Common name:  Oxygen weed
Scientific name:  Lagarosiphon major
Management programme:  Site-led


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

Why are they a pest?

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 and ponds. It now causes trouble for our beautiful lakes. Lagarosiphon has the potential to threaten the aquatic environment because its fast growth means it displaces and shades out aquatic native plants. Thick areas of lagarosiphon can disturb water flows and cause local deoxygenation (chemical reaction removing oxygen) of water. Lagarosiphon can choke and block water bodies, having a negative effect on how the area looks, its recreational activities (such as boating, water-skiing and swimming), and water supply intakes. If lagarosiphon is left uncontrolled, large beds can form and wash ashore, leaving unpleasant heaps.  

What does it look like?

Lagarosiphon is an underwater, bottom-rooted, weed that can grow up to 5 metres tall upon reaching the water surface. The leaves are dark green and have tiny, pointed edges. The leaves are spirally around the stem and are curved backwards or downwards. It produces tiny pinkish flowers but, as there are only female plants in New Zealand, they don’t set seed. It spreads through pieces of the stem being carried on water currents, boats, fishing gear, aquarium and pond escapes and people planting it.

What are the rules?

Lagarosiphon is managed under a site-led programme in our plan. 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, Wanaka 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 pest plan is to reduce the amount of lagarosiphon in Lake Wanaka 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 Wakatipu, and lagarosiphon establishing in lakes, and rivers and tributaries where it is not already present.

How can I control it?

ogether 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 any river or lake:

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:

Cleaning option 



Dishwashing detergent or nappy cleaner 

10% mix (1 litre to 10 litres of water) 

Soak or spray all surfaces and leave wet for at least 10 minutes 


2% mix (200mls to 10 litres of water) 

Soak or spray all surfaces for at least 1 minute 

Hot water above 60°C 

Soak entire item 

Soak for at least 1 minute

Hot water above 45°C 

Soak entire item 

Soak for at least 20 minutes 



Until solid 

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

If it’s wet, it’s a threat.

Management programme