How to Eradicate Submerged Weeds from a Dam

Step 1

Manually remove as much of the submerged weed as possible

Before treatment it is important to remove as much weed as possible using one of the methods below. If you're struggling to see where your weeds are most concentrated, use a non-toxic dam dye to darken the water so you can better see the plants.

Aquatic Weed Hand Cutter

For hardier plants: The Aquatic Weed Hand Cutter is a high quality, comfortable-to-use tool that makes short work of submerged weeds. For deeper areas, we recommend duct taping the cutter to a pool net extension pole.

Lake Bottom Blanket

For spot treatments: Lake Bottom Blankets kill aquatic weeds by blocking their access to sunlight. This starves them to death because they can’t photosynthesise food.

Beachroller

For soft-celled plants: Beachroller features a 3-blade design that rolls along the floor of your dam, cutting and ripping up weeds. The tool also tills the soil which slows regrowth.

Dyes

For reducing sunlight: Dam dyes tint the colour of your water which reduces how much sunlight reaches the bottom where the weeds are growing. This can reduce the growth of submerged weeds by limiting photosynthesis.

Step 2

Increase aeration

Aeration means adding oxygen to your water. This provides ideal conditions for “good” aerobic bacteria to grow in number. These “good” guys reduce nutrients in your dam, e.g. nitrogen and phosphorus, and eat sludge on the floor which is fertiliser for aquatic weeds. With enough “good” bacteria reducing nutrients in your dam, new or regrowing weeds find it difficult to survive.

Avalon Nursery near Ballarat was having a problem with red azolla and blue-green algae in one of their irrigation dams.

We recommended installing a 1.5hp solar-powered Air-O-Lator Carnival Aerating Fountain. Within days of installing the aerator, the azolla and blue-green algae began to retreat. After just 6 weeks, the weed and algae were completely gone. The smell had disappeared too.

Step 3

Add Biostim probiotics

Biostim is a probiotic that feeds the “good” bacteria in your dam, growing their populations. As mentioned above, these guys consume nutrients like nitrogen and phosphorus and eat the sludge on the floor of your dam. Eventually they create (nutrient-depleted) conditions that makes it tough for aquatic weeds to grow, no matter how much sunshine your dam receives. There simply isn't enough food to go around.

In extreme cases, take Step 4 to kill your aquatic weeds immediately.

We typically don’t encourage use of herbicides because they're not selective in keeping the "good" plants while killing the "bad". But in extreme cases, you may have no choice. In this situation, we recommend you follow the instructions below.

Step 4

Herbicide (+ Biostim Accelerator)

If killing your aquatic weed immediately is your only option, we suggest you use an herbicide (our recommendation is WaterTreats Aquatic Weed Killer or Knockout Aquatic Herbicide) in combination with Biostim Accelerator. Biostim Accelerator is a 100% natural biocatalyst which improves the effectiveness of herbicides, reducing how much you need to use⁠.

How Biostim Accelerator works

Biostim Accelerator

WaterTreats Aquatic Weed Killer

Knockout 360 Aquatic Herbicide

Remember, you must combine herbicide use with Step 2 and 3 if you want to prevent aquatic weeds from returning. The reason is if you don’t improve numbers of “good” bacteria by aeration and Biostim then you’re leaving the food supply in the water which the next generation of weed or algae can use. They simply take the place of what you’ve just killed. This leads to a chemical cycle of needing to continually kill the next generation of weed. Over time this can really impact the health of your dam.

"Since WQS installed the aerator, we've not had a single algae problem. In our smaller ponds that are too small for aerators we used Biostim pellets on their own and have noticed a huge reduction in algae.”

Andy Hart

Horticultural Curator—Department of Environment, Water and Natural Resources, Adelaide

“We had a nutrient-rich stormwater lake that was having continuous problems with algae. We were previously treating the nutrients with a liquid solution that would just end up getting flushed away with the outgoing water. Scott recommended we switch to Biostim pellets which were exactly what we needed.”

Giles Pickard

Environment Project Officer, City of Subiaco

“Visiting ducks and our pet geese were continuously fouling the dam. WQS recommended a number of systems to improve the water quality. We are very happy with our final choice, the windmill aeration system. This combined with the Biostim pellets and liquid are cleaning up the dirty dam. Everything WQS said would happen has happened!”

Greg Lewis

Canterbury, Victoria

“We had an urgent problem—our old irrigation system had blockages from weeds. Our dam was also riddled with black sludge. Scott recommended both aeration and biologicals and within six weeks the dam became so clear I could see to the bottom of it for the first time in years! I was chuffed that we could fix the issue without the use of chemicals.”

Michael Grant

Owner, Grant’s Citrus Farm