Watermeal: What It Is and What To Do About It

     If you’re seeing lots of tiny little green dots in your pond or lake, you’re probably dealing with watermeal. A member of the duckweed family, watermeal is a free-floating weed that can grow aggressively and completely cover the surface of a body of water. It’s called watermeal because of its close resemblance to cornmeal in both look and feel – small, grainy particles that often clump together to form a film at the surface of a body of water. That film may not look like much at first, but it can rapidly expand and thicken to nuisance levels.

Where Does Watermeal Come From?

     Watermeal is common in nutrient-rich, highly productive backwaters of wetlands, swamps, and other calm, shallow water. Watermeal generally pops up around the edges of a body of water in the beginning of the spring season and will begin to spread more quickly as summer gets closer. It can be introduced through transfer by waterfowl, turtles and other local fauna or by overflow of water from an infested site to a downstream water body.

The Problem with Watermeal

     Some fish—like tilapia—are known to eat it, along with ducks, but it isn’t considered an important food. As watermeal covers more and more of a pond’s surface, it begins to block out sunlight to submerged plants, which will in turn reduce or prevent their growth. It can also reduce the levels of oxygen that is getting into the water, which can harm fish or live organisms.

Treating Watermeal

     Because it is one of the most difficult invasive species to deal with, getting watermeal under control can be a challenge. And if it’s not handled right away, it can take over the entire surface of water. Because it’s easy to recognize, you should be able to get treatment in as soon as you spot it.

     Depending on climate factors and how quickly it’s spreading, you may experience a thin layer of it, or a thick, chunky mat that covers the water completely. Because of its density, applying a simple spray herbicide won’t do and it usually needs in-water herbicide treatment for best results

     Before it overruns your entire surface, treat at an early growth stage with effective watermeal herbicides such as Sonar® or Galleon®. Even with strong herbicide management, watermeal can be a persistent problem in productive ponds. In highly productive ponds with a history of watermeal issues, consider phosphorus inactivation with Phoslock to limit nutrient availability and reduce watermeal growth.


Mark Heilman is Senior Aquatics Technology Leader for SePRO Corporation. As a blogger for StewardsOfWater.com, he shares his knowledge and passion about aquatic ecology and protecting our water from aquatic invasive species.