Frogbit (Limnobium laevigatum)

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
• creates dangerous conditions by hiding the water surface and making it appear like firm ground
• can limit recreational activities such as fishing, swimming or boating.

What does it look like?

Frogbit is a perennial floating waterweed.  

Leaves are:

• bright green
• usually rounded at the tip
• 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
• are firmly anchored in the substrate if the water is less than 30 cm deep.
• One major root grows quickly downwards from the base of the leaves. This root is 2 mm thick and up to 30 cm long.
• Minor roots are 1 mm thick and up to 10 cm long and they grow slowly from the major root. 

Similar species

Frogbit looks similar to two other weed species:

• Spongeplant (Limnobium spongia), which is also prohibited matter but not yet known to occur in Australia. Spongeplant has larger fruit (4-12 mm diameter) than frogbit and usually has a pointed leaf tip rather than rounded.
• Water poppy (Hydrocleys nymphoides), which has large yellow flowers up to 8 cm wide and larger leaves up to 12 cm wide.

There is also a native plant called frogbit (Hydrocharis dubia) that occurs on the North Coast of NSW. This plant has larger leaves (up to 6 cm long). Its flowers are 2-3 cm across with 3 round white petals and yellow centres.

 

Where is it found?

The first known naturalised occurrence of frogbit in NSW was at Green Point near Forster in 2017. Naturalised and cultivated plants have been found in seven regions in NSW: Greater Sydney, Hunter, South East, North Coast, Northern Tablelands Central Tablelands and the Western region. 

All known infestations in NSW have been treated immediately to help 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, 2017-2023)
  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 seeds can remain viable for at least 3 years. The fruit splits open releasing the seeds which 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 a waterbody 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

See Using herbicides for more information.

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PERMIT 92971 Expires 31/03/2028
Metsulfuron-methyl 600 g/kg (LYNX WG herbicide)
Rate: 10 g / 100 L water (add wetter at 200 mL per 100 L)
Comments: Only for enclosed water bodies. Areas within 400 m of potable water supply uptakes are excluded. ONLY apply as a foliar spot spray during non-frost periods using a high volume sprayer/knapsack fitted with calibrated spray equipment. Apply a maximum of 3 applications per year at minimum intervals of 90 days. WARNING: Very toxic to aquatic plants and algae. See permit for more restraints and critical use comments.
Withholding period: A nil withholding period is applicable. However, it is recommended not to graze treated areas for 3 days to ensure product efficacy.
Herbicide group: 2 (previously group B), Inhibition of acetolactate and/or acetohydroxyacid synthase (ALS, AHAS inhibitors)
Resistance risk: High

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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: 9 (previously group M), Inhibition of 5-enolpyruvyl shikimate-3 phosphate synthase (EPSP inhibition)
Resistance risk: Moderate

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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: 9 (previously group M), Inhibition of 5-enolpyruvyl shikimate-3 phosphate synthase (EPSP inhibition)
Resistance risk: Moderate

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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: Do not use treated water for human consumption, livestock watering or irrigation purposes for 10 days after application. Do not graze or cut sprayed vegetation for stock food for 1 day after application. See label for harvest withholding periods.
Herbicide group: 22 (previously group L), Inhibition of photosynthesis at photosystem I via electron diversion (PSI electron diversion)
Resistance risk: Moderate

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Flumioxazin 15 grams /tablet (Clipper herbicide®)
Rate: 1 tablet for every 37.5 cubic metres of water to achieve 400 parts per billion.
Comments: For use on dense or established weed populations in enclosed water bodies, deeper than 0.5 m and larger than 37.5 cubic metres, or margins of larger, still water bodies. Throw tablets directly into the water to achieve uniform distribution of the herbicide. See label for restrictions.
Withholding period: 14 days before using treated water to irrigate food crops. See label for withholding periods for other uses of treated water.
Herbicide group: 14 (previously group G), Inhibition of protoporphyrinogen oxidase (PPO inhibitors)
Resistance risk: Moderate

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Flumioxazin 15 grams /tablet (Clipper herbicide®)
Rate: 1 tablet for every 75 cubic metres of water to achieve 200 parts per billion.
Comments: For use on low density, establishing or re-establishing weeds in enclosed water bodies, deeper than 0.5 m and larger than 37.5 cubic metres, or margins of larger, still water bodies. Throw tablets directly into the water to achieve uniform distribution of the herbicide. See label for restrictions.
Withholding period: 14 days before using treated water to irrigate food crops. See label for withholding periods for other uses of treated water.
Herbicide group: 14 (previously group G), Inhibition of protoporphyrinogen oxidase (PPO inhibitors)
Resistance risk: Moderate

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Flumioxazin 15 grams /tablet (Clipper herbicide®)
Rate: Spray 12 - 15 L of solution per 100 m2. Solution = 1 tablet per 50 L water + 0.5-1.0 % adjuvant/surfactant.
Comments: Spray on dense or established weed populations in enclosed water bodies, deeper than 0.5 m and larger than 37.5 cubic metres, or margins of larger, still water bodies. See label for restrictions.
Withholding period: 14 days before using treated water to irrigate food crops. See label for withholding periods for other uses of treated water.
Herbicide group: 14 (previously group G), Inhibition of protoporphyrinogen oxidase (PPO inhibitors)
Resistance risk: Moderate

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Flumioxazin 15 grams /tablet (Clipper herbicide®)
Rate: Spray 12 - 15 L of solution per 100 m2. Solution = 1 tablet per 100 L water + 0.5 -1.0% adjuvant/surfactant.
Comments: Spray on low density, establishing or re-establishing weeds in enclosed water bodies, deeper than 0.5 m and larger than 37.5 cubic metres, or margins of larger still water bodies. See label for restrictions.
Withholding period: 14 days before using treated water to irrigate food crops. See label for withholding periods for other uses of treated water.
Herbicide group: 14 (previously group G), Inhibition of protoporphyrinogen oxidase (PPO inhibitors)
Resistance risk: Moderate

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Flumioxazin 15 grams /tablet (Clipper herbicide®)
Rate: Inject solution into water body. 1 tablet per 37.5 cubic metres. Each tablet dissolved in at least 20L of water + 0.5 - 1.0% adjuvant/surfactant
Comments: For use on dense or established weeds in water bodies less than 0.5 m deep or with a volume less than 37.5 cubic metres. Dissolve tablets in water (at least 20L per tablet) mix thoroughly and then inject the solution directly into the water body.
Withholding period: 14 days before using treated water to irrigate food crops. See label for withholding periods for other uses of treated water.
Herbicide group: 14 (previously group G), Inhibition of protoporphyrinogen oxidase (PPO inhibitors)
Resistance risk: Moderate

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Flumioxazin 15 grams /tablet (Clipper herbicide®)
Rate: Inject solution into water body. 1 tablet per 75 cubic metres. Each tablet dissolved in at least 20L of water + 0.5 - 1.0% adjuvant/surfactant
Comments: For use on low density, establishing or re-establishing weeds in water bodies less than 0.5 m deep or with a volume less than 37.5 cubic metres. Dissolve tablets in water (at least 20L per tablet) and additives in a spray tank, mix thoroughly and then inject the solution directly into the water body. See label for restrictions.
Withholding period: 14 days before using treated water to irrigate food crops. See label for withholding periods for other uses of treated water.
Herbicide group: 14 (previously group G), Inhibition of protoporphyrinogen oxidase (PPO 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 pest 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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Reviewed 2023