Frogbit (Limnobium laevigatum)

PROHIBITED MATTER: If you see this plant report it. Call the NSW DPI Biosecurity Helpline 1800 680 244
Also known as: Amazon frogbit

Frogbit is a floating water weed with small, round glossy leaves. Plants grow very quickly, forming dense mats over water bodies.

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How does this weed affect you?

Frogbit is a fast growing, floating freshwater weed that: 

  • forms large dense mats across the water's surface
  • prevents native water plants from growing
  • reduces light, food and shelter for fish and other aquatic animals
  • can block waterways and irrigation channels.

What does it look like?

Frogbit is a perennial floating waterweed.  

Leaves are:

  • bright green
  • up to 4 cm across
  • glossy on top.

Young leaves are round, spongy on the underside and float lying flat on the water surface. As the leaves mature they lose their spongy underside become more oval shaped and can extend up to 50 cm above the water.

Flowers are:

  • about 13 mm wide
  • white, greenish-white or yellowish
  • either male or female (male flowers are on longer stalks often above the female flowers).

Fruit are:

  • a fleshy berry-like capsules with up to 100 seeds
  • 4–13 mm long and 2–5 mm in diameter.

After the female flowers have been pollinated, the flower stalk bends over so that fruit are formed in the mud or underwater.

Seeds are:

  • 1 mm long
  • slightly flattened 
  • hairy.

Stems: 

There are two types of stems:

  • short stems that are mostly branched and have leaves
  • long unbranched runners up to 50 cm long with no leaves that produce a daughter plant on the end of the runner. 

Roots

  • are hairy
  • grow quickly downwards from the base of the leaves
  • are 2 mm thick and up to 20 cm long
  • have minor roots that grow slowly from the major roots that are 1 mm thick and up to 10 cm long.

Where is it found?

The first known occurrence of frogbit in NSW was at Green Point near Forster in 2017. It has also been found at Fairfield, Smithfield, Greystanes, Plumpton, Georges River, Prospect Creek and Bulahdelah.

All known infestations in NSW have been treated immediately to eradicate this weed.

It has been found for sale several times in aquarium shops, at markets and online. This plant must not be sold in NSW. 

Frogbit is native to Central and South America.

What type of environment does it grow in?

Frogbit grows on the surface of freshwater rivers, ponds, dams, lakes, canals and other aquatic habitats. It can grow in shade and full sun. It grows in a wide range of environments but prefers water that is 15–28°C with a pH of 6–8.  It can tolerate slightly saline water. 

Maps and records

  • Recorded presence of Frogbit during property inspections (Map: Biosecurity Information System - Weeds, 2017-2020)
    These records are made by authorised officers during property inspections under the Biosecurity Act 2015. Officers record the presence of priority weeds in their council area and provide this to the NSW Department of Primary Industries. Records reflect the presence of the weed on the date of inspection.

How does it spread?

Illegal dumping of aquarium or pond plants in waterways has been the main cause of Frogbit infestations in NSW. 

By seed

The fruit splits open releasing the seeds. The seeds germinate underwater then float to the surface. The tiny seedlings are moved by water flow, wind and can attach to birds or watercraft.

By plant parts

Frogbit spreads within an area from daughter plants. It can spread to new areas when daughter plants or plant parts are moved by water currents, birds or by attaching to watercraft or equipment.

References

Anderson, L. and Akers, P. (2011). Spongeplant: A new aquatic weed threat in the Delta. Cal-IPC News, 19(1):4-5.  

Cook, C. D. K. (1998). Hydrocharitaceae. In Flowering Plants·Monocotyledons (pp. 234-248). Springer, Berlin, Heidelberg.

Cook, C. D., & Urmi-König, K. (1983). A revision of the genus Limnobium including Hydromystria (Hydrocharitaceae). Aquatic Botany, 17(1), 1-27.

van de Witte, Y. (2020). Invasive Species Compendium: Limnobium laevigatum. CAB International. Retrieved 18 November 2019 from: www.cabi.org/isc. https://www.cabi.org/isc/datasheet/115273  

More information

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Control

Please do not attempt to treat or dispose of this weed yourself. Report this plant if you see it anywhere in NSW by calling the helpline listed at the top of this page immediately.

NSW DPI will lead an initial response for the treatment and disposal of the plant to stop it from spreading

Checking

Check for plants in fish ponds, aquariums, fish tanks, water features, dams and waterways.

Herbicide options

WARNING - ALWAYS READ THE LABEL
Users of agricultural or veterinary chemical products must always read the label and any permit, before using the product, and strictly comply with the directions on the label and the conditions of any permit. Users are not absolved from compliance with the directions on the label or the conditions of the permit by reason of any statement made or not made in this information. To view permits or product labels go to the Australian Pesticides and Veterinary Medicines Authority website www.apvma.gov.au

See Using herbicides for more information.


PERMIT 9907 Expires 31/03/2025
Glyphosate 360 g/L (Only products registered for aquatic use)
Rate: One part product to 50 parts water
Comments: Spot spray
Withholding period: Nil.
Herbicide group: M, Inhibitors of EPSP synthase
Resistance risk: Moderate


PERMIT 9907 Expires 31/03/2025
Glyphosate 360 g/L (Only products registered for aquatic use)
Rate: One part product to 9 parts water
Comments: Splatter gun
Withholding period: Nil.
Herbicide group: M, Inhibitors of EPSP synthase
Resistance risk: Moderate


Diquat 200 g/L (Reglone®)
Rate: 5 L of product per megalitre of water
Comments: Apply by injection below the surface or as a surface spray.
Withholding period: 1 day in pasture, 10 days in treated water.
Herbicide group: L, Inhibitors of photosynthesis at photosystem I (PSI inhibitors)
Resistance risk: Moderate


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Biosecurity duty

The content provided here is for information purposes only and is taken from the Biosecurity Act 2015 and its subordinate legislation, and the Regional Strategic Weed Management Plans (published by each Local Land Services region in NSW). It describes the state and regional priorities for weeds in New South Wales, Australia.

Area Duty
All of NSW General Biosecurity Duty
All plants are regulated with a general biosecurity duty to prevent, eliminate or minimise any biosecurity risk they may pose. Any person who deals with any plant, who knows (or ought to know) of any biosecurity risk, has a duty to ensure the risk is prevented, eliminated or minimised, so far as is reasonably practicable.
All of NSW Prohibited Matter
A person who deals with prohibited matter or a carrier of prohibited matter is guilty of an offence. A person who becomes aware of or suspects the presence of prohibited matter must immediately notify the Department of Primary Industries
All species of Limnobium are Prohibited Matter

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For technical advice and assistance with identification please contact your local council weeds officer.
For further information call the NSW DPI Biosecurity Helpline on 1800 680 244 or send an email to weeds@dpi.nsw.gov.au

Reviewed 2020