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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 Ultra PondWeed Defense® 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 Sonar™ A.S. since it 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 Sonar™ A.S.!

To answer the question above, treat with Sonar™ A.S. 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 Sonar™ A.S. such as: It works very slowly so there is no chance of oxygen loss that could harm your fish or other aquatic life. Sonar™ A.S. 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: Sonar™ A.S. 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 Sonar™ A.S. treatment to track turnover and shade the pond from sunlight which can also reduce the life span of Sonar™ A.S.

2 Responses

  1. My pond iscovered with green seed looking things and has a vine gr4owing on it. My husband passed away and i am trying to keep his pond up. Don’t know how yo treat. it has roots about a foot long under the water. Is this duckweed?

    • Hi Betty,

      Duckweed has a tiny little root hair on each leaflet but does not have any defined root system like you are describing. You may want to take a look at our Weed ID guide and see if any of those weeds look like what you have. If you still aren’t able to identify it there will be information on the weed ID on how to send us a sample for identification. I wouln’t want to make any treatment suggestions until we can get a better idea on the type of weed we are dealing with.

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