Aquatic Weeds

Update April 22


BoPRC has advised residents that they would not take action on the weed wash up at Okawa Bay on a public amenity basis because it was really a private amenity one which they would need to address themselves if they wanted to have the weed removed commenting as follows:

“While I agree with you that rotting weed isn’t pleasant, I don’t consider it to be toxic. Although it certainly creates an unpleasant amenity effect for those private dwellings. Our scientist confirmed with me that it’s unlikely to be a driver of the algae blooms that have been present. However, once any weed dies and breaks down to the bed of the lake it does become available as nutrient, which is a driver in algae blooms. The presence of lake weed may actually be an indicator of good water quality (clarity), i.e. if there was an algae bloom there wouldn’t be sufficient light for the weed to grow.”

They also clarify responsibilities around weed on the Te Arawa Lakes - including weed harvesting, spraying, spraying limits and managing new incursions of weed on lakes that do not already have invasive weeds present.

Emails on the subject from BoPRC and Okawa Bay representative Richard Amery can be read here.

Update December 21


LRCA scrutinises the twice yearly notification by Boffa Miskell (who manage the spray programme on behalf of LINZ), to gain an understanding of the areas that have been approved for spraying in the Spring and Autumn spraying, and the areas that have been excluded.

There are two prime reasons for excluded areas. The first is that there is little weed, and the second is ‘dirty weed’ identified by the the diver inspection.

We have asked for an explanation of why dirty weed causes areas not to be sprayed.

Some Good News

The pre spray inspection found that Lake Rotoiti had a significant amount of clean weed. Therefore the largest control programme ever is being undertaken with just over 75 hectares of weed to be targeted.

Regarding Dirty Weed

This is an issue experienced across all New Zealand lakes. There are generally a few reasons as to why an application of herbicide is ineffective.

* The weed is too dirty, and organic particles in the water and on the weed deactivate the active ingredient in the chemical Diquat before it can attach itself to the plant material.

* The herbicide has been applied but has drifted off target and settled outside of where the plants are. This can be common and can be caused by currents, and the thermal stratifications of the different temperatures in the water body. This is very hard to avoid as we cannot monitor water currents at every location across all the lakes we work in.

* The plants need to be actively photosynthesizing to absorb the chemical. If its early in the morning or a really overcast day, then this can also affect results. The plants will decrease how much energy they are taking in and therefore how much chemical they absorb.

The weed can still be healthy and covered in sediment. It might also seem clean once you have swum around as the sediment has been disturbed and will be suspended in the water column and no longer on the weed.

If you see the article and photos here from divers at Lake Rotoiti this shows weed at a 4 on the dirtiness weed scale. This weed will not be getting treated this round.

When spraying you need the weed to be in condition 1 or 2 or you will get poor results.

Therefore, the main way to implement an effective spray programme is to have clean weed. How do we get clean weed? That’s essentially up to mother nature.  If we had a chemical that we could apply and get 100% kill rate every time, then we would not have a weed issue. Unfortunately, we don’t have that luxury at present. As mentioned on the phone, the EPA is working on getting a new chemical registered for use in New Zealand that could control dirty weed. However, this will more than likely need a consent to be used in large public water bodies which is another hurdle before we can effectively use it.

If you are concerned about weed in your bay, please notify LRCA, as we can discuss with LINZ.

Update November 21


Lake weed is a problem in shallower water around the lake. Weed like Hornwart enjoys the cleaner water due to the wall in Okere channel that directs Rotorua water into the Kaituna river.

LRCA has regular discussions with LINZ (which has responsibility under Settlement Act), Boffa Miskell (manages the programme), and BoPRC (Biodiversity responsibilities), particularly to highlight the problem areas brought to LRCA attention by members, and to query when some areas are not sprayed.

Below is the email received this week confirming the spring lakeweed spraying plan, which is the largest area to be sprayed.

You can open the Rotoiti map here and zoom in on your bay. In each location with historical weed, there is a red circled zone. The yellow shaded areas identify the areas to be sprayed this time.

You will see there are some areas with no yellow shading, meaning no spray this time, and other areas where only part of the red circle are being sprayed.

We have spoken to LINZ (and Boffa Miskell) who are engaged by LINZ to manage the spray programme, and record below the reason they provided for minimal OR no spraying in some locations.

Hot Springs - 0 hectare – weed evident at drop off only – outside zone

Manupiura Bay 0 ha - minimal weed

Tapuakura (Curtis Road) - 0 ha - Weed identified at each end of bay, but evidence of sediment that would prevent effective attachment of diquat gel to the weed, so no spraying.

Hauparu Bay – 1 ha - weed evident at eastern end only

Whangamoa Road (west end) 0 ha – hornwort around rocks, dirty with sediment

Kuharua (Lees Pt) – 0 ha - patches of relatively dense native underwater plants – outside scope of spray programme

We understand that diquat will only adhere and be effective on healthy weed that is free of sediment. Before each spray plan is determined, divers inspect every location to determine evidence and condition and weed. We are seeking more information about this to share with our membership.

You are welcome to advise LRCA of concerns about the spray programme and we can discuss with LINZ and Boffa Miskell.

From: BML Biosecurity

Sent: Sunday, 31 October 2021 12:53 PM

Subject: Aquatic Weed Spraying - Te Arawa/Rotorua Lakes

Kia ora koutou,

Aquatic weed spraying is scheduled to be carried out in the Te Arawa/Rotorua Lakes from Monday 8th November and will continue intermittently until Friday 10th December 2021.


This work will be carried out via boat-based boom spraying using the herbicide diquat in gel form at the following sites at Rotoiti:

Wairau Bay

Ōkahutoroa Bay/Cherry Bay

Ōkawa Bay

Teal Bay

Te Weta Bay

Ōtaramarae Boat Ramp

Te Ārero Bay

Gisborne Point

Ōkere Inlet

Te Akau Peninsula

Te Waiiti Point

Te Ruato Bay

Hauparu Bay

Here is a map showing where work will be carried out.

All work is dependent on suitable weed, water and weather conditions and is subject to change without notice.


Advertisements will been placed in the Rotorua Daily Post to inform lake users of the control work.


Warning signage will also be erected at official boat ramps 24 hours before work is taking place.


If you have any questions in relation to this work, please feel free to call us on 0800 638 943 or keep up to date with treatment progress on the LINZ website.


Further updates on projected works will be provided on Sunday 7th November once we have a better idea of the weather forecast.


Ngā mihi,


 Boffa Miskell.



Review of Aquatic Weed Control for Amenity Purposes on Rotorua Lakes

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