and Port Waikato (4,155 ha),were identified as a key ecological site for pest control on private land, with a priorityranking of 21 out of 109 (Wildland Consultants Ltd 1999). While the vegetation isheavily modified and dominated by exotic species, the site is considered to be a goodrepresentative example of a lowland river system that supports nationally uncommonand rare plant and animal species. The site also meets the criteria for a wetland ofinternational significance (Cromarty and Scott 1995).4.2.4 Lake Waikare and Whangamarino WetlandThe current extent of yellow flag iris in Lake Waikare, and the current and potentialextent of yellow flag iris in Whangamarino Wetland are shown in Figure 4 (note thatthe location of yellow flag iris is only indicative at Lake Waikare).Yellow flag iris has been found scattered around the margins of Lake Waikare.Occurrences appear to be recent and are probably from seed that has entered the lakevia the Te Onetea Stream (Craig Purvis, Department of Conservation, pers. comm.).At Whangamarino Wetland, yellow flag iris has only been found at two locations. Asingle plant was found next to an old causeway at the northern end of the Reao Armin February 2011. A large clump is also present on the bank of the WhangamarinoRiver just before the confluence with the Maramarua River, and has probably beenthere for several years.© 201117Contract Report No. 2714

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Plate 15: Punga Punga Wetland near Pukekawa contains large areas of potentialhabitat for yellow flag iris. Photograph: Craig Purvis.Plate 16: An area within the Punga Punga wetland with a low density ofyellow flag iris. Photograph: Craig Purvis.© 201120Contract Report No. 2714

Plate 17: An area of medium density yellow flag iris in a backwater of the WaikatoRiver, opposite the Meremere Dragway. Photograph: Craig Purvis.Plate 18: A high density area of yellow flag iris next to kahikatea forest at WestHayward Road Bush, a key ecological site for pest control on private land.Photograph: Craig Purvis.© 201121Contract Report No. 2714

Plate 19: Namuheiriro Island, near Tuakau, is potential habitat for yellow flag iris.Currently only scattered clumps occur at the western edge of the island.Photograph: Craig Purvis.Potential habitat at Whangamarino Wetland includes areas of swamp and marshwhere water levels are unlikely to exceed 30 cm depth for more than several weeks ata time. It also includes the margins of fens where the willow canopy has beenremoved by aerial spraying and which are likely to receive floodwaters periodically.The recent repair of the weir has probably raised water levels enough to preventyellow flag iris establishing in large areas of open marsh, but has probably made someareas of swamp and fen more vulnerable.Both Lake Waikare and the Whangamarino Wetland have high conservation values.Despite Lake Waikare being highly modified by flood protection works, it still retainssignificant habitat for a range of indigenous fauna including threatened avifauna(Wildland Consultants 2009). The Whangamarino Wetland (7,100 ha) is recognisedboth nationally and internationally for its high conservation values, and is one of fourRamsar sites in the North Island. It is one of the few remaining wetlands of its typeand size, being a complex of four wetland types: marsh, swamp, fen, and bog. Itcontains a number of threatened plants and animals, including the largest populationsin New Zealand of the nationally endangered Australasian bittern (Department ofConservation 2010). Australasian bittern prefer habitat dominated by open water andreeds, which could partly be displaced if yellow flag iris spreads into the large areasof potential habitat at Whangamarino Wetland.© 201122Contract Report No. 2714

5. MANAGEMENT OF YELLOW FLAG IRIS5.1 Control methodsIn New Zealand, the method recommended for controlling yellow flag iris is chemicalcontrol using either glyphosate or metsulfuron. Manual control is not considered to beeffective (Williams and Champion 2008). While the herbicides recommended areeffective at killing yellow flag iris, repeat applications are usually required to combatre-infestation from nearby sources, and to control seedlings at sites where a seed bankhas developed. At Lake Rotoroa in Hamilton, yellow flag iris seeds germinatedprofusely on bare areas left as the rhizomes decayed, and continued to do so for up tofive years following the initial herbicide treatment (Kemble Pudney, Hamilton CityCouncil, pers. comm.).5.2 Cost of controlThe cost of controlling yellow flag iris will depend on a range of factors, including thedensity and length of time it has been present at a location, access to the site, and thesensitivity of surrounding vegetation. It is therefore difficult to estimate the cost ofcontrolling the current extent of yellow flag iris currently present in the LowerWaikato River catchment.Table 3 summarises cost estimates for herbicide control of different densities ofyellow flag iris, based on the experience of the restoration team at WildlandConsultants Ltd, Philip Mabin (West Waikato Weeds Ltd), and Department ofConservation (Waikato Area Office).Table 3: Cost estimates (labour and herbicide) for chemical control of yellow flag irisat different densities.Density of Yellow Flag Iris Cost per HectareHigh (cover > 40%) $1,350+Medium (cover 10 - 40%) $475-$1,350Low (cover < 10%) $340-$675Scattered (isolated patches) $100-$340Aerial spraying with herbicide (c.$450 per hectare) could substantially reduce controlcosts and could be used in areas where damage to indigenous vegetation would beminimal. Suitable areas include wet pasture (e.g. Te Onetea spillway), and areasalmost exclusively dominated by exotic species (e.g. alluvial floodplain south of theHuntly Bridge, on the true right bank).Control of yellow flag iris has been undertaken in the Lower Waikato Rivercatchment in the past year, providing further indication of costs. Cost of labour (288hours) to control yellow flag iris along c.23 km of shoreline at Lake Waikare inNovember 2010 was $18,720 (Craig Purvis, Department of Conservation, pers.comm.). Searching for and controlling yellow flag using gun and hose or knapsackwas done alongside the Waikato River, on the true left between Ngaruawahia andHuntly and on the true right between Ngaruawahia and just north of Taupiri inAutumn 2011. This cost $7,753 and included labour (155 hours), mileage, andherbicide costs (Wendy Mead, Waikato Regional Council, pers. comm.).© 201123Contract Report No. 2714

