Native to: Australia and Southeast Asia
Likely an accidental escapee from the aquaria trade and/or hitchhiker. Landoltia punctata looks very similar to the native giant duckweed, Spirodela polyrhiza. It is frequently found growing in rivers, ponds, lakes, and sloughs nearly throughout the state. The common name “dotted duckweed” is confusing when applied to this plant because there are distinct red dots on many individuals of the native giant duckweed, Spirodela polyrhiza, but there are none on this species. (The term “dotted” apparently refers to the barely visible ridge of “dots” on the leaf surface of Landoltia punctata.) A good comparison of the native species vs. Landoltia punctata with defining characteristics can be found here.
Habit: tiny free-floating aquatic plant.
Fronds: narrowly egg-shaped, longer than wide, mature fronds ~1-3 mm wide.
Roots: numbering from 2-5 (commonly seen as 3-4), all protrude through the prophyllum.
Nerves: barely visible, numbering 3-7 (commonly seen as 3-5).
Distribution in Florida: throughout
Growth rates are extremely high and populations can double in size in 1 to 3 days under optimal conditions. The diminutive size of duckweed allows plants to easily “hitchhike” on water currents, waterfowl and watercraft, which contributes to its spread. Similar to filamentous algae, duckweed can form dense surface mats that are several layers thick and may include mixtures of different species. However, duckweed’s ability to decrease light penetration and intensity and to consume nutrients can actually inhibit algal growth. Dissolved oxygen concentrations below duckweed mats are often low, which can influence the type and abundance of invertebrate and fish populations. Duckweed mats can also reduce aesthetics and recreational uses of water resources because their excessive growth covers the surface of the water. Duckweed usually causes problems in smaller bodies of water such as backyard ponds, canals, wetlands and other static sites.
Thoroughly clean all boating gear, equipment, shoes, and clothing before leaving infested water bodies.
Floating booms and suction devices can be used to remove duckweed, and rakes can be used when wind and currents cause colonies to accumulate near banks or in isolated small areas. However, mechanical harvesting is typically limited to smaller (less than ½ acre) water bodies.
Grass carp have been used to manage small infestations of duckweed, although high stocking rates (50 to 75 per acre) of small fish (4 to 6 inches) are needed to have an impact. It is important to remember that small grass carp are very susceptible to predation, so most stocking recommendations specify grass carp that are at least 10 to 12 inch long to reduce predation. However, grass carp that are this large have lost the ability to strain small plants from the water and have little utility for duckweed control.
Diquat/Fluridone. Reach out to your local UF IFAS Extension for further assistance with management recommendations. Additional management recommendations can be found in Biology and Control of Aquatic Plants Chapter 2.14.