Duckweeds (Lemna species)
Duckweeds are small free-floating plants often forming dense mats on the surface of still or slow flowing water. They grow best in eutrophic (nutrient rich) waters. The leaves are small, often not exceeding 5mm in length (except Spirodela polyrhiza which can be up to 1 cm), either single (Lemna minuta and Lemna gibba) or in groups of two or three (Lemna minor).
The leaves of L. gibba are always swollen, whereas those of L. minor can be slightly swollen under some circumstances. L. minuta has very small leaves and short roots. The leaves of L. minuta tend to be more ellipsoidal than those of L. minor and the
plants do not stick together on the surface, behaving like individuals rather than as a mat.
The plants grow mainly by vegetative reproduction, two daughter plants bud off from the adult plant. This form of growth allows very rapid colonisation of new water and re-colonisation after
treatment in previously infested areas. The doubling times in high summer can be as short as 2-3
days. L. minuta is becoming more prevalent as it tends to winter better than other species,remaining green throughout winter.
Control
Mechanical control - Removal of Duckweed using our apmhibious Truxor weed cutter will have an instant effect and can last for a reasonable length of time although continuous removal is often necessary.
Chemical control - Glyphosate can be applied from our boat, but this will only control the leaves that are above the surface.
Biological control - Grass carp will eat Lemna species. Further information on the use of Grass Carp for Aquatic Weed Control is available from the Environment Agency in R&D Note 57.
Environmental control - The use of shade has been successful in reducing the amount of duckweed growth although very deep shade is often required. Shade can be achieved by planting trees on the south side of a waterbody. Lemna does not compete well with other floating leaved plants such as waterlilies and planting species with floating leaves can substantially reduce the nuisance level of duckweeds. Duckweeds prefer still water and increasing the disturbance of the water surface can reduce the amount of duckweed. This can be achieved by the use of a fountain or, on canals, by increasing the amount of boat traffic. The minimum amount of boat traffic required to eliminate Lemna is reckoned to be about 1500 boat movements per annum. Boat traffic of less than this will reduce the competitive ability of the species and may aid in their eventual elimination.