Many weeds are increasing in distribution and invasiveness. Photo: By Lupin (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons
More than 100 new weeds have been added to the Department of Conservation's list of environmental threats.
DoC has added 112 species to its [https://www.doc.govt.nz/globalassets/documents/science-and-technical/sfc340.pdf List of Environmental Weeds in New Zealand 2024].
The list includes weeds increasing in distribution and invasiveness, such as Chilean myrtle, Bangalow palm, holly fern, purple-flowered iceplant and the vine bomarea.
These species all originated as garden plants, the agency said.
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 number of introduced plant species in New Zealand is 10 times greater than the number of native species," DoC Science Advisor Dr Kate McAlpine said in a statement.
The new list comprises 386 environmental weeds, an 18 percent rise from the 328 weeds on the previous list in 2008.
Plants are categorised as environmental weeds if they are established in the wild and have significant impacts in natural ecosystems.
"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," said McAlpine.
The report also carries 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 will 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."
DoC recommends people use iNaturalist, a nature recording app, to post weed observations that can help agencies understand their distrubution around the country.