A banned aquatic weed that forms dense mats that can double in size in days and pose a drowning risk has been discovered in two home garden ponds in Northland, prompting a warning from biosecurity experts.

Salvinia (Salvinia molesta) was thought to have been officially eradicated from the region, however, Northland Regional Council biosecurity staff have in recent weeks stumbled across the plant in both the Hikurangi area, near Whangarei, and near Okaihau in the Far North District.

In both cases the rapidly-growing pest plant - also known as kariba weed - had been found in home garden ponds.

Sara Brill, a biosecurity officer with the Northland Regional Council, says salvinia is typically spread by pond or aquarium owners and is a real threat to Northland's lakes and large ponds.

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She says the plant is a real nasty and well deserving of its banned status under both the Biosecurity Act and the regional council's Regional Pest Management Strategy which make it illegal to sell, propagate or distribute it.

"Salvinia quickly forms extensive mats, completely smothering waterways and badly affecting water quality. Overseas experience is that these mats kill off native plants, block dams and waterways, impede drainage and disrupt recreational activities and can create a very real drowning risk for people and animals."

Ms Brill says salvinia reproduces solely by stem fragments and can double in area within just 10 days.

Anyone who sees it in Northland - or suspects it may be present - must legally report it to either the regional council or MAF Biosecurity New Zealand (MAF BNZ).

"Staff will ensure ponds are cleared thoroughly and will do follow-up checks as necessary. There is no charge for this."

Ms Brill says salvinia is a small, free-floating aquatic fern with branched, horizontal stems that lie just below the water surface.

"Plants are usually up to 30cm long, and have green to bronze spongy leaves that occur in pairs, while leaf shape varies with the age and environment of the plant. Young leaves are oval, about 12 mm across and lie flat on the water, often resembling duckweed and as the plant matures, leaves become thick and fold at the mid-rib."

The upper surface of the leaf is water repellent and covered with distinct white hairs with an egg beater-like tip. Salvinia has no true roots, but has a root-like
structure underneath each leaf pair and as the plant matures, these 'roots' resemble wet hair.

Ms Brill says information on salvinia is available online from 'pest plants' section of the Northland Regional Council's website via www.nrc.govt.nz/nasties or from the MAF BNZ website via: www.biosecurity.govt.nz/files/pests/salvinia/salvinia-fact-sheet.pdf