Weed Strikeforce and Kauhuria Te Waihora

New Zealand has some of the most beautiful lakes and rivers in the world, but we need to stop the spread of freshwater pests so that our waterways stay that way.

It only takes one pair of tramping boots, or a wet lifejacket to give a hitchhiking freshwater pest a ride to a new location, where it can establish and cause damage. Once there, the detrimental effect the pests can have can diminish the quality of our recreational experience.

Freshwater pests like didymo spoil river activities for people and they also have the potential to affect the habitats of freshwater plants and animals. A number of aquatic pests, including purple loosestrife, Beggar’s Tick and plants like Carex pendula, reed canary grass, and Phragmites affect water margins, that are, or could be, of significance to Te Waihora and its tributary streams and drains. Lagarosiphon, Egeria and hornwort could be an issue if they got established in the Te Waihora catchment.

There are a number of measures people can employ to reduce the risk of freshwater pests.

Check and take debris off your equipment and clean it with soapy water before moving to another waterway to stop unknown pests moving about. Find out more about check, clean and dry.

Gorse seeds first arrived in New Zealand in the early 1800s in response to the English settlers’ desire for hedging. A natural lack of control agents in New Zealand, combined with high seed production, unpalatability to stock and invasive, colonising growth habitats, have allowed gorse to become the widespread problem it is today.

Gorse (Ulex europaeus), is now a major invasive plant species in New Zealand and millions of dollars are spent on its control. As one of the country’s most widely recognised agricultural weeds, it covers around 700,000 hectares – a total of 5% of New Zealand land area, excluding existing indigenous forest and vegetated alpine and sub-alpine areas.

Gorse seed can lie dormant in the ground for up to 50 years and unfortunately, when adult gorse plants are removed, ideal conditions are created for that dormant seed to germinate.

Biological control of gorse has been investigated since the 1920s but large-scale control of this pest plant has not been achieved. One method that does appear to have some success is the germination of native tree and plant seedlings that can grow up through leggy, adult gorse plants, eventually cutting out its light and replacing it. This technique has been used successfully at Hinewai Reserve on Banks Peninsula.

The ice plant (Carpobrotus edulis) is long-lived and is a common sight around some parts of Te Waihora, particularly along Kaitorete Spit and the beach areas just south of the spit. The species reproduces by seed and vegetatively via its fleshy stems.

Carpobrotus edulis and its hybrids are classed as unwanted organisms in New Zealand and are listed on the National Pest Plant Accord. This native to South Africa is known by a host of names – Hottentot-fig, sour fig, Cape fig and many more – but in New Zealand it is most commonly called ice plant and it has a long history as a pretty ornamental garden plant.

It thrives in many sub-tropical regions throughout the world and in a number of those it has escaped cultivation and become invasive, creating a serious ecological problem by forming vast, mono-specific zones, lowering biodiversity and competing directly with several threatened or endangered plants for nutrients, water, light and space.

In New Zealand, the species has become naturalised in coastal areas, sandy pastures, along cliff tops, sand dunes and roadside cuttings, and it frequently displaces other vegetation. It can also change the structure of sand dunes. The South African ice plant species (Carpobrotus edulis), hybridises with and replaces the much smaller, pale pink-flowered native ice plant (Disphyma australe).

Carpobrotus edulis is a creeping, mat-forming succulent species and a member of the stone plant family (Aizoaceae), one of about 30 species in the Carpobrotus genus. C. edulis is easily confused with its close relatives, with which it hybridises easily, although it can be distinguished by the colour of its flowers – large, yellow or light pink blooms. Carpobrotus chilensis has smaller, deep magenta flowers.

The fruit of Carpobrotus edulis is edible when fresh and reportedly has a strong, astringent, salty, sour taste. The leaf juice is astringent and mildly antiseptic and has been used as a remedy for everything from sore throats and skin rashes to constipation and tuberculosis.

Viper’s bugloss or blueweed (Echium vulgare) is an invasive weed in Australia and New Zealand. In the summer months, it is frequently seen growing around Te Waihora where it has successfully displaced many other plants. This densely bristled annual or biennial herb has a long tap root and grows to around 50-90cm high.

The pretty blue-pink flowers of the Viper’s Bugloss belie the fact that it is unpalatable to livestock and that its bristles can cause skin irritation.

Echium vulgare is a species of Echium native to most of Europe, and western and central Asia. The flowers start pink and turn vivid blue and are 15–20 mm in a branched spike, with all the stamens protruding.

Stamens remain red and stand out against the blue flowers, producing a good honey flow when most other nectar sources have dried up.

Purple loosestrife was previously sold as a garden plant but has been declared an unwanted organism in New Zealand by the Biosecurity Act 1993 and consequently is banned from sale, propagation or division.

Characterised by its pretty, tall, purple-pink flower spires, the plant can grow as high as three metres and have as many as fifty stems per plant. It flowers from December to February and it can produce over two million seeds per plant per year – and most seeds can last several years. The seeds are dispersed by water but may also be spread by birds and by people sharing garden cuttings.

Purple loosestrife thrives in damp places, particularly around wetlands, garden ponds or along river and lake margins. Once established, it quickly forms a dense stand that excludes most other vegetation. It also has the potential to clog waterways and impede drainage

Within the Te Waihora catchment, purple loosestrife is found in some private gardens and along many of the drains that run into the Huritini/Halswell River. Te Papa Atawhai (Department of Conservation) and Environment Canterbury control these areas every season with glyphosate. Since starting control of purple loosestrife, its density within the area has reduced dramatically.

Yellow 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 dense vegetation 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 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.

There is a large infestation of yellow flag iris at Waitātari/Harts Creek – easily visible from the walkway leading out to the bird hide. There are scattered populations and individual plants all around the catchment along waterways (e.g., Waikiriri/Selwyn River near the upper huts, Huritini/Halswell River canal). To protect the high value ecological areas, scattered plants will be controlled where feasible (e.g., with landowner permission) around the lake margins. Sightings can be reported to Environment Canterbury (biosecurity or biodiversity teams) or recorded on the iNaturalist NZ Mātaki Taiao website. https://inaturalist.nz/.

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.

Reed canary grass was first recorded in Te Waihora lake shore wetlands in 2009. Reed canary grass is becoming a significant weed species around Te Waihora.

Reed canary grass (Phalaris arundinacea), is a tall perennial grass that grows to around two metres in height. Its leaf blades are flat and often harsh on both sides; and it has long, creeping rhizomes. Spreading flower/seed panicles rise from hairless stems and can be green, purple or brown.

Reed canary grass is an internationally renowned weed of wetlands, where it forms dense sprawling stands up to 1m tall that smother and impede the regeneration of other vegetation.

The dense mats impede access and drainage, causing silt accumulation and flooding. It can replace other aquatic margin vegetation and degrade habitat for aquatic fauna. It has the potential to take over wetland margins and the margins of other water bodies and drains, and it represents a significant threat to wetlands where it can replace native species.

The plant is very hardy, it grows rapidly and it spreads easily – through both seeds and rhizomes. It is difficult to eradicate once established. It particularly favours moist, fertile or wet soils, including under the shade of willows and open kahikatea forest, wet grassland, margins of water bodies, wet waste areas and roadsides.

It is tolerant of freezing conditions. It has been found in Canterbury gardens and ponds and its variegated version (with cream-striped leaves), is cultivated as an ornamental plant. Unfortunately, it too, has ‘escaped’ and spread widely in many places.