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Dirty Weed Affects Spray Plan

2 December 2021


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.