Duckweed vs. Algae

Written by Peyton Warmack-Chipman
Published: November 7, 2022
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Duckweed and algae (singular alga) are green aquatic plants that grow on the surface of ponds and other still bodies of water. Having similar habitats as they do, duckweed and algae are sometimes found together. Though algae is relatively well-known, duckweed is less familiar to most landlubbers. These two plants have many commonalities, but are very easy to tell apart. Let’s read on to learn how to spot the differences between algae and duckweed.

Duckweed vs. Algae at a Glance

ClassificationAquatic plantEukaryotic organism
LooksTiny, oval-shaped green leaves that float on waterOne large, stringy green mass made of tiny threads
TextureSolid leaves that feel like most aquatic plantsSlimy and soft but won’t break apart easily
Harmful?No, as long as it doesn’t take over the body of waterNo, as long as it doesn’t take over the body of water


Duckweed is an exceptionally small aquatic plant that grows in large colonies. From afar it looks like a large green mass and can be mistaken for algae. Duckweed is related to pond lillies and grows in similar ways.

Duckweed is a free-floating plant: its roots descend into the water, while its leaves float on top. Duckweed spreads by producing a new plant which eventually breaks from the original and grows on its own, allowing this plant to spread readily.

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Bright green duckweed floating on the water

Duckweed is a free-floating plant: its roots descend into the water, while its leaves float on top.

© Arunotai


Algae is an umbrella term that refers to many types of single-cell, photosynthesizing organisms that grow in standing water. Common algae, Viridiplantae, is often called green algae or colloquially, pond scum.

Algae grows in single cells that connect with each other to form long stringy threads, which also connect and create the large green plant mass that we recognize as algae.

bright green algae covering the surface of a pond

Unlike duckweed, algae grows in single cells that connect with each other to form long, stringy threads.


Duckweed vs. Algae: Appearance

From quite a distance, duckweed and algae appear to be similar, flat green masses on the water’s surface, but moving closer they look vastly different.

Duckweed leaves are a fraction of an inch, making it difficult to discern that they’re individual plants until you’re quite close. Moving the duckweed, either with your hand or a stick, will result in the leaves spreading apart easily.

Algae, on the other hand, form a connected mass of thin threads that become tangled and inseparable when disturbed. Algae form on the bottom of a pond, eventually building up to the surface, making it common to see algae underwater, whereas duckweed is always on the surface.

The oval-shaped leaves of duckweed leaves are light green and resemble mini lily pads. Algae is often darker green, especially the farther below the surface it is.

Duckweed vs. Algae: Texture

Duckweed and algae are easily distinguishable by their dissimilar textures. To determine whether the floating green substance in a pond is duckweed or algae: touch it!

Neither duckweed nor algae are poisonous to touch, but a its considered a best practice to wear gloves when going in for the grab, to avoid possible allergic reactions, the presence of other toxic plants, or water bacteria.

Duckweed’s leaves will separate easily, and when plucked from the water, its slender, thread-like roots will be exposed. Duckweed roots grow up to one inch in length. Once replaced, the duckweed will resume floating on the water’s surface.

Algae, however, is much harder to put your hand through, and it’s practically impossible to separate one strand from the greater mass. Algae, once removed from a pond when dropped back in, will sink down to the bottom.

Algae is slippery and slimy, while duckweed’s leaves are smooth and solid.

Where They Grow

Duckweed and algae grow in warm, stagnant bodies of water. Having the same growing conditions, they’re often found in similar locations, if not in the same pond!

Both duckweed and algae consume nitrogen and phosphorus, nutrients which are found in ponds and other still bodies of water, accumulating through animal waste and plant decay.


Duckweed and algae are both beneficial to water ecosystems, rarely causing problems, as both play a role in balancing and maintaining these ecosystems, where too much nitrogen and phosphorus can cause problems.

Duckweed in particular helps circulate these nutrients in the water, as its roots absorb the nitrogen and phosphorus to feed the plant. By leaching these elements from the water, duckweed helps prevent toxic cyanobacteria. For this reason, duckweed is often grown as a natural water clarifier!

Duckweed and algae can also be habitats for other single-cell organisms or micro-invertebrates living in the water.

Duckweed or algae growing in large masses on the surface creates a protective layer that blocks some sunlight from entering the water, giving plants that thrive in darker conditions a place to grow, which promotes biodiversity.

However, when duckweed and algae are not controlled, problems may result.

Issues with Duckweed or Algae

Generally speaking, duckweed and algae are signs of a healthy pond ecosystem. They aren’t harmful or toxic. The problems arise when either grows out of control and takes over the pond’s surface.

When duckweed or algae grow to the extent that the water’s surface is completely covered, leaving minimal light to penetrate the water, plant and animal life will likely suffer.

Duckweed and algae are both photosynthetic, primarily subsisting on sunlight. At night or during stretches of cloudy days, these plants draw from the oxygen in the pond. Normally, this isn’t a problem, but if there’s a large amount of these aquatic plants in the water, they can consume too much oxygen, pulling it from other plants that need it as well.

It’s important to stay vigilant when growing Duckweed or algae. Both are vigorous growers that will overtake a stock pond or an ornamental pond in the blink of an eye.

Getting Rid of Duckweed and Algae

Since both duckweed and algae grow in standing water, the best way to prevent or get rid of these aggressive growers is to create movement in the water. This can be done in a variety of ways, naturally or by purchasing artificial means, such as pumps and filtration systems.

There are many types of water pumps that can increase movement and aeration. Keeping the water moving is the goal, no matter how it is achieved.

Removing the duckweed or algae by hand is an option: a time consuming one. Because duckweed plants aren’t connected, it’s difficult to get all of them and, even though algae is a connected mass, it grows all the way to the bottom of the pond and can be very heavy!

Barley straw is known to help improve aeration in still bodies of water. It’s common for farmers to put bundles of straw, wrapped in wire, in ponds on their land in winter or early spring, to keep the growth of duckweed and algae controlled.

About Cyanobacteria

Though often mistaken for one another, duckweed and algae can also be mistaken for cyanobacteria. While duckweed and algae are both nontoxic, cyanobacteria produces toxins that can kill pond ecosystems and make the water unsafe for drinking. So how is cyanobacteria different from duckweed and algae?

A type of bacteria, cyanobacteria is incredibly small and isn’t noticeable until there’s a lot of it, appearing as a very thin, blue-green film on water.

This bacteria multiplies very quickly and can easily grow to a point where it is uncontrollable. Once it is established , the cyanobacteria will release toxins that disrupt the pond’s ecosystem, destroying plant life. Thankfully, duckweed and algae are both virtually harmless, and easily identifiable.

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The photo featured at the top of this post is © Arunotai

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About the Author

Peyton has always loved playing outside, as a kid and still well into her 20's. The connections between our lives, other animals, and all the plants around us has always fascinated her and fueled a drive to learn so much about the natural world. Through curiosity and experience, her knowledge has grown, specifically on medicinal plants and regenerative agriculture. Her favorite animal is the Holland Lop rabbit, after learning they're the greatest pet you could ever have.

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