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Freshwater pest plants

Didymo and freshwater weed pests

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Freshwater pest plants

Gisborne is home to many waterways which are at risk of becoming contaminated with freshwater pests and weeds.

Keep our waterway clear of dydimo, phormidium, alligator weed and other pests by adopting the 'Check Clean Dry' method before leaving a waterway.

Read A Guide to Freshwater Pest Plants in our District [PDF, 3.6 MB]


Gisborne district is free from Didymosphenia geminata, common name didymo or rock snot. And we need to keep it that way.

Didymo is an invasive freshwater alga that grows in rivers, streams and lakes.  Didymo continues to devastate many rivers in the South Island, where it favours cold, clear, shallow running water. Didymo or rock snot

Didymo can be spread by just one microscopic algal cell in a single drop of water.  Even if you cannot see it you could easily spread it.  It's believed didymo has not yet reached the North Island, but if it does it could cause significant damage to fresh water native fish, trout, plants and insects in our region.  It makes rivers, streams and lakes unsightly and most unpleasant for swimming, boating and fishing.  We all need to be extremely careful when going from one waterway to another.  Water users are legally obliged to prevent the spreading of didymo.

Didymo can form large mats on the bottom of lakes, rivers and streams which have a smothering effect on rocks, submerged plants and other materials. This can affect stream habitats and sources of food for fish and make recreational activities like kayaking, fishing, boating and swimming unpleasant.

Didymo is native to temperate northern hemisphere areas. Until didymo was found in the lower Waiau River of the South Island in 2004 it had not previously entered the southern hemisphere.

If you suspect you've sighted didymo contact MAF's hotline ph: 0800 80 99 66.

Freshwater weed pests

We need to keep freshwater weeds out of our waterways. If you see any please take a note of the location, take a photo if you can, and contact Department of Conservation(external link)

Niwa Report - Vegetation Survey of Aquatic and Wetland Sites in Gisborne District [PDF, 6.8 MB]


Lagarosiphon is an invasive introduced freshwater weed. It grows quickly in depths of 2-4 metres. The weed is currently in Lake Waikaremoana but is actively controlled by the Department of Conservation.
Please don't dump this plant from your aquarium or pond into any waterway.


Hornwort is a freshwater perennial weed. It's widely established in the North Island, but currently not in the Gisborne district. Please help keep it out of Gisborne district.


Hydrilla is an exotic freshwater weed, recognised as the world’s worst submerged weed. It's currently limited to 4 lakes in Hawkes Bay. Please help to keep it out of Gisborne district.
For more information on weed pests visit Department of Conservation's website.

The 'Check Clean Dry' method to contain freshwater pests

'Check clean dry' has been adopted to prevent didymo and freshwater pests infecting Gisborne's waterways.

When leaving a waterway ‘check’ for and remove any visible clumps from gear, leave it at the site or in a rubbish bin.
‘Clean’ your gear - use 5% dishwashing liquid or nappy cleaner or 2% bleach, soak for at least 1 minute.
If items are not cleaned, they need to be completely 'touch dry’ for at least 48 hours before entering another waterway.

Go to Ministry for Primary Industries' website for recommended cleaning methods.

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