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I have a small floating weed in my pond. I think it is duckweed, how do I know and how do I treat it? | Pond & Lakes Q&A


I have a small floating weed in my pond. I think it is duckweed, how do I know and how do I treat it?

I have a small floating weed in my pond. I think it is duckweed, how do I know and how do I treat it?
Jason – Raleigh, NC

Duckweed can be a real nuisance if not identified and treated correctly. As it is a prolific grower it can quickly make your pond or lake look more like a golf course in a relatively short period of time. Duckweed is a small floating weed with a single root hair extending from the bottom of each individual leaf. Each green leaflet is about 1/8” of an inch in size and you should be able to fit about 5 to 10 on the tip of your finger. Duckweed can sometimes be confused with watermeal which is also a small green floating weed. Watermeal differs from duckweed in that it is much smaller and has a grainy or almost sandy feel to it if you hold it in your hands.

You can treat duckweed with two different methods. The first method is by spraying contact herbicide like Ultra PondWeed Defense® & Treatment Booster™ PLUS directly onto the floating masses with a pond sprayer. This method typically yields fast results but tends to be a quick fix that ends up resulting in new growth reforming over just a few weeks. If you need to whip your pond into shape for a planned day or two event, then spraying your pond with a contact herbicide may be an appropriate treatment for you.

For longer lasting control of duckweed you can treat the pond with Sonar™ A.S. aquatic herbicide. This product works by inhibiting the plants ability to produce carotene and as a result chlorophyll is degraded by the sunlight and the weed dies. There are however a few things you will need to check before adding it to your pond to ensure a successful treatment. Most importantly, Sonar™ A.S has a 30 day irrigation restriction meaning that if you water your plants or grass with your pond water you will not be able to do so for at least 30 days. Secondly, Sonar™ A.S needs to maintain a high concentration in the pond for up to 90 days. If your pond is prone to overflow or has an inlet/outlet chances are that the Sonar™ A.S will rinse out of your pond to quickly making the treatment less effective. A good way to visually check your water loss is to color the water body with pond dye. Dye will typically remain in your pond for 2-4 weeks in normal conditions. If your pond looses color sooner then it is a great indicator that too much water is exiting the pond. As Sonar™ A.S is degraded by sunlight it is important that you dye your pond while you are chemically treating the water body. When applying Sonar™ A.S use a pressurized tank sprayer and submerge the spray nozzle to apply the herbicide beneath the surface of the pond where it is safe from evaporation and sun exposure.

The best time to use Sonar™ A.S is early in the spring a couple of weeks before you normally see duckweed forming in your pond. This will give the herbicide a chance to establish itself in the pond and discourage plant growth before it gets out of control.

Pond Talk: Have you experienced Duckweed in your pond?

4 Responses

  1. Have a weed growing in our farm pond. It has yellow flowers in it. What is it & shoild I treat the pond or let it go, & what do I treat it with. It grows very fast.

    • Hi Millie,

      There are a few possibilities as to what this weed may be. Is the majority of the plant above the surface of the water? If so I would reccomend Avocet PLX. If the majority of the plant is underwater a submerged herbicide may be more suitable. You are always welcome to mail us a sample of the weed in a dry papertowel or e-mail us a picture of the weed for identification. Some weeds are natural and healthy for a pond but if the weed is starting to get out of control it may be necessary to use other means of control to keep your pond in check.

  2. I have it in spades. I have the Whitecap, but I have a culvert outlet. With Spring rains, it is flowing out pretty fast. The Catch-22 is that if I wait for it to get dryer, the duckweed will have a chance to get established again.

    My current plan is to wait a bit ’til the rains subside, dam the culvert and add the Whitecap.

    What do you think?

    • Paul, I think you are on the right track. It will be even more difficult to control the duckweed with constant outflow from rain, the chemical effectiveness will be greatly reduced. You are better off waiting until you can keep the pond better contained.

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