Finding solutions to invasive purple weed in Manawatū

Source: NZ Department of Conservation

At Lake Papaitonga in the Horowhenua, a group of Manawatū rangers took on a challenge that had frustrated previous control efforts: how do you find and eliminate purple loosestrife in a complex wetland environment?

Purple loosestrife in flower.

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Purple loosestrife. 📷: DOC

Combining skills and teamwork

Historically, helicopters have been the primary tool for both surveillance and control of purple loosestrife, however this method was proving inefficient. The answer was a combination of ingenuity, software coding, drone technology, practical field skills and a healthy dose of teamwork.

By bringing together different tools and expertise, the team developed a smarter, more efficient, and cost-effective way to detect and control this invasive weed.

Drone image of staff rowing to wetland.

The team attempted to row to infestation sites (drone image). 📷: Ruby Bennett

Turning drone images into action

After earlier contractor-led drone surveys struggled to identify purple loosestrife plants, Biodiversity Ranger Stephen Conn wondered if there might be a different approach. Drawing on his experience with graphic software, he investigated whether open-source tools could automatically detect flowering purple loosestrife in drone imagery.

His theory was straightforward: if the distinctive purple flowers stood out from the surrounding vegetation, perhaps software could help pinpoint areas that needed a closer look.

Drone image of wetlands with purple loosestrife plants circled.

Drone images were scanned for purple loosestrife – the purple circle indicates a plant/s. 📷: Stephen Conn

Working alongside Biodiversity Project Lead Ruby Bennett, who is trained in drone operations, and with support from the wider team, Stephen developed and tested a process that used graphic software to scan hundreds of drone photos and highlight shades of purple.

After several days of coding, testing, and refining the process, the results were impressive. The system could process more than 700 images in under ten minutes, quickly identifying areas that warranted ground inspection and control.

From high-tech to low-tech

Finding the plants was only half the battle.

Getting to some of them presented another challenge. The lake’s water depth is too shallow for a rowboat and the mud too deep to traverse by foot. To reach a significant infestation site, the team needed another solution.

That’s when fellow Biodiversity Ranger Tim Gilbertson and Stephen brainstormed at the lake’s edge and agreed upon a mode of transport from the opposite end of the technology spectrum: a large tyre inner tube. Inflated and attached to a rope, it became a safe and simple but effective way to transport equipment—and rangers—across a small stretch of water to reach the infestation site.

Sometimes the best solutions are also the simplest.

Purple loosestrife in flower.

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Purple loosestrife in flower.

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Purple loosestrife. 📷: DOC

Ranger on boat in the wetlands.

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A low-tech travel solution to get to the infestation site! 📷: Tem Gilbertson

Results on the ground

The new approach delivered immediate results, this season, the team controlled around 950 plants across the reserve.

The ground-based control also brought additional benefits. Apart from reduced spray drift, minimising impacts on non-target species, it allowed flowers and seed heads to be removed before treatment, helping prevent further spread.

Looking ahead

The methods trialled this season, while highly successful, can be improved upon. There is the possibility that the processed images could be fed back into more traditional drone data processing software to improve results and allow for other modes of weed control. This could potentially be applied to other weed control projects.

What began as a challenge that had stumped earlier control efforts has become a great example of innovation in action. By combining ranger expertise, drone surveys, open-source software, and even an inflatable inner tube, the team has developed a practical method that makes the goal of eliminating purple loosestrife from Papaitonga Reserve feel genuinely achievable.

Finding solutions to invasive purple weed in Manawatū

Combining skills and teamwork

Turning drone images into action

From high-tech to low-tech

Results on the ground

Looking ahead

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