Spirodela polyrhiza - Duckweed

The great duckweed, Spirodela polyrhiza, is widespread worldwide and native to France. This deciduous aquatic plant grows quickly with small, round floating leaves, although larger than the lesser duckweed. It thrives in bright light and easily colonises freshwater with high nitrate levels, making it rich in organic matter. Excessive growth can cause problems for rooted aquatic plants by depriving them of light. It is best reserved for use in aquariums or artificial outdoor ponds.

Spirodela polyrhiza (syn. Lemna major) belongs to the Arum family. While this floating plant is common in cool, temperate climates, it is absent from arctic and subarctic climates and high mountains. Spirodela polyrhiza is found in North America, Asia, and  Europe and is rare in Australia and South America. This organism reproduces quickly under the right sunlight and temperature conditions. It has excellent potential for water purification and can be used as organic fertiliser or food. However, it can also rapidly accumulate certain non-biodegradable pollutants and spread them to other parts of the environment. It is a small floating plant kept afloat by an internal air reserve. Its flat round or slightly oval leaves measure 3 to 10 mm in diameter. The leaves are light green on top and darker underneath. Each mature leaf has a single central root that grows up to 3 cm long. These white to green roots usually grow straight into the water. New plants form from small leaves that bud and separate. In cool and temperate areas, when the water temperature drops below 6 to 7 °C, the duckweed makes starch granules stored in a "turion structure". The plant dies, and the turion sinks into the water for the winter. A new plant grows in spring or early summer, floating on the surface, and the seasonal cycle starts again. It's essential to check the structure of the leaves to tell them apart from Lemna minor.

Spirodela polyrhiza can be used in artificial ponds or aquariums to clean water and create shaded areas that certain plants or fish like. The water should stay under 25°C in aquariums, and the cover should be slanted to prevent droplets from forming. It's important not to put this plant in natural environments because it can harm other plants and animals. It grows quickly and forms thick mats on the water's surface, blocking light and air. This can use up oxygen and create a lot of organic matter that needs oxygen to break down. Duckweeds also take in a lot of mineral salts, which can make the water less healthy.