Exotic weed spreads its tentacles in Waipa waterways

Illegal trade: Blackmarket trade in the Brazilian aquatic weed salvinia (kariba weed) is putting the region’s waterways under threat. ntsGPhoto: Supplied.
Illegal trade: Blackmarket trade in the Brazilian aquatic weed salvinia (kariba weed) is putting the region’s waterways under threat. ntsGPhoto: Supplied.

A blackmarket trade in a Brazilian aquatic weed is threatening to choke waterways in the Waipa District that could flood land and create a hazard where people and livestock could drown.

Salvinia (kariba weed) was introduced to New Zealand nearly a century ago for fishponds and is now found in Northland, Auckland, Waikato and Bay of Plenty.

A Waipa District Council worker discovered a small amount of the invasive plant in a stormwater pond near Te Awamutu last month but failed to recognise the species and when he returned six weeks later, he found the pond choked by the plant.

"In six weeks it had gone from pretty much nothing to the entire 50 metres by 20 metres pond," Waikato Regional Council biosecurity officer Darion Embling said.

"That shows just how quickly salvinia can multiply and take hold if it isn't identified and dealt with promptly."

Salvinia was a fast grower which formed large, dense floating mats on ponds, drains, lakes and still water and could double in area within 10 days.

The plant attracts mosquitoes, kills native plants, blocks dams and irrigation systems and removes oxygen from the water.

Hamilton aquarium shop owner Cliff Frankis said the plant had been sold in stores "years ago". Tough biosecurity laws now prohibited its sale and distribution, but the plant had its admirers.

"They restricted it by not being allowed to be sold in shops but you can't stop Joe Bloggs down the road from growing it in his pond."

He said the plant made a good environment for fry and fish breeders and plant enthusiasts would trade between themselves.

"People have it in their ponds because its got fine roots that the fish spawn on. That's why they grow it," he said.

"They clean their pond out and some of it goes down the drain along with some bits of plant."

But Ministry for Primary Industries (MPI) response manager Emmanuel Yamoah said salvinia has been declared an unwanted and notifiable organism under the Biosecurity Act 1993, which made it illegal to sell, propagate and distribute the plant.

The Te Awamutu discovery had MPI officials and Waipa District Council concerned that the trade in both salvinia and another invasive weed, water hyacinth, would cause damage to local waterways and native plant and fish species. "We are concerned at the extent of the spread of these aquatic weeds," Dr Yamoah said.

Both salvinia and water hyacinth were among the world's most invasive weeds and growing and sharing them was illegal and undermined eradication efforts.

Salvinia is a small, free-floating fern with green-brown leaves and distinct white hairs and roots that look like wet hair. The plant floats just below the water surface and grows up to 30cm long.

Small infestations of the weed could be controlled by removing, drying and incinerating the plants or disposing of the plant by deep burial. Larger waterways with infestations are treated with sprays.

Penalties included fines and imprisonment.


Scientific name: Salvinia molesta AKA: giant salvinia

Type: Aquatic fern

Native to: Southeastern Brazil

Free-floating, does not attach to the soil Prefers slow-moving waters such as lakes, ponds.