Water star grass (Heteranthera zosterifolia)

Water star grass is a water weed with white to pale blue flowers that can grow both under and above the water. It grows well in warm water and can form dense mats, reducing water quality.

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How does this weed affect you?

Water star grass forms thick mats on the water's surface that:

• smother and outcompete native plants
• reduce light, oxygen levels and water quality
• reduce food and habitat for fish and other aquatic animals
• limit water flow, increasing the risk of floods
• limit access to water for recreational use.

What does it look like?

Water star grass is a perennial aquatic plant. Plants usually start growing underwater, rooted in the mud. These plants look like star-shaped grass and send out long stems to the water’s surface. 

Leaves:

Leaves on underwater plants:

• grow in a star shaped rosette
• are grass-like with pointed tips
• are up to 50 mm long and 7 mm wide
• are stalkless.

 Leaves on floating plants are:

• often oval-shaped with rounded tips
• about 4 cm long
• on stalks up to 8 cm long.

 Flowers:

• are white to pale blue, darker at the center
• have a tube 1 cm long that spreads into 6 lobes/petals
• usually grow in pairs but sometimes in groups of up to 10.

Fruit are:

• a capsule
• 0.5-1 mm long.

Seeds:

• are winged
• 8-14 per capsule.

Roots:

Plants form roots at nodes. They are usually rooted in the mud.

Similar looking plants

Water star grass looks similar to kidney-leaf mud-plantain (Heteranthera reniformis), which has kidney-shaped leaves and does not have any underwater leaves.

Where is it found?

Water star grass grows near Port Macquarie on the North Coast and at Morriset in the Hunter region. It has not been found anywhere else in Australia.

Water star grass is native to South America.

What type of environment does it grow in?

Water star grass is an aquatic weed that grows in or on still freshwater.  It prefers warm water and in shallow water it can form very dense mats. It can grow in wetlands, drains, farm dams and other damp areas. 

Maps and records

• Recorded presence of Water star grass during property inspections (Map: Biosecurity Information System - Weeds, 2017-2024)
  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 seeds

Plants produce seeds. Spread is most likely caused when pond or aquarium waste is dumped in wetlands and waterways. Seed could also spread by wind, waterflow out of ponds or by animals especially water birds.

By plant parts

Stem fragments can break off adult plants and start new infestations. Broken stem fragments can be moved by floods, birds and sticking to boats, machinery and people. 

References

PlantNET (The NSW Plant Information Network System). Royal Botanic Gardens and Domain Trust, Sydney. Retrieved 02 July 2021 from: https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Heteranthera~zosterifolia

Technigrow Australia (2012). WeedWatch - Water star grass (Heteranthera zosterifolia). Retrieved 02 July 2021 from http://www.technigro.com.au/wp-content/uploads/2016/10/52-Water-Stargrass.pdf

Zlatkovic, B. K., & Bogosavljevic, S. S. (2020). Risk analysis of alien plants recorded in thermal waters of Serbia. Weed Research, 60(1), 85-95.

More information

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Control

Physical removal

Remove individual plants or small infestations. Take care to remove all roots and stem fragments. Try not to break the stems when removing plants. Contact your local council for advice on how to dispose of these plants.

Chemical control

Larger infestations can be controlled with herbicides. Herbicides can be applied via tablets, injection, spot spraying or a splatter gun. See label for detailed instructions.

Herbicides must be registered for aquatic use.

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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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. Alternatively, if weeds grow in clusters, concentrate the tablet application on the densest areas. 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. Alternatively, if weeds grow in clusters, concentrate the tablet application on the densest areas. 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 20 L 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 20 L 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 20 L 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 20 L 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.
Greater Sydney	Regional Recommended Measure* (for Regional Priority - Prevention) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found.
Hunter	Regional Recommended Measure* (for Regional Priority - Prevention) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found.
North Coast	Regional Recommended Measure* (for Regional Priority - Eradication) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found.
South East	Regional Recommended Measure* (for Regional Priority - Prevention) Land managers should mitigate the risk of the plant being introduced to their land. Land managers should eradicate the plant from the land and keep the land free of the plant. A person should not deal with the plant, where dealings include but are not limited to buying, selling, growing, moving, carrying or releasing the plant. Notify local control authority if found.
	*To see the Regional Strategic Weeds Management Plans containing demonstrated outcomes that fulfil the general biosecurity duty for this weed click here

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Reviewed 2023