5.3 Priority areas for controlThe current extent of yellow flag iris in the Lower Waikato River catchment is suchthat eradication is probably not a realistic goal, especially in areas of highest densities(i.e. Huntly to Manuatahi Island, north of Rangiriri). However it may be possible toeradicate it from areas where current densities are very low and the chance of reinfestationare not high due to the lack of upstream sources. These areas wouldinclude:Margins of the Waikato River between Ngaruawahia and the southern outskirts ofHuntly (before the industrial area);Lake Whangape;Weavers Lake.In addition, a high priority should be assigned to areas of high conservation value thatcontain large areas of potential habitat but where infestation levels are currently stilllow even though the risk of re-invasion is high. These areas are listed below, in orderof relative priority:Whangamarino Wetland;Lake Waikare;Opuatia Wetland;Lake Whangape;Namuheiriro Island (near Tuakau);Te Weranga Okapu Island (near Tuakau).Of lesser importance, but also a high priority, are tributaries of the Waikato River thatfeed into some of the areas of high conservation value listed above. These wouldinclude:Te Onetea Stream;Opuatia Stream;Whangape Stream.There are other areas with high conservation values where the current extent andpotential distribution of yellow flag iris is not well known. Investigation of these sitesis recommended before determining further priorities. These areas are listed below:Lake Waahi (highest ranking riverine lake in the Waikato District for biodiversitymanagement, Wildland Consultants 2009);Punga Punga Wetland (only the area adjacent to Waikato River was included inthe current survey);Areas of indigenous forest and wetlands on the alluvial floodplains and riverineislands on the Waikato River between Tuakau and Port Waikato.In addition to the above approach, a high priority should be given to controllingyellow flag iris in the vicinity of sites where alligator weed (Alternanthera© 201124Contract Report No. 2714

philoxeroides) is present in the Lower Waikato River catchment. These species occurin very similar habitats, and preventing the spread of alligator weed into densethickets of yellow flag iris is likely to reduce future costs of controlling alligator weedat these sites.6. DISCUSSIONYellow flag iris is widespread in the Lower Waikato River catchment. The earliestrecord of its’ presence in the catchment is from a survey of Lake Hakanoa, at Huntly,in January 1990 where it occurred as ‘scattered clumps on the north shore’ (de Wintonand Champion 1993). It seems likely that this was the original source of theinfestation in the Lower Waikato River catchment but it is possible that it may alsohave been planted in ponds or on the banks of the Waikato River in the vicinity ofOhinewai, given its current density and distribution.Over the last 5-10 years it appears that yellow flag iris has undergone a dramaticincrease in abundance in the lower Waikato River. This is probably due to seedproduction reaching levels that allow it to dominate the seed bank in areas of suitablehabitat. High levels of nitrogen in waterways of the Lower Waikato River catchmentundoubtedly enhance the spread of this nitrophilic species.The cost of controlling yellow flag iris is likely to be substantial, particularly if theopportunity is taken to revegetate control sites to prevent re-invasion. However thereare also substantial financial and ecological costs if its’ spread continues given theextent of potential habitat which includes large areas of high conservation value andagricultural production.ACKNOWLEDGMENTSPhilip Mabin (West Waikato Weeds Ltd) and Craig Purvis (Department of Conservation),assisted in the helicopter survey, and provided photographs, reports and costings of yellowflag iris control. Richard Guest (Wildland Consultants Ltd) also assisted with information oncontrol costs. Kemble Pudney (Hamilton City Council) provided information on the yellowflag iris eradication programme at Lake Rotoroa.REFERENCESChampion P.D., Clayton J.S. 2001: Border control for potential aquatic weeds: Stage 2. Weedrisk assessment. Science for Conservation 185, Department of Conservation,Wellington. 30p.Champion P.D., de Winton M.D., de Lange P.J. 1993. The Vegetation of the Lower WaikatoLakes. Volume 2: Vegetation of thirty-eight lakes in the Lower Waikato. NIWAEcosystems Publication No. 8. Hamilton. 137p.Cooper M.R., Johnson A.W. 1984: Poisonous plants in Britain and their effects on animalsand man. Her Majesty's Stationery Office, London, England. 305 p.© 201125Contract Report No. 2714

