Duckweed and Watermeal are a common nuisance for small ponds that receive a lot of runoff. The reason is that these floating plants draw their nutrients directly from the water, so ponds that are nutrient rich are prone to their development. Ultimately to control Duckweed and Watermeal you need to reduce the nutrient load in the water.
Duckweed on the left and Watermeal on the right
The most common sources for the nutrients in the water are lawn fertilizers and soil erosion from runoff. Other sources are wildlife droppings (goose poop), faulty septic systems, decaying leaves from trees, and just about anything else that is organic that gets into your pond.
Regarding lawn fertilizers, if you use them anywhere within the water shed area of the pond, the fertilizer needs to be ZERO PHOSPHOROUS. Low phosphorous will not do, it needs to be Zero!
To gauge whether soil erosion is a problem for you, take a look at the pond before and after a heavy thunderstorm. If soil erosion is a problem, the pond will look dirtier after a thunderstorm. If it is not a problem, the pond will actually look better after the storm.
Once you have addressed the source issue of the nutrients, the next thing to do is to Aerate the pond, which helps the pond clean itself. Your average pond runs out of oxygen on the bottom in the summer, so stuff just accumulates and rots on the pond bottom. If you aerate you will bring oxygen to the bottom, which will allow beneficial aquatic organisms to go to work at breaking down the organic debris, which reduces the nutrient load of the pond. Aeration alone may not be enough to solve Duckweed and Watermeal problems, but it certainly is a major piece of the puzzle.
After you have taken the first 2 steps, you will have to decide if you want to take the herbicide or mechanical removal approach to controlling Duckweed and Watermeal. If you choose herbicide, Propeller is the best product on the market. Be sure to read the label thoroughly and apply as directed. If you don’t want to use herbicides, then there is a 50 foot long Seine Net that is available for removing the floating vegetation. You can anchor one end on shore and loop out into the pond with the other end to corral the floating weeds and bring them in for removal. Once brought in by Seine, we commonly use Dip Nets to scoop the Duckweed and Watermeal from the pond. Once on shore, you should let it dry out a bit and then you can compost it or utilize as fertilizer.
Once you have the Floating weeds removed from the pond, one way or another, then it is time to apply a Phosphorous Remover to prevent those weeds from returning as quickly. These products react with Phosphorous in the water and bind it up making it unavailable for plants and algae to use for growth. This is a great thing to do every time after you treat and/or remove any kind of aquatic vegetation.
There is no guarantee that Duckweed and Watermeal will not return after these steps, but if you get a handle on the nutrients it will be a much longer period of time before they do.