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In 1940s New Zealand, cordgrass was planted to reclaim wetlands for grazing; it spread across tidal mudflats, forcing decades of eradication work
In the 1940s, New Zealand planted cordgrass for pasture. It now damages the estuary mudflats birds rely on, and officials are still fighting it.
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Some fixes can backfire in a big way. In New Zealand, a hardy grass was introduced to turn marshy coastlines into grazing land. It eventually took over parts of the coast. In February 2025, New Zealand’s Department of Conservation (DOC) issued a media release. Cordgrass, or spartina, was planted across the country in the 1940s to stabilize land for reclamation projects. Now, the agency calls it an aggressive and long-lasting invader of tidal mudflats. That’s a big change for a once helpful plant.
Why people planted it
Spartina is a fast-growing, salt-tolerant plant that can grow to be two meters tall. These features appealed to settlers who wished to create more cultivable land areas. It seemed like an ideal strategy since all the plant needed to do was bind the mud, which would be transformed into pasture areas for grazing animals. In 1983, scientists William Lee and Trevor Partridge wrote in a New Zealand Journal of Botany paper that in estuaries, cordgrasses had been used extensively to defend shorelines and recover land. They said the idea predated the 1940s plantings described by DOC, with the first planting in New Zealand at Foxton in 1913.
How it took over the estuary
Their study tracked the grass in one estuary and rechecked survey lines and a plot from 1973. When the researchers returned in February 1982, they found that in newly colonized areas, the grass spread at rates of up to 5.3 meters a year. The area covered by spartina increased from 10.5 square meters in 1973 to 207.5 square meters in 1982 on a plot of 20 by 20 meters. That’s nearly 20 times more. Mud accumulation was up to 12 millimeters per year in dense stands.
Deep Dive
That’s important because the estuary’s mudflats were an important food source for 34 bird species, including 16 migratory waders. The authors observed that dense spartina was little used by animals. Decades later, DOC described a similar problem. It said the grass traps silt and sand, which damages the open mudflats needed by birds such as the bar-tailed godwit and dotterel, and some shellfish. The clumps also raise the level of the land and can change coastal ecosystems, the release adds.
Decades of fighting back
Removing the grass has taken many years. According to Lee and Partridge, herbicide testing had been going on since 1980. For more than four decades before the DOC’s 2025 release, spraying was being tested. The 2025 release said DOC planned to spray herbicide from a helicopter in Coromandel Harbour later that month. Monitoring of the harbor’s water quality would follow, as required under the consent DOC holds to use the herbicide. According to DOC ranger Jamie Carey, “When it comes to spartina, ‘do nothing’ is not an option.” He warned that the plant would spread if it were left unattended and the harbor would lose valuable habitat for native species.
A lesson from the mudflats
The story shows how a clever plan in one decade can cost the next. In 1983, Lee and Partridge already knew that uncontrolled proliferation represented a danger to the diversity of life in the estuary. The grass did its job and trapped the mud. That very efficiency became an obstacle for open mudflats and the fauna living in them. This control had been carried out over many years, and the DOC’s release in 2025 shows that it was still required. The story is a reminder to consider how landscape changes may spread over time.