Coops H., Van Der Velde G. 1995: Seed dispersal, germination and seedling growth of sixhelophyte species in relation to water level zonation. Freshwater Biology 34: 13-20.Cox G.W. 1999. Alien species in North America and Hawaii: Impacts on natural ecosystems.Island Press, Washington D.C.Cromarty, P. and Scott, D.A. (Eds) 1995: A Directory of Wetlands in New Zealand.Department of Conservation, Wellington, New Zealand.de Lange P.J. 2006: Indigenous vascular flora of Opuatia Wetlands: www.nzpcn.org.nz/publications/Opuatia%20Vascular%20Plant%20List.pdf, accessed 26/1/2011.Department of Conservation, 2010: Yellow Flag Iris (Iris pseudacorus) Survey of the LowerWaikato River. Internal document, Docdm-571032, Waikato Area Office, Hamilton.Jacono C.C. 2001: US Geological Survey Invasive Fact Sheet: Iris pseudacorus.http://nas.er.usgs.gov/taxgroup/plants/fact%20sheets/Iris%20pseudacorus.pdf.Accessed 8 February 2011.King County Noxious Weed Control Program. 2007: Best management practices: yellow-flagiris (Iris pseudacorus) Iridaceae. http://your.kingcounty.gov/dnrp/library/water-andland/weeds/BMPs/yellow-flag-iris-control.pdf.Accessed 8 February 2011.Kirk T. 1878: On the naturalised plants of Port Nicholson and the adjacent district.Transactions of the New Zealand Institute 10: 362-78.Ramey V. 2001: Iris pseudacorus L. Non-Native Invasive Aquatic Plants in the United:States Center for Aquatic and Invasive Plants, University of Florida and Sea Grant.http://aquat1.ifas.ufl.edu/seagrant/iripse2.html. Accessed 20 April 2011.Raven P.H., Thomas J.H., 1970: Iris pseudacorus in western North America. Madrono 20:390-391.Simon B. 2008: Aquatic Weeds Management Fund Final Report: Yellow Flag Iris Controland Eradication. Washington State Department of Agriculture, USA.Sutherland W.J. 1990: Biological Flora of the British Isles No. 169 Iris pseudacorus L.Journal of Ecology 78: 833-848.Thomas L.K. 1980: The impact of three exotic plant species on a Potomac Island. NationalPark Service Scientific Monograph Series, Number 13.Tu M. 2003. Element Stewardship Abstract for Iris pseudacorus L. The Nature Conservancy,United States of America. http://tncweeds.ucdavus.edu. Accessed 25 August 2011.Wildland Consultants 1999: Key ecological sites for pest control in private tenure in WaikatoRegion - Waikato District and Part Franklin District. Wildland Consultants LtdContract Report No. 236. Prepared for Environment Waikato. 411p.Wildland Consultants 2008: Whangamarino Wetland Vegetation 2008. Wildland ConsultantsLtd Contract Report No. 2020. Prepared for Department of Conservation.© 201126Contract Report No. 2714

Wildland Consultants 2009: Significant Natural Areas of the Waikato Region: LakeEcosystems. Wildland Consultants Ltd Contract Report No. 2091b. Prepared forEnvironment Waikato. 436p.Wildland Consultants 2010: Biodiversity enhancement opportunities for EnvironmentWaikato land in the Lower Waikato Zone: Volume 2. Wildland Consultants LtdContract Report No. 2368. Prepared for Environment Waikato. 160p.Wildland Consultants 2011a: Vegetation Assessment of Lake Rotongaro WildlifeManagement Reserve. Wildland Consultants Ltd Contract Report No. 2712. Preparedfor Department of Conservation. 27p.Wildland Consultants 2011b: Opuatia Peat Bog Management Plan. Wildland Consultants LtdContract Report No. 2621. Prepared for Waikato Regional Council. 34p.Wildland Consultants (in preparation): Restoration potential of Department of Conservationadministeredland at Lake Whangape. Wildland Consultants Ltd Contract ReportNo. 2713. Prepared for Department of Conservation.Williams P.A., Champion P.D. 2008: Biological success and weediness of existing terrestrialpest plants and aquatic weeds in Northland. Landcare Research Contract ReportLC0708/080. Prepared for Northland Regional Council.© 201127Contract Report No. 2714