Spirodela polyrhiza aka Greater Duckweed

Greater duckweed requires virtually no hands-on care once placed in a suitable water body. It needs full sun and still, nutrient-rich water, but thrives without soil, fertilizer, or regular intervention. The main management challenge is controlling its aggressive spread, which can cover the entire water surface if left unchecked.

Individual thalli are tiny — roughly 5–10 mm wide and 7–9 mm long — and float flat on the water surface in a horizontal, mat-forming habit. The species is a fast grower; colonies can expand rapidly across a pond surface through continuous vegetative budding.

Greater duckweed is cold-hardy to at least USDA zone 4–5, surviving winter as sunken dormant turions. Active growth requires water temperatures above approximately 15°C; at lower temperatures, plants convert to turions and sink. It can persist in tropical conditions through zone 11.

As a fully aquatic plant, greater duckweed has no conventional humidity requirement. It floats on water and obtains moisture directly from its aquatic environment; aerial humidity is not a relevant care factor.

Spirodela polyrhiza thrives in nutrient-rich water containing nitrates and lime. It extracts nutrients directly from the water column; additional fertilizer is generally not needed. In wastewater or pond environments, it naturally absorbs excess phosphate and nitrate, sometimes too effectively, which can deplete nutrients needed by other organisms.

In autumn, as water temperatures drop, greater duckweed converts to dormant starch-filled turions that sink to the pond bottom. No intervention is needed during winter dormancy. In spring, once water warms above about 15°C, turions resurface and vegetative growth resumes rapidly. In warm climates (zones 9–11), active growth may continue year-round.

Greater duckweed rarely flowers. When flowering occurs, two small pouches form on the edge of the thallus: one produces a single pistillate (female) flower; the other produces two staminate (male) flowers, each with a single stamen. The flowers are inconspicuous and less than 1 cm in size. The plant relies almost entirely on vegetative reproduction.

The accepted scientific name is Spirodela polyrrhiza; Spirodela polyrhiza is a widely used orthographic variant. One variety has been formally described: Spirodela polyrrhiza var. masonii Daubs. The species was formerly classified in the family Lemnaceae and under the name Lemna polyrrhiza L.

It provides food and shelter for ducks, turtles, fish, and aquatic invertebrates. Its aggressive growth should be factored in before introduction to natural water bodies, as it can spread via animal dispersal to adjacent wetlands.

Propagation occurs almost entirely through vegetative reproduction: parent fronds produce daughter fronds that bud off and separate throughout spring and summer. In autumn, the plant forms turions — dense, starch-packed structures — that sink to the pond bottom, remain dormant over winter, and re-float in spring to generate new fronds. Flowering and seed set occur very rarely. To propagate intentionally, simply transfer fronds to a new water body during the growing season.

Slow growth in greater duckweed typically signals insufficient light (full sun is required), cold water temperatures below 15°C, or a lack of dissolved nutrients (nitrates and phosphate) in the water. Shaded or nutrient-poor ponds will limit colony expansion.

Greater duckweed has few serious pests in its natural aquatic environment. Its main management issue is invasive overgrowth — under favorable conditions (full sun, warm, nutrient-rich water) it spreads aggressively and can cover an entire water surface, reducing oxygen levels for other aquatic life. Spread is facilitated by waterfowl, mammals (beavers, muskrats), and water currents carrying sticky fronds or turions between water bodies.

Greater duckweed rarely reproduces sexually. When flowers are produced, they are small and enclosed within thallus pouches; pollination is self-compatible (self-pollinating). Vegetative reproduction is the dominant strategy and accounts for most colony growth and spread.

Greater duckweed has an edibility rating of 1 out of 5 (PFAF). The thallus (leaf-like frond) is the edible part and has sometimes been used as food. However, large-scale human consumption is generally not recommended: the plant readily hyperaccumulates heavy metals (arsenic, mercury) from its water source, may harbor pathogenic bacteria such as E. coli and Clostridium botulinum, and contains calcium oxalate crystals linked to kidney stone formation.

In traditional East Asian medicine, greater duckweed (Spirodelae Herba) is used as an antipyretic, diaphoretic, diuretic, antipruritic, cardiotonic, and carminative. Traditional indications include fever, measles, eczema, pruritus, oedema, and difficulty in urination. Modern research (20+ studies to 2026) confirms anti-inflammatory activity in vitro, identifies the flavonoid vitexin as an active compound modulating allergic responses via calcium signaling, and documents improvement of atopic dermatitis with topical application. A fibrinolytic protease with antithrombotic potential has also been identified. The medicinal rating is 2 out of 5 (PFAF).

Greater duckweed has significant non-food, non-medicinal applications. It is used in phytoremediation — hyperaccumulating heavy metals (arsenic, mercury) and removing up to 99.6% of phosphate and 90% of nitrate from contaminated water. It has potential as a feedstock for bioethanol production (up to 50% higher yield per area than corn) and is used as high-protein animal feed for fish (including tilapia) and poultry in small-scale aquaculture. The other-uses rating is 0 of 5 per PFAF, though its industrial and ecological applications are well documented in research literature.

Greater Duckweed’s native range is Europe, including Britain, south and east from Scandanavia to N. Africa and Asia. America, Australia.

Greater Duckweed should be watered regularly, allowing the soil to dry out between waterings.

Greater Duckweed loves a well-draining soil. Perlite and vermiculite help with drainage, while coco coir adds organic matter, so a good potting soil mix will have all three. You can improve store-bought soil by adding some perlite to it.

Greater Duckweed requires abundant, bright and direct light. Placing it less than one foot from a window is the best way to make sure the plant receives enough light to survive and thrive. Depending on your region, however, the current weather can affect the placement of the plant in your home. For example, in some areas the sun may be too direct and intense, so it's best to place it farther away from the window. You may also need to adjust your placement according to the season as the sunlight can change intensity over the course of the year. If you live in a region with significant temperature fluctuations, such as a desert, you may need to place the plant a bit farther away from the window to avoid extreme heat or cold. With some care and attention, your Greater Duckweed will be sure to bring you many years of beauty and enjoyment.

Ploi does not possess verified information regarding the toxicity of this particular plant. In the event that you, a member of your family, or a pet ingests plant material with an unclear toxicity level, it is highly recommended to seek the advice of a medical professional.