Spirodela polyrrhiza (Greater Duckweed, sometimes called Giant Duckweed) is a tiny floating aquatic plant found on nearly every continent except Antarctica, thriving in still and slow-moving nutrient-rich waters worldwide. It is the largest genus in the duckweed group, and its round, pea-sized fronds — roughly 0.5–1 cm across, light to medium green on top with a reddish underside and a tuft of 7–21 roots — make it easy to tell apart from smaller duckweeds like Lemna minor. An excellent beginner plant that is genuinely easier to manage than the common duckweeds most aquarists dread.
Care tip: Greater Duckweed is a floating plant — it needs no substrate and simply floats loose on the surface. Its fronds are noticeably larger than common duckweed, so thinning by hand or net is quick and easy — a real advantage over smaller, harder-to-corral duckweed species.
Greater Duckweed draws CO₂ directly from the air, so no injection is needed, and it has no substrate requirement at all. It grows fastest in nutrient-rich water, making it a genuinely useful nitrate and phosphate sponge in tanks that run a little dirty. New fronds bud from small pouches along the edge of each parent frond and eventually detach to form independent plants, so a healthy colony multiplies quickly under good light and nutrient levels. If nutrients run low or temperatures drop, the plant can form small turions that sink and lie dormant, resurfacing and regrowing once conditions improve — a normal part of its life cycle, not a sign of failure.
In the aquarium, Greater Duckweed provides open, natural-looking surface cover that shades the water column, dims light for shade-loving plants and fish below, and gives fry and shrimp somewhere to hide right at the surface. Because the fronds are larger than typical duckweed, coverage reads as a looser, pond-like scatter rather than a solid green film, and it is far simpler to skim back when it starts crowding the tank. It suits low-tech, nature-style, and blackwater setups particularly well, and pairs naturally with other floaters like Salvinia or Frogbit for a layered surface look.
Duckweeds in the genus Spirodela are botanically classified within the arum family (Araceae), in the subfamily Lemnoideae, alongside Lemna and Wolffia. Greater Duckweed's cosmopolitan distribution and wide tolerance for temperature, pH, and hardness make it one of the hardiest floating plants available — equally at home in a tropical planted tank, an unheated betta bowl, or an outdoor pond. For aquarists who want the nutrient-absorbing benefits of duckweed without the frustration of the tiny, hard-to-remove common species, Greater Duckweed is the practical choice.
| Difficulty | Easy |
| Placement | Floating |
| Max Height | 0.2–0.4 inches (frond diameter) |
| Light Level | Low to Moderate |
| Temperature | 65–82°F |
| pH | 6.0–7.8 |
| CO2 Needs | Not Required |
| Origin | Nearly worldwide (temperate to tropical regions on every continent except Antarctica) |
| Notes | The largest of the common duckweeds — round, reddish-backed fronds with 7–21 roots each. Far easier to net and control than Lemna minor. May form dormant turions in cold or nutrient-poor conditions; this is normal. |
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