Hornwort does not root itself into sediment and reproduces quickly, so once it's found in waterways, it can be difficult to control. Photo: SUPPLIED/Trevor James
The Canterbury regional council is on the lookout for a pesky freshwater weed that was eradicated from the South Island more than a decade ago, after it was discovered being sold online from Christchurch.
Aquatic plant hornwort (Ceratophyllum demersum) had feathery, dark green leaves in rings around the stem and was considered the worst submerged weed in Aotearoa.
It spread rapidly when stem fragments broke off and reproduced in lakes, rivers and streams.
Hornwort was often used as a plant for aquariums or domestic indoor fish tanks, but Environment Canterbury warned that it could be devastating to marine eco-systems if it escaped into waterways by being flushed down the drain.
Its principal biodiversity advisor, Laurence Smith said it was widespread in the North Island, but the pest was eradicated in the South Island in 2013.
But he said a Department of Conservation officer found an online sales listing for hornwort from a domestic urban garden on the outskirts of Christchurch.
"The location of the original site was on the edge of Christchurch in a domestic urban garden, and it has apparently only been shared from that location once," Smith said.
"So we're pretty confident that that person hasn't been selling it widely or giving it away.
"But what we are concerned about is where they sourced it from so and we've got several options there; locations that we could investigate just to make sure it isn't occurring in the natural environment in Canterbury, so that work's going to be underway shortly."
Hornwort (Ceratophyllum demersum) is ranked New Zealand’s worst submerged weed, often smothering all other aquatic plant beds, including native and exotic. Photo: SUPPLIED/Environment Canterbury
Smith said hornwort was often called [
https://www.rnz.co.nz/news/national/316386/blitz-on-lake-karapiro-water-weed "pond weeds"] in listings on Facebook marketplace or TradeMe - but could have major effects on the environment.
"It's a plant that can shade out native vegetation and alter habitats, and diminish biodiversity," he said.
"It's something that can impact on fish species as well as vertebrates and other plant species, that all rely on that sort of shelter for to live in and for food.
"I think it can de-oxygenate water as well which would have dire impacts on freshwater species. So that's why it's very important that if we do find any sites within Canterbury that they're eliminated quickly so that it's spread into our natural environment."
Hornwort is native to North America and was first recorded in Napier in 1961. It has spread to much of the North Island, but two clusters in the South Island were eradicated. Photo: SUPPLIED/Rohan Wells
Smith said anyone who reported sightings of it on their property could go into the draw for a prize pack towards their aquarium - and the council would also remove it.
Reports can be made to reportpests.nz
