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Duckweed
Family Lemnaceae


wide shot of duckweed on pondDuckweed is the smallest flowering plant. It floats on the water surface in a bright green layer.

Duckweed has no stems and no leaves. Some may have tiny roots. It is basically a green sphere, sometimes called fronds but having nothing in common with leaves or fronds (their proper name is thallus). Some comprise a pair of "fronds" with tiny roots.
Mangrove and wetland wildlife at
Sungei Buloh Nature Park
Main features: Tiny, 1-20mm in diameter.

Bark: None.

Leaves: None.

Flowers: White, tiny occurring inside a cavity in the spherical body. Usually requires cross-pollination.

Fruits: Tiny, called utricle, contains one seed. The flower and fruit are nearly invisible to the naked eye.


Status in Singapore: Common.

World distribution: Global except poles and deserts.

Classification: Family Lemnaceae. 35-40 species.
Duckweed does not flower frequently, and usually reproduces by budding on the margin or base of the "fronds". Each "frond" can only do this a limited number of times before dying of old age, whereupon they turn yellow as they lose their chlorophyl. In temperate climates, Duckweed survives the winter by producing buds (called turion) that sink to the bottom of the pond. Duckweed is found almost everywhere except permanently frozen poles and driest deserts. But most species are found in tropical and warm temperate regions. Duckweed grows both in sunlight and in shade, and can grow in slightly brackish water. Duckweed grows best in shallow nutrient-rich pools (e.g., with decaying vegetation), sheltered from wind and currents.

Duckweed disperse by water or are transported by water birds. The plants are also small enough to be carried along with weather like tornadoes. They have been found inside hailstones!

Duckweed have the highest growth rate of any higher plant. In ideal conditions (high levels of nitrates and phosphates), the surface area covered by duckweed can double in less than 2 days. The Indian species, Wolffia microscopica, can bud off a new daughter every 30-36 hours. Thus one tiny plant could, theoretically, in four months produce offspring equal to the volume of the earth!

Role in the habitat: Duckweed is an important food for wild waterfowl and fish both directly and as a source of food for small creature that are in turn eaten by the birds and fish. As it grows, Duckweed absorbs nutrients from the water. Thus it has a useful role in controlling the growth of algae, both by removing nutrients and by shutting out sunlight as the Duckweed covers the water surface. Algae absorbs oxygen and as it decays, it further reduces oxygen levels. Algal blooms can thus severely affect aquatic life. By shading the water, Duckweed also keeps it cool and thus allow for more dissolved oxygen. And by covering the water surface, it minimises water loss through evaporation.

Uses as food: Because Duckweed floats and require little structural fibres (5-15%), it has more nutrition by weight compared to other vascular plants: protein (15-25% in natural conditions, 15-45% when cultured under ideal conditions), fat, nitrogen, and phosphorus. It also contains higher amounts of essential amino acids than most plants. In fact, it most closely resembles animal proteins. It also contains large concentrations of trace minerals that make Duckweed good supplements for animal feed. In addition, they are easily cultivated on small strips of wasteland and easily harvested. In fact, Duckweed is eaten by people in Thailand. There are also projects to look into the cultivation of Duckweed as feed for fish (carp), poultry (chickens, ducks), livestock (pigs). Duckweed is also an effective "crop". For the same amount of nutrients, Duckweed grows on 10% the area needed for soyabeans, and 20% that of corn. Because it has such low fibre, the whole plant can be used, unlike other crops where only a small part of the plant can be eaten.

Other uses: By absorbing nutrients, Duckweed also has potential as a natural water purifier, converting waste water and sewage into pure water and edible Duckweed with little resulting sludge. The only drawbacks are that a large surface area is required; and Duckweed cannot handle toxic wastes and heavy metals. But Duckweed can become a serious weed in nutrient rich, shallow ponds.

LINKS
REFERENCES
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  • Science Club, River Valley High School, "A Guide to the Flora and Fauna of Schools in Singapore", Hillview Publications, 1991 (p. 67: description, habits, habitat, photo).
 
By Ria Tan, 2001