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How Do I Treat Duckweed? | Ponds & Lakes Q&A

How Do I Treat Duckweed?

How Do I Treat Duckweed?

Tim - Rock Hall, MD

Duckweed is a very small floating plant about the size of a pencil eraser. It has shoe-sole shaped leaves with a small, hair-like root hanging below. Since duckweed floats on the surface of the pond, it is commonly mistaken for and treated as if it is algae. Unfortunately, algaecides have no effect on duckweed but there are a couple treatments you can do to keep the prolific duckweed at bay.

The Short-Term Solution
To treat duckweed as well as other aquatic weeds on a short-term basis, you can use Pondweed Defense® or RedWing™. Both are great for a one-time treatment or for spot treating. These are, however, only short-term solutions for duckweed. Due to their aggressive nature, duckweed can grow back in as little as 3 to 4 weeks. This is why we suggest a longer-term solution.

The Long-Term Solution
One of the only products on the market to provide long-term, season-long control of duckweed and other aquatic weeds is called WhiteCap™. Instead of having to spray the plant directly, WhiteCap™ is sprayed directly into the water. WhiteCap™ works by preventing photosynthesis in aquatic plants and by doing so causes them to slowly parish. Treating duckweed and other aquatic plants in this fashion allows for complete control of the entire water body for the season instead of just a few weeks.

Novasource WhiteCap SC Aquatic Herbicide

Controlling Pondweeds: Duckweed – Pond & Lake Q & A – Week Ending March 28th

Picture of Duckweed Floating in a Pond.

Pond & Lake Q & A

Q: Last year I had an uncontrollable case of duckweed in my pond. It covered my entire 1/2 acre pond! I’ve heard of your product called WhiteCap and wanted to use it this spring. I don’t see any signs of duckweed yet, but was wondering if it is too early to treat.
- Cameron of Michigan

A: Duckweed can take over a pond. This prolific grower can come from many sources although most commonly brought in on the feet of waterfowl such as ducks, geese or even herons. The small plant can stick to the feet or the feathers of such birds and be carried for miles. Duckweed can start out slow and in some cases take several seasons to become a problem, although I would recommend treating for it as early as possible.

There are a few options when treating duckweed. You can use a fast-acting aquatic herbicide such as PondWeed Defense or RedWing although this will only give you temporary relief and require multiple applications of spraying the duckweed directly. These are not usually suggested unless your treatment areas are not contained or you have heavy water turnover. When possible I always suggest WhiteCap. WhiteCap is the least expensive method of treating an entire pond, is easy to apply and lasts an entire season.

We recommend to use 32 oz or 1 quart per surface acre (4-6′ deep) when treating for duckweed. This means Cameron’s 1/2 acre pond can be treated with just 16 oz of WhiteCap!

To answer the question above, treat with WhiteCap in the early spring before the duckweed even appears. This will allow you to get ahead of it before it reaches the surface. Although you don’t generally see it Duckweed is actually growing at the bottom of your pond long before you see it at the surface.

There are also other nice benefits to WhiteCap such as: It works very slowly so there is no chance of oxygen loss that could harm your fish or other aquatic life. WhiteCap is also very effective at controlling most other nuisance aquatic plants. So when your treat for your duckweed you will be controlling most other species as well.

Please Note: WhiteCap needs to stay in your pond for up to 90 days. It is not recommended to use it in ponds with a heavy overflow or when during times of heavy rain. If you are unsure of your pond’s turnover (or amount of water that is exchanged) use Nature’s Blue Pond Dye to shade the water column and track the time it takes for the color to disappear. You may also use Nature’s Blue Pond Dye during a WhiteCap treatment to track turnover and shade the pond from sunlight which can also reduce the life span of WhiteCap.

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