The Duckweeds & Mosquito-ferns

Greater Duckweed Greater Duckweed Common Duckweed Carolina Mosquito-fern

What are they?

Once considered to form their own family, the Lemnaceae, duckweeds are now generally considered to be in the arum family, Araceae. Duckweeds are amongst the smallest flowering plants in the world, though the flowers are tiny and seldom seen, while some species rarely - if ever - flower. Such plants propogate vegetatively, with small sections breaking off and becoming new plants. Mosquito-ferns are true ferns, but are included here with the duckweeds as they often grow together and are superficially similar in their habits.

Where are they found?

Duckweeds are a very familiar sight in mid and late summer when they often carpet the surface of small ponds and other areas of still water with a layer of green. The mosquito-ferns similarly grow in dense colonies, but often turn rich reddish-purple in the Fall.


The identification of duckweeds is difficult and it is quite likely that there are more species to be found in the Cape May area. A hand lens is generally required for most species. Identification involves the number of veins, presence or absence of rootlets on the underside (and their number) and subtle differences in the overall shape of the plant. One technical term that it is useful to know is Thallus. In this instance, a thallus is a kind of amorphous 'blob' of plant material, which can neither be considered a stem or a leaf, but a mixture of both.

Greater Duckweed      Spirodela polyrrhiza

(Common Duckmeat) A widespread but localized species, typically found in still, more or less permanent waters of ponds and lake margins. Underside is usually red, which can be visible from above as a red border. Many rootlets on underside.
Greater Duckweed Greater Duckweed Greater Duckweed
Typical thallus
Typical thallus

Common Duckweed      Lemna minor

A widespread species in still water ponds. Usually not red on the underside (though may be lightly so at times) and rootlets are single. Thallus has three weak veins.
Common Duckweed Common Duckweed Common Duckweed
Typical plants

Carolina Mosquito-fern      Azolla caroliniana

Probably native, though some consider it introduced from the tropics. Popular as a garden pond plant and at least some plants probably originate from nurseries or gardens. Plants usually green early in the season, turning reddish by mid-summer.
Carolina Mosquito-fern Carolina Mosquito-fern Carolina Mosquito-fern Carolina Mosquito-fern
Typical plant