Spongeplant (Limnobium spongia)

Spongeplant is a water weed with small, spongy glossy leaves. Plants grow very quickly, forming dense mats over water bodies.

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How does this weed affect you?

Spongeplant is a fast growing, 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
• can restrict recreational activities and movement of boats in waterways.

What does it look like?

Spongeplant is a perennial water weed that can be free floating or rooted in mud.

Leaves are:                                                                  

• bright green and shiny above
• thick and leathery
• oval or heart shaped with a pointed tip
• 1–10 cm long and 1–8 cm wide
• on slender ridged stalks

Young leaves have a central disk with red spongy cells on the underside. As the leaves mature they lose the spongy cells and can extend up to 50 cm above the water. 

Flowers are:

• usually 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:

• fleshy berry-like capsules with up to 200 seeds
• 4–18 mm long and 4–12 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:

• 0.9–2.3 mm long
• slightly flattened
• covered in small hairs/bristles up to 0.4 mm long.

Stems:

There are two types of stems.

• Short stems that are mostly branched and have leaves.
• Long unbranched stems, reaching up to 50 cm, that have no leaves along their length and produce a daughter plant on the end.

Roots:

• are white and hairy
• are floating or 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 looking plants

Spongeplant looks like two other weed species:

• Frogbit (Limnobium laevigatum), which is known to occur in NSW. Frogbit has smaller fruit (2–5 mm diameter) and has a rounded leaf, never with a pointed leaf tip.
• Water poppy (Hydrocleys nymphoides), which has large yellow flowers up to 8 cm wide and larger leaves up to 12 cm wide.

Where is it found?

Spongeplant has not been found in Australia.

It is native to the south eastern states of the United States of America (USA) .

What type of environment does it grow in?

Spongeplant grows in fresh water and prefers areas where winter temperatures stay above 0°C. It can grow in regions with winter temperatures below 0°C but there are no green parts present over winter. Plants can be free floating or rooted in mud. It tolerates both shade and full sun.

In the USA it grows in lakes, ponds, swamps, wetlands and along the edges of rivers, canals and ditches.

Maps and records

• Recorded presence of Spongeplant 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?

By seed

Each fruit can produce more than 200 seeds which are released under the water or in the mud. Seeds sprout under water and then the small seedlings float to the surface where they can then be spread by water currents. Numerous small bristles on the seeds could help them stick to birds, other animals or water craft. Seeds could also be spread in contaminated mud.

By plant parts

Spongeplant spreads within a water body from daughter plants. Spread to new areas can occur when young plants or plant parts are moved by water, birds or attaching to watercraft or equipment.

References

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.

Les, D. H., & Mehrhoff, L. J. (1999). Introduction of nonindigenous aquatic vascular plants in southern New England: a historical perspective. Biological invasions, 1(2), 281-300.

UF/IFAS (2021). Limnobium spongia. Centre for Aquatic and Invasive Plants. Retrieved 13 April 2021 from: https://plants.ifas.ufl.edu/plant-directory/limnobium-spongia/

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.

Prevention

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

Do not dump aquarium plants.

Herbicide options

See Using herbicides for more information.

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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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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 2021