It’s hard to spell and even harder to get rid of. Lagarosiphon was once sold as a pretty decoration for aquariums or ponds and is thought to have been accidentally introduced to our waterways through disposal into drains or ponds. It now causes trouble for our beautiful lakes.

Lagarosiphon is a fast-growing freshwater weed that spreads easily through pieces of stem being carried on water currents, boats, fishing gear, aquarium and pond waste, and deliberate plantings.

Once established, it:

• shades out and takes over aquatic native plants
• chokes and blocks water bodies
• affects water supply intakes
• interferes with recreational activities (swimming, boating, water-skiing, etc.)

Lagarosiphon is present in Lakes Dunstan and Roxburgh and parts of Lake Wānaka. It is also found in the Clutha River/Mata-Au, Kawarau River, and Pūerua River. Isolated plants are regularly removed from Frankton Arm in Lake Whakatipu, which is thought to result from weed transfer by boats from other waterways in the region.

It spreads through the dispersal of stem fragments, either naturally by water movement or by human activity such as boating, fishing gear, machinery, aquarium and pond waste, and deliberate plantings.

• Size: Can grow up to 5 metres long; its long, brittle stems form dense mats when they reach the water's surface.
• Appearance: The leaves are dark green with tiny, pointed edges. They are arranged spirally around the stem and curve backward or downward. In its native habitat, lagarosiphon produces tiny pinkish flowers; however, as there are only female plants in New Zealand, it does not flower or set seed here.
• Distinctive features: A submerged, bottom-rooted weed.
• Habitat: Found in still or slow-moving freshwater environments — lakes, ponds, canals, riverbanks, reservoirs, and similar habitats.

Lagarosiphon is managed under a site-led programme in our Regional Pest Management Plan (2019-2029).

Head to ORC ArcGIS to see a map of the areas covered by the lagarosiphon site-led programme.

•  Any person leaving the waters of lakes Dunstan, Wānaka or Roxburgh or from the Clutha River/Mata-Au and the Kawarau River must immediately remove and safely dispose of all fragments of lagarosiphon from boats, equipment and all other items in their possession.
• Anyone in Otago must destroy and safely dispose of all lagarosiphon from any pond or aquarium on their land.

The goal of the Regional Pest Management Plan (2019-2029) for lagarosiphon is to: 

• Reduce the amount of lagarosiphon in Lake Wānaka and the Kawarau River through progressive containment over the next 10 years.
• Keep the levels of lagarosiphon in Lake Dunstan at a stable level.
• Prevent lagarosiphon establishing in Lake Whakatipu.
• Prevent lagarosiphon establishing in lakes, rivers and tributaries where it is not already present.

Together with other stakeholders, we are working to curb the spread of lagarosiphon and have removed many tonnes since it was first spotted in our waterways in the early 1970s.

Make sure you don’t spread any aquatic pests by following the Check, Clean, Dry method. 

To prevent the spread of freshwater pests, including lagarosiphon, whenever you move between waterways you must check, clean and dry any equipment that comes into contact with water.

Before you leave:

Check — remove any plant matter from your gear and clothing and leave it at the site. Don’t wash plant material down the drain.

Clean — soak or scrub all items using one of the below treatments:

Note: 60° C is hotter than most tap water; 45°C is uncomfortable to touch. Allow longer times for absorbent items.

Dry – ensure your gear is completely dry to touch, inside and out, then leave to dry for at least another 48 hours before you use it.

Do not dump aquarium plants or pond plants into natural waterways — even small fragments (~1 cm) can start a new infestation. 

www.orc.govt.nz/lagarosiphon