How to Use Herbicide on Floating Weeds (Azolla and Duckweed)

Floating weeds like azolla and duckweed are a massive problem for dams, lakes and ponds. Dealing with them can take up a lot of time. I want to talk to you about how to use herbicide on floating weeds so you can DIY spray at home if that’s the direction you choose to go.

Why You Need to Act Fast

Recently, I was supposedly on holiday on the beautiful sunny coast in Queensland, but I got a call from a good customer who needed help with a spray. So I went out there to do what I could do. They’d had a recent cyclone go through, and so there was a lot of runoff — a lot of nutrients came into the water, and it clearly spiked a massive bloom. 

When I got there, the dam was full of azolla. It’s a very invasive floating weed, and if you don’t get to it quickly, it can take over the entire water surface. If I hadn’t got to it when I did, in another week the dam would have been completely covered. Whenever you see azolla, you want to deal with it straight away.

Choosing the Right Sprayer

As I was on holiday, I didn’t have my typical equipment or the stuff I’d normally use, so I had to use what she had on hand. I recommended she use a sprayer on wheels, only about 30 litres, and she got it before I arrived. I chose it because you can move around without lugging it, which makes it easier to use.

You want a sprayer that’s not too fine or too coarse. You want a decent amount of spray, because if it’s too small, spray drift will get onto the lawn and kill your grass. But you don’t want it like a jet because it’s just going to get into the water and stir it all up — it won’t contact the plant very well. 

Picking the Right Herbicide

Picking the right herbicide is essential. You need to make sure you select the appropriate herbicide for the plant that you’re treating. For our client, we couldn’t use aquatic glyphosate because when it hits the water, it’s done. It’s not really effective against azolla or duckweed.

It’s not the herbicide of choice. I used WaterTreats Aquatic Weed Killer. I also sprayed the water with a product called Biostim Accelerator, which degrades the rotting vegetation much faster than if you don’t use it.

Spraying the Dam

You are going to need some sort of boat, kayak, blow-up dinghy — something to get that sprayer out onto the dam. If it’s a smaller infestation, that’s when you utilise the wind. For our client’s dam, we used their aerator as well. If we had got to this infestation when it was a lot smaller, we would have had the luxury of using the aerator to push it out to the sides and then potentially wouldn’t need to get into a boat, because we could go around spot spraying instead.

But when you don’t get on top of it quickly, and the azolla goes everywhere, there’s no way around it because the aerator or the wind isn’t going to push out as far as you need it to actually contact with the spray. 

You can do it with a modern backpack sprayer, you can spray from a boat or a kayak — whatever works best. But whichever method you choose should be comfortable for you.

Tips for Mixing

Here are some tips when you’re mixing chemicals into a spray container: 

– Always put the water in first, then the chemical. If there’s a splash, it’s mostly water coming up, not the chemical.

– Make sure you have enough product for coverage: You’ve got to be mindful of how much spray is in a 25L tank — how much surface area can you treat before it runs out?

It’s essential that you have enough product to cover your whole dam or lake. You need enough solution to cover your entire area evenly. If you need one litre of product, you need to figure out how to spread that one litre across your whole surface, so you might need to put it in 40, 50, or even 60 litres of water to get an even spray. 

The Legal Stuff

If you look at this guide and think it’s too hard and you want someone else to do it, that’s fine — just make sure they’re licensed.

By law in Australia, you can’t just get someone — your local landscaper or gardener — to go out and spray. If you’re paying them for it, they need to be licensed, and they’ll get in trouble if they’re caught. And that’s not mentioning whether or not they understand chemical handling, dosing, and doing the right thing for the environment.

Results and Long-Term Planning

You can get rapid results with herbicides. For our client, the azolla was already starting to brown off by the time I left — that’s about an hour. But all that dead stuff is going to drop down to the bottom of the dam, and that’s its own management challenge.

This is where we use Biostim Tablets or Biostim Pellets to clean up all the stuff at the bottom. We do this because if you leave it there, it’s just going to cycle the nutrients back into the water to feed the next generation of weeds.

In this instance, we used a herbicide because we needed to, but it’s not the only thing you want to use. Our client’s dam had aeration already, so we needed to increase the number of good bacteria to clean up the waste to stop the azolla from coming back. That’s why we use the Biostim products.

Want to Learn More?

If you’re interested in learning more about water management or exploring practical water quality solutions, get in touch with us. We can assess your site, offer FREE expert advice, and design and provide products to help fix your specific water quality issue.