How to Identify Duckweed in Ponds

Duckweed is tiny and often looks like green algae.

Duckweed is an invasive aquatic plant that can be found all over the United States, according to the University of Florida. This floating plant is the tiniest flowering plant in the world, and it thrives in still waters such as ponds and lakes. A prolific spreader, it can quickly cover the surface of the water if left unchecked. Not only is this unattractive, but it blocks sunlight from reaching plant and animal life below the surface of the water. There are three types of duckweed native to the United States: small, giant and dotted.

Things You'll Need

• Gloves or spoon
• White piece of paper
• Magnifying glass

Instructions

1. Scoop up some duckweed from the water. Put on gloves if you don't want to touch the water with your skin, or use a spoon. Only take a small amount, such as you can scoop up with two fingers.

2. Spread the duckweed out on a white piece of paper so that you can examine it. You may need to use a magnifying glass to easily see it.

3. Look at the leaves. Leaves that are shaped like a shoe rather than round are most likely Lemna valdiviana, or small duckweed. Landoltia punctata, or dotted duckweed, also has shoe-shaped leaves. If the leaves are round with red undersides, they are most likely Spirodela polyrhiza, or giant duckweed. Some giant duckweed leaves also have a red dot on top.

4. Count the roots. While small and dotted duckweed both have shoe-shaped leaves, small duckweed only has one root that dangles down from the joined leaves. Dotted duckweed, on the other hand, has between two and five roots that hang from each leaf, according to the University of Florida. Giant duckweed has large clumps of roots (up to nine) hanging from each leaf.

5. Look at the size of the plant. Small duckweed is only around 4 mm in width, according to Iowa State University. Giant duckweed is not giant, but it is larger than small duckweed at around 10 mm in width. Dotted duckweed is closer in size to giant duckweed.

References

• University of Florida: Small Duckweed
• University of Florida: Giant Duckweed
• University of Florida: Dotted Duckweed
• Iowa State University: Duckweed

Resources

• Missouri Botanical Garden: Duckweed Pictures