Yellow waterlily (Nymphaea mexicana)

Also known as: water lily, yellow water lily, Mexican waterlily, water lilies

Yellow waterlily has large floating leaves and bright yellow flowers. It forms dense infestations on waterways that reduce water quality and outcompete native plants.

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How does this weed affect you?

Yellow waterlily forms dense infestations that:

• reduce water quality
• outcompete native aquatic plants
• reduce habitat for native aquatic animals
• increase siltation 
• restrict water flows
• increase breeding habitat for mosquitoes
• limit access and recreational use of waterways.

What does it look like?

Yellow waterlily is a perennial water weed with large floating leaves and flowers that grow above the water. The leaves and flowers grow from an underwater rhizome. Rhizomes grow vertically, are knobby, and up to 30 cm long and 4 cm thick. Long white horizontal stolons are produced at the top of the rhizome.

 Leaves are:

• usually floating but may be above the water when crowded
• green on the top side, sometimes with brown markings
• green or red on the underside
• up to 25 cm in diameter
• circular with a slit near the stalk
• waxy
• wavy along the margins
• on thick fleshy stalks up to 1.5 cm in diameter.

Flowers are:

• bright yellow with many petals
• up to 12 cm in diameter
• raised above the water
• open during the day and closed at night (live for one week).

Seeds are:

• rarely produced in Australia
• 5 mm long.

Roots:

Roots can form from rhizomes where they touch the mud. Floating storage roots can also grow from the end of stolons. These roots:

• grow in bunches
• are small, yellow and banana shaped.

Similar looking plants

Yellow waterlily looks similar to:

• White waterlily (Nymphaea alba), which does not have brown marks on the leaves and its flowers are white, pink, red or orange.
• Giant waterlily (Nymphaea gigantea), which has leaves up to 75 cm in diameter and blue or white flowers
• Cape waterlily (Nymphaea capensis), which has white, blue or bluish- pink flowers and its leaves do not have wavy edges.

Yellow waterlily has been crossed with other species to produce hybrids that have yellow or orange flowers. Commercial names of hybrids include Sioux, Yellow Comanche and Texas dawn. 

Where is it found?

In NSW yellow waterlily has been found in the Central and Northern Tablelands, Greater Sydney, Hunter, North Coast and Riverina regions.

Yellow waterlily is native to Mexico and southeastern USA.  It was introduced to Australia in the early 1900 as an ornamental plant.

What type of environment does it grow in?

Yellow waterlily grows in still or slow moving waterways up to 2 m deep. It prefers nutrient rich water.

Maps and records

• Recorded presence of Yellow waterlily 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 plant parts

Yellow waterlily is mostly spread by plant parts. Stolons produce new plants at the nodes, which detach when small. Most small plants are spread by water but they can also be spread via boats and other recreational equipment. Birds may also spread the plants.

By seed

Seeds are rarely found in Australia. The flowers are pollinated above the water then the stems droop over and the fruit is formed under water. Seeds are released into the water if present.

References

Bonilla-Barbosa, J., Novelo, A., Orozco, Y. H., & Márquez-Guzmán, J. (2000). Comparative seed morphology of Mexican Nymphaea species. Aquatic Botany, 68(3), 189-204.

Champion, P.D., Burnett, D.A. and Petroeschevsky, A. (2008). Risk assessment of tradeable aquatic plant species in Australia. Prepared for NSW Department of Primary Industries. National Institute of Water and Atmospheric Research, O’Connor, Western Australia.

Hofstra, D. E., Champion, P. D., Dugdale, T. M., Fridman, M., Baker, R., & Finlay, M. (2013). Comparison of use rates and treatment timing with glyphosate to control Mexican water lily. J. Aquat. Plant Manage, 51, 69-76.

Johnstone, I. M. (1982). Yellow waterlily (Nymphaea mexicana) in Lake Ohakuri, North Island, New Zealand. New Zealand Journal of Botany, 20(4), 387-389.

Naidu, P. (2018). The invasion ecology of Nymphaea mexicana Zucc.(Mexican waterlily) in South Africa (Doctoral dissertation, MSc. Thesis. Rhodes University, Grahamstown, Eastern Cape, South Africa).

PlantNET (The NSW Plant Information Network System). Royal Botanic Gardens and Domain Trust, Sydney. Retrieved 3 February 2023 from https://plantnet.rbgsyd.nsw.gov.au

Sainty, G. R., & Jacobs, S. W. (2003). Waterplants in Australia (No. Ed. 4). Sainty and Associates Pty Ltd.

More information

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Control

Successful weed control relies on follow up after the initial efforts. This means looking for and killing regrowth or new seedlings. Using a combination of control methods is usually more successful.

Prevention

To reduce the risk of spreading yellow waterlily to new locations:

• Don’t run boat/watercraft motors