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Aquatic Pest Plants

Aquatic Pest Plant Control Programme

Invasive pest plants are negatively impacting stream health in Nelson. Nelson City Council is implementing an invasive pest plant control programme to stop their spread and reduce further damage to the region's waterways.

The main plants being targeted are Water celery, Vietnamese parsley and Parrot’s feather.

Treatment will begin in the week starting 25 October 2021, and will be repeated in March to June 2022, taking fish spawning, migration and weather conditions into account. Treatment of each stream area will be done within a day, starting with Orphanage Stream which is the most impacted by the pest plants. Physical signs will be placed near areas that are being treated. Please do not harvest plants or animals (e.g. tuna) from the treated area for at least two weeks after treatment. Keep dogs and children away from the area during treatment. Triclopyr is low in toxicity if inhaled and is not absorbed well through the skin, however Council asks that people please take care and avoid the areas being treated.

These pest plants form dense mats that block sunlight and deoxygenate the water, smother indigenous plants and trap sediment, and reduce habitat for stream life, including fish, eels (tuna) and macroinvertebrates. Choked streams can also exacerbate flood impacts, threatening infrastructure such as bridges and pathways.

Attempts to remove these aquatic pest plants by hand or mechanical removal have had limited success. These plants are easily spread from seeds and root sections carried along the watercourse. Remaining stems and seedlings germinate and lead to recolonisation and regrowth, reaching nuisance proportions within a single growing season.

National Institute of Water and Atmospheric Research (NIWA) has advised that herbicide application is likely to be the most successful control technique and has the potential to eradicate infestations over time. Use of Triclopyr triethylamine (Garlon 360) is recommended because it does not persist in the environment and has given excellent field control results. It readily degrades in water and soil and has low toxicity to fish and aquatic invertebrates.

Following the successful trial to control water celery in a section of Orphanage Stream in June 2021, Council will conduct a more extensive control of this and other aquatic pest plants in Nelson streams from mid-late October 2021, using this approved herbicide.

Triclopyr triethylamine (Garlon 360) will be applied with a backpack sprayer (100ml/10L) to control Water celery (Apium nodiflorum) in Orphanage Stream, Saxton Creek, Jenkins Creek, Orchard Stream, Arapiki Stream, and Brook Stream; Vietnamese parsley (Oenanthe javanica) in Poorman Valley Stream; and Parrot’s feather (Myriophyllum aquaticum) in Hillwood Stream and a site along Cable Bay Road.

This plant control is carried out under Resource Consent RM205133 and Environmental Protection Authority permissions, using GROWSAFE-trained contractors and industry best practice. Council will be monitoring stream health during and after herbicide application.

Pest plants targeted:

Water celery (Apium nodiflorum)

Water celery (Apium nodiflorum also known as Heloscadium nodiflorum) is a major weed in in the North Island, majorly obstructing both flowing and still bodies of water. In recent years, it has become firmly established in Nelson waterways, and is especially prolific in Stoke streams. Water celery poses the following significant threats:

  • It forms dense mats that block sunlight and deoxygenate the water, smothering local flora
  • The thick mats trap sediment and reduce habitat for stream life, including fish, eels (tuna) and macroinvertebrates
  • Choked streams can exacerbate flood impacts, threatening infrastructure such as bridges and pathways
  • Water celery is so successful that it outcompetes and displaces indigenous plants.

Vietnamese parsley (Oenanthe javanica)

 This invasive weed appears to be in the first stages of naturalisation in New Zealand. It is edible so is likely to be cultivated for culinary as well as ornamental purposes. Vietnamese parsley was detected in Poorman Valley Stream in 2017 and is increasingly smothering areas of this well-loved waterway.

 Parrot’s feather (Myriophyllum aquaticum)

Parrot’s feather (Myriophyllum aquaticum) is listed as an unwanted organism under the Biosecurity Act 1993 meaning it is illegal to sell, distribute or propagate it. It forms dense floating mats, shading out existing native species and preventing new seedlings of native species from establishing, and replaces species that usually grow on the margins of waterbodies. Large clumps can cause flooding if dislodged.

Herbicide details: Triclopyr triethylamine (Garlon 360)

Triclopyr triethylamine is a selective systemic herbicide used to control woody and broadleaf plants as recommended by NIWA. It readily degrades in water and soil, and does not persist in the environment.

Triclopyr also has a low potential for bioaccumulation. It is described as having a moderate to high toxicity in mammals; moderate toxicity in birds; and low toxicity in fish and aquatic invertebrates.

Risk assessments detailing the ecological values have been undertaken for each of the streams and the monitoring recommended to mitigate the herbicides use (see Table 1 below).

Monitoring details:

Water samples will be taken within 2 hours of application, and again after the first heavy rain following treatment – approximately midway in the treatment area and 100m downstream of the spray area – to assess whether trace concentrations of dissolved herbicide are detectable and within Environmental Exposure Limits (EELs) for flowing water.

Annual monitoring at the Council’s State of the Environment (SoE) sites will be used to provide context to the monitoring data and potential effects of future years of herbicide application. Native fish communities will be assessed over time by annual surveys of Stoke streams for fish spawning, mainly for bullies (October) and inanga (March-May).

Application areas:

 

 Figure 1: Total length of approximately 2.1 km of Orphanage Stream (bright and dark green) proposed to be sprayed with triclopyr (GarlonTM 360) for the control of water celery (Apium nodiflorum)

Figure 2: Total length of approximately 1.9 km of Saxton Creek (dark green) proposed to be sprayed with triclopyr (GarlonTM 360) for the control of water celery (Apium nodiflorum)

Figure 3: Total length of approximately 2 km of Jenkins Creek (dark green) proposed to be sprayed with triclopyr (GarlonTM 360) for the control of water celery (Apium nodiflorum)

Figure 4: Total length of approximately 1.2 km of Orchard Stream (dark green) proposed to be sprayed with triclopyr (GarlonTM 360) for the control of water celery (Apium nodiflorum)

Figure 5: Total length of approximately 950 km of Arapiki Stream (dark green) proposed to be spot sprayed with triclopyr (GarlonTM 360) for the control of water celery (Apium nodiflorum), dashed green line represents culverted sections of the stream

Figure 6a: Brook Stream, from Cummins Street to Westbrook Terrace (2.2 km marked in dark green) proposed to be spot sprayed with triclopyr (GarlonTM 360) for the control of water celery (Apium nodiflorum)

Figure 6b: Brook Stream, from Westbrook Terrace to the Maitai River (2.1 km marked in dark green) proposed to be blanket sprayed with triclopyr (GarlonTM 360) for the control of water celery (Apium nodiflorum)

Figure 7: Total length of approximately 1.45 km (in green) of Poorman Valley Stream proposed to be sprayed with triclopyr (GarlonTM 360) for the control of Vietnamese parsley (Oenanthe javanica)

Figure 8: 560 m of the tributary of Hillwood Stream (dark green) proposed to be spot sprayed with triclopyr (GarlonTM 360) for the control of parrot’s feather (Myriophyllum aquaticum)

Figure 9: Approximate area (1,400 m2) of a farm drain, located at 113 Cable Bay Road (dark) proposed to be blanket sprayed with triclopyr (GarlonTM 360) for the control of parrot’s feather (Myriophyllum aquaticum)

Table 1: Risks and mitigation for proposed use of Triclopyr for aquatic pest plant control in Nelson