The list of weeds threatening New Zealand’s native plants and ecosystems has grown, with 112 species added to the latest watch list.
The Department of Conservation’s List of Environmental Weeds in New Zealand 2024 include new entrants Chilean myrtle, Bangalow palm, holly fern, purple-flowered iceplant, and the vine bomarea which had all increased in distribution and invasiveness.
“Like most environmental weeds in New Zealand, these species all originated as garden plants,” said Department of Conservation’s science advisor Kate McAlpine.
She said New Zealand had more than 25,000 exotic plant species, and the number of introduced plant species was 10 times greater than the number of native species.
“Introduced plants have very few barriers to naturalisation in New Zealand, and many are quick to ‘jump the garden fence’ via bird or wind dispersal, or even people dumping their garden waste at the edge of their local bush reserve.”
Plants were categorised as environmental weeds if they were established in the wild and had significant impacts in natural ecosystems, of which the new list comprised 386 environmental weeds, an 18% rise from the 328 weeds on the previous (2008) list.
The new list also includes two seaweeds: undaria and caulerpa. Undaria is known as ‘the gorse of the sea’, while caulerpa was first found in New Zealand in 2021 and has the potential to smother marine ecosystems.
The report also carried a warning about “sleeper” weeds such as Himalayan wineberry and octopus tree.
“Sleeper weeds aren’t on the list because they’re not established in the wild, or at least not yet. But these are forecast to become fully invasive, with significant impacts on our native species and ecosystems.”
McAlpine said climate change would only aggravate the weed problem.
“As the climate warms, many environmental weeds will be able to live in parts of New Zealand that were previously too cold. The disturbance created by extreme weather events also tends to favour invasion of environmental weeds.”