Spirodela polyrhiza

Greater Duckweed

Greater Duckweed

Flora, fauna, earth, and sky...
The natural history of the northwoods

Name:

• Spirodela, from the Greek speira (speira), "cord or thread", and delos (delos), "evident or visible," referring to fascicles of roots.
• polyrhiza, from the Greek polus (polus), "many", and `riza (rhiza), "root; stem, origin"; hence, "many roots"
• Common Name, from its size relative to that of the other common North American duckweed, Lemna minor.
• Other common names include: Duck-meal, Giant Duckweed, Spirodele polyrhize (Qué), Stor andmat (Swe), Stor andmat (Nor), Stor Andemad (Dan), Isolimaska (Fin), Teichlinse (Ger), Spirodelka Mnohokoreòová (Slovak)

Taxonomy:

• Kingdom Plantae, the Plants
• Division Magnoliophyta, the Angiosperms (flowering plants)
• Class Liliopsida, the Monocotyledons
• Subclass Arecidae, the Arum
• Order Arales, the Arum
• Family Lemnaceae, the Duckweeds As a result of their adaptation to aquatic habitats, they are among the smallest and simplest of the flowering plants, floating monocotyledons, only 1-15mm in size.
• Genus Spirodela, 3 species worldwide. Spirodela is the largest and least simplified of the Lemnaceae.
• Taxonomic Serial Number: 42599
• Formerly known as Lemna major, Lemna polyrhiza

Description:

• A diminutive floating aquatic perennial, often forming a solid cover on the surface of quiet waters.
• Leaves and Stems merged in common structure typically called a frond or thallus, though neither term is correct by strict botanical definition. Turions sometimes present; rootless, brown/olive, circular/reniform, 1-2 mm in diameter.
• Frond flattened, orbicular-ovate in outline; green above with reddish-purple underside. The largest North American Duckweed, the fronds grow to ¼" or more in breadth, with 7-12 veins, rarely as many as 15. Two budding pouches are located on either side of the basal end. Fronds solitary or in connected clonal clusters of 2 to 5.
• Roots 6-12 per frond, rarely more.
• Flower of 2-3 stamens and a single pistel in a membranous saclike spathe, hidden away inside the budding pouches. Flowers uncommon.
• Fruit tiny, 1-1.5 mm, laterally winged to apex.
• Seed minute, with 12-20 distinct ribs.
• Because Duckweed is largely made up of metabolically active cells with very little structural fiber, the tissue contains twice the protein, fat, nitrogen, and phosphorus of other vascular plants. Each frond absorbs nutrients through the whole plant and not through a central root system, directly assimilating organic molecules such as simple carbohydrates and various amino acids. With the entire body of the duckweed composed of non-structural, metabolically active tissue, most photosynthesis is devoted to the production of protein and nucleic acids, making them very high in nutrional value.

Identification:

• Identifiable as a Duckweed by its diminutive size and free-floating habit.
• Distinguished from Lesser Duckweed (Lemna minor) by its much larger fronds, reddish-purple under surface, and multiple roots.
• Distinguished from Ivy Leaf Duckweed (Lemna trisulca) by its large round fronds, reddish-purple under surface, and multiple roots.

Distribution:

• Widespread in North and Central America, Europe, Africa, Asia, and northern Australia; in South America mostly replaced by Spirodela intermedia.

Habitat:

• Fresh water ponds, marshes, and quiet streams.

Associates:

• Natural populations of duckweeds are usually mixtures of several species.

History:

Uses:

• Very important in the ecosystem as an essential link in the food chain.
• Duckweeds are useful as a water crop as they can acclimatize themselves to almost all growing conditions, with some thriving in manure-rich or eutrophic waters. They reproduce quickly, extending over large surface areas, and are easily harvested. Their high fat and protein content makes them a source of food for animals and poultry.
• Duckweeds have potential in wastewater treatment, absorbing excess nutrients from surface waters, including phosphorus and ammonias, reducing suspended solids, and reducing biochemical oxygen demand.

Reproduction:

• Reproduces by seed and vegetatively, thought during the growing season, nearly all plants arise by vegetative reproduction.
• Unlike the ordinary leaves of most plants, each duckweed frond contains buds from which more fronds may grow. These buds are hidden in pouches along the center axis of older fronds. As they grow, new fronds emerge through slits in the side of their parent fronds. Until they mature, daughter fronds may remain attached to the parent frond. Rapidly growing plants often have three or four attached fronds.
• In fall, budding pouches produce smaller, rootless, dark green or brownish daughter plants called turions. These dense, dormant, starchfilled structures settle to the bottom to overwinter.
• Although Duckweed can set seed and produce fruit like other flowering plants, flowers and seed are uncommon.
• Plants may overwinter as turions, or as seeds, sinking and resting at the bottom of the pond until they germinate.
• Duckweed reproduce at twice the rate of other vascular plants.

Propagation:

• By benign neglect

Cultivation:

• Hardy to USDA Zone 3 (average minimum annual temperature -40ºF)
• Easily grown in garden ponds, where it removes excess nutrients from the water while providing surface shade, both of which act to inhibit algae growth.
• Like most water plants, does best in full sun.
• Goldfish, and other carp, are particularly fond of duckweed. Keep a "starter batch" in a separate, fish free container or pond, if the fish are too voracious.
• Can be invasive in garden ponds without goldfish, but can be controlled rather easily by surface skimming with a hand skimmer from a swimming pool supplier. Just throw it in the compost - it's rich in nutrients.

Links:

• Den virtuella floran, from the Naturhistoriska riksmuseet, Stockholm
• The Charms of Duckweed
• Key to the Lemnaceae of Western North America, index to genera from Wayne Armstrong, Oregon State University

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