Control of Duckweed species with Roundup Pro Biactive

Duckweed  Duckweed close

Background – The Duckweed family

The Duckweed family consists of a number of species, characterised by their small size and free-floating nature. They are some of the smallest flowering plants in the world. Duckweeds have neither stems nor leaves, but a round, slightly oval-shaped body, called a "frond" and a small root-like structure, known as a "rootlet," hanging down from underneath. They reproduce mainly vegetatively, with two daughter plants budding from the side of the mother plant, which enables rapid propagation.

Still or slow moving, nutrient-rich or eutrophic water, (especially with leaf litter), favour rapid colonisation and Duckweeds form dense mats on the surface, turning the water green.

They are spread by visiting ducks and wildfowl, by floating downstream and by the introduction of new plants in contaminated pots.

Despite being an eyesore in many ornamental water bodies, duckweeds are useful food for ducks, geese and Carp and they provide shelter for many other aquatic creatures such as frogs, toads, newts, fleas, beetles and snails. The tremendous growth rate and ability to reduce polluting nutrients has meant they have been used by environmentalists to remove unwanted substances from water and by aquaculturists as fish food.

Duckweed species

Greater Duckweed, (Spirodela polyrhiza, syn. Lemna polyrhiza). The largest species has fronds up to 10mm across and several roots per frond. It is rather local to lowland England.

Rootless Duckweed, (Wolfia arrhiza, syn. Lemna arrhiza). The smallest species is a rootless Duckweed, with very swollen fronds, only 1mm across. Very local in the South East of England.

Common Duckweed or Lesser Duckweed, (Lemna Minor), The most widespread species and most frequent cause of weed problems.  Fronds are 2-4mm across and tend to grab each other in groups of two to four, forming the thick mats. Flowering occurs in early summer, showing up as a yellowish hue with microscopic flowers in tiny pouches. Tiny fruits are oval-shaped. The only species to flower regularly in the UK

Least Duckweed, (Lemna Minuta), has very short roots, but is otherwise very similar to L Minor. The frond shape is more ellipsoidal and the plants tend to float singly rather than sticking together to form a mat. This species is becoming more prevalent, since being discovered in 1977. It overwinters well, remaining green and is becoming more prevalent throughout the UK.

Fat Duckweed or Gibbous Duckweed, (Lemna gibba). Locally common in lowland England only, it has a slightly fatter, swollen frond than the Common Duckweed, (Lemna Minor).

Ivy Duckweed, (Lemna trisulca). This has thinner fronds with a complex branched structure and is more likely to be submerged. It is rarely a nuisance.

Flowering is rare in the UK, except in Common Duckweed.

Control Methods


Dragging a floating boom across the affected area can remove large amounts of the weed and booms stretched across inlets can prevent infestation from upstream. There are machines which can be used on larger lakes. Collected weed can be composted very successfully.

Continuous removal is often necessary and re-colonisation can be rapid, but the method can be adequate in some situations and is useful in combination with methods of control.


Severe shade will reduce the growth of Duckweeds, so planting trees to the south of a water body may help and competition from other floating plants like water lilies will also reduce their dominance.

Increasing the disturbance of the water by boat traffic or fountains will also reduce the weed.


Grass carp will eat Lemna species. Contact the Environment Agency for more details

Chemical control

Roundup Pro Biactive is the most widely used chemical for control of Duckweeds. It is very safe to the aquatic fauna and can be used in the presence of all species of fish.

Care should be taken when controlling very severe infestations not to cause de-oxygenation of the water as large quantities decompose at once. It may be better to treat in several stages to minimise any possible effect on fish.

Integrated Control

Control With Roundup Pro Biactive

The best option is to remove as much weed as possible by mechanical means and then spray the remainder with Roundup Pro Biactive, where species are susceptible.

L.Minuta and L.trisulca will not be well controlled by Roundup ProBiactive.

Monitor and retreat as necessary. Re-spray before the mat becomes too thick for good coverage, there is no restriction on the number of applications per year.

MethodDose Rate of Roundup ProBioDose Rate of Roundup ProVantageApplication Advice
Overall Spray* 6l/ha in 80-250l water 4.5l/ha in 80-250l water
  • Spray at or shortly after flowering in June/July before the mat becomes too thick
  • Apply from the bank using a telescopic lance or from a boat. Care is needed not to cause wash off with the bow wave. Do not exceed 8kph (5mph)
Spot Spray* 30ml per litre of water in a knapsack sprayer 23ml per litre of water in a knapsack sprayer

* treatment applies for all species except L.trisulca and L.Minuta

Spot Treatment in Knapsack Sprayer

Most knapsack sprayers are supplied with a set of 4 deflector nozzles giving different swath widths but all delivering 200l/ha of water at 1 bar pressure and a walking speed of 1 metre per second.

Area SprayedRate of Roundup ProBioRate of Roundup ProVantageVolume of Water
50 Square Metres 30ml 23ml 1 litre
500 Square Metres 300ml 230ml 10 litres
1,000 Square Metres 600ml 460ml 20 litres

NB: Do not add Mixture B NF adjuvant for aquatic spraying, starch based additives to aid sticking may be used