Aquatic Pest Plants

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.

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) 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 a successful trial to control water celery in a section of Orphanage Stream in June 2021, Council has undertaken more extensive control of this and other aquatic pest plants in Nelson streams on an annual basis from October 2021, using this approved herbicide.

Triclopyr triethylamine (Garlon 360) has been 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 Streamd.

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

Pest plants targeted:

Water celery (Apium nodiflorum)

Water celery (Apium nodiflorumalso 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.

Monitoring details:

Water samples are 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 (EEL) for flowing water.

Annual monitoring at the Council’s State of the Environment (SoE) sitesis 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).