Common name: Oxygen weed

Botanical 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 is it 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 5m 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.

Click here to see images



Site-led programme

Site-led programmes have rules for specific pests that only apply in that area. Site-led areas have special biodiversity and other values to protect.


Regional Pest Management Plan




What are the rules?

Lagarosiphon is managed under a site-led programme in our plan. Click here 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?

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.

You can help stop it spreading further by making sure you leave the weed behind.

Always check any gear that you’ve taken in the water but especially your:

  • Fishing gear
  • Propeller
  • Anchor
  • Trailer

Make sure you don’t spread any aquatic pests by following the Check, Clean, Dry method. Click here for more information on Check, Clean, Dry.






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