Duckweed: How to Distinguish From Algae

We get calls all the time from pond owners worried about an algae bloom, only to find out they’re actually dealing with duckweed. It’s a super common mix-up—both can cover the surface of your pond, and if you’re not familiar with what to look for, it’s easy to confuse the two. The problem? They require totally different approaches to manage. In this post, we’ll help you tell the difference between duckweed and algae so you can figure out what’s going on in your pond and treat it the right way.

What is Duckweed?

Duckweed is made up of tiny, round green plants that float freely on the surface of still or slow-moving water. Unlike rooted aquatic plants, duckweed doesn’t grow from the bottom of the pond—it floats on top, often clustering together to form thick, green mats.

At first glance, it’s easy to mistake duckweed for algae. They’re both green, they both float, and they both love warm, nutrient-rich water. In fact, they often show up in the same pond, especially when the water isn’t moving much and there’s plenty of phosphorus or nitrogen to feed on.

But while they might look alike, duckweed and algae behave quite differently—and knowing which one you’re dealing with can make all the difference when it comes to treatment.

How to Distinguish Duckweed from Algae

Not sure what’s growing in your pond? Try this simple test: just dip your hand into the water where the growth is. If it’s duckweed, the tiny plants will scatter and float apart—they move individually when disturbed. Algae, on the other hand, usually sticks together in slimy clumps. Even if it breaks apart when you touch it, it often sinks rather than staying on the surface.

Another clue? Duckweed always floats. No matter how much you stir the water, it’ll stay on top. Algae can drift lower in the water column, especially when disturbed.

Interestingly, duckweed has even been used as a natural water clarifier—it absorbs nutrients like nitrogen and phosphorus, along with some toxins and pathogens. But keep in mind: when duckweed dies off, it releases those nutrients right back into the pond. So while it might help in the short term, it’s not a long-term fix.

While harvesting duckweed can help reduce its impact, it’s important to dispose of it properly—otherwise, all those absorbed nutrients end up right back in the pond. The catch? Duckweed grows fast, and physically removing it is both time-consuming and tough to keep up with. Once it takes off, it can be a real challenge to manage.

Algae, on the other hand, is a bit more complicated. It often gets a bad rap, but not all algae is harmful. In fact, some forms of algae—like planktonic algae—are a natural and essential part of a healthy pond. They sit at the bottom of the food chain and support a variety of organisms at higher trophic levels. So while too much algae can cause problems, a little bit plays an important role in the overall ecosystem.

While some algae is good, trouble starts when there’s too much of it. Large algae blooms can actually lower the amount of dissolved oxygen in your pond—especially at night. That’s because algae, like all plants, needs oxygen to survive when there’s no sunlight for photosynthesis.

When oxygen levels drop too low, it can put serious stress on your pond’s ecosystem. Fish and other aquatic life need that oxygen to breathe, and in extreme cases, an algae bloom can lead to fish kills or other health issues in the pond.

How to Reduce and Prevent Unwanted Growth

Adding water aeration to your pond or water body can help duckweed and algae. The oxygen bubbles from the aeration device will disturb the water’s surface, creating an environment where duckweed cannot thrive. Algae require carbon dioxide to flourish, so aeration prevents algae blooms.

Additionally, aeration can assist in the decomposition process of algae when it eventually dies naturally or otherwise. Fish and aquatic life also benefit from increased dissolved oxygen over the summer season, which an aeration system can provide. You can read more about dissolved oxygen and water aeration in our blog, “Water Aeration Is The Key To A Healthy Pond”.

Floating fountains, in combination with aeration, can be beneficial. The movement disrupts the weeds, and the dissolved oxygen reduces the nutrient load, which helps diminish duckweed and algae.

The nutrient load in the pond also impacts duckweed growth, so beneficial bacteria need to be applied in combination with the aeration methods to bring down the nutrient levels.

Lastly, opening up the area around the pond, especially if it is enclosed by cattails or other bushes or trees, can help the wind reach the pond and disturb the water.

At first glance, algae and duckweed can be pretty difficult to distinguish as they have several similar features. However, we hope this post has alleviated some confusion and highlighted their defining differences.

If you have any further questions, feel free to contact us, and one of our water experts will reach out!