Endothall helping in the fight against invasive aquatic weeds


New research on the effectiveness of the herbicide endothall shows favourable results in the battle to rid lakes and rivers of New Zealand’s most invasive aquatic weeds, including hydrilla, hornwort and lagarosiphon (an oxygen weed).

Until 2004, the herbicide diquat dibromide was the only product registered and used in New Zealand for submerged aquatic weed control. Though effective in controlling most aquatic weeds, diquat has had a limited effect on some of our worst aquatic weeds, especially hydrilla. It’s also largely ineffective in turbid waters because it binds strongly to suspended sediments in the water, rendering it inactive.

Endothall was approved for use by ERMA in 2004 as an alternative herbicide. A contact-type herbicide that causes defoliation and stem die-off in aquatic plants, endothall has been used in the United States for aquatic weed control for over 40 years.

NIWA aquatic plants specialists have been testing endothall’s effectiveness in New Zealand-specific conditions. Initial results are proving very promising.
Endothall has been used to effectively control New Zealand’s worst aquatic weed, hydrilla, as part of a MAF Biosecurity New Zealand eradication programme centred on four Hawke’s Bay lakes. Subsequent studies have shown the herbicide can eradicate hornwort and oxygen weed as well.

“Using endothall to remove lagarosiphon hasn’t been trialled before, because it doesn’t grow in the US. The suppliers of endothall and water managers were keen to see how effective endothall was on lagarosiphon in South Island lakes before they look at large scale applications,” says NIWA Principal Scientist, Aquatic Plants, Dr John Clayton.

“What we’ve found is that endothall can potentially eradicate this pest species, even at very low doses. It also targets the root crown of the plants, preventing regeneration. That’s something that diquat can’t do because it’s deactivated by the sediments. Better yet, it has very little effect on our native flora.”

NIWA has also used endothall to eradicate hornwort from a small lake in Timaru, which was the only known infestation in the South Island. NIWA is also trialling it on hornwort in a 9 hectare lake at well below recommended label rates. Again, endothall proved useful, with 95 percent of the hornwort decaying within two months of treatment, and beneficial results recorded for water quality and native flora.

“What’s really important from the results is that our three top ranked aquatic weeds are highly susceptible to endothall and that is very encouraging news in the fight against these invasive species.

“Aquatic weeds have a huge impact on our waterways, smothering native plants, affecting food and shelter for fish and other wildlife, clogging water ways and power station intakes, and making recreational activities like boating and swimming very difficult.  Any new tools we can use in the fight against them will be beneficial to our environment and our economy,” Dr Clayton says.
NIWA is now looking at the effectiveness of endothall in larger waterways and monitoring its usefulness in flowing water.

A recent Rural Delivery story covers NIWA's work in helping to beat back hornwort, and an accompanying video (credit:  Showdown Production) can be watched on the TVNZ website.


Fast facts about Endothall


  • Is a short-lived contact herbicide used to control aquatic weeds.
  • Has been used in the US since the 1960s and was registered for use in New Zealand in 2004.
  • Is trade marked as Aquathol® K (in liquid form) and Aquathol® Super K (pellets/granular form).
  • Can destroy invasive aquatic weeds within 4-6 weeks, once the tissues of the plants come into contact with the herbicide.

Fast facts about invasive aquatic weeds

Over 70 freshwater aquatic plants introduced into New Zealand have now naturalised here and many have become problem weeds. Most New Zealand lakes, rivers and streams are affected by at least one of these species.

Invasive aquatic weeds smother native submerged plants, affecting food and shelter in the lake for fish and other wildlife, and often make recreational activities such as swimming and boating very difficult. Currently lake managers are fighting to control these types of weeds using a mixture of mechanical, chemical (herbicides) and biological control measures.

About hornwort:

  • Scientific name: Ceratophyllum demersum.
  • Is an introduced plant and was first recorded in natural waters near Napier in 1961.
  • Can grow to a depth of 8-10 metres or more and up to 10 m tall.
  • Does not have roots but is well anchored to the lake or stream bottom via buried stems.
  • Is presently confined to the North Island only, with five South Island sites eradicated. It is part of Biosecurity New Zealand’s National Interest Pests
  • Response programme aimed at exclusion of hornwort from the South Island.

About hydrilla:

  • Scientific name: Hydrilla verticillata (or Indian star vine).
  • Is an introduced submerged, branched perennial plant that was first recognised in New Zealand in the 1960s.
  • Can grow to the surface from depths of 4 metres, forming dense canopies.
  • Occurs only in the Hawkes Bay region of the North Island.
  • Eradication methods by NIWA have now cleared 99 percent of the plant from New Zealand waters, and the risk of spread is now negligible.
  • About the oxygen weed lagarosiphon:
  • Scientific name: Lagarosiphon major
  • Is an introduced submerged perennial plant that grows in lakes and streams.
  • Can grow to depths of 6.5 metres.Has a tiny pink flower.
  • Has been recorded throughout New Zealand and its distribution is still increasing, particularly in clear South Island lakes.
Top: A lake in Southland is sprayed with endothall to remove lagarosiphon (an oxygen weed). Bottom: Following the application of endothall, all the lagarosiphon has gone and native aquatic plants have their habitat back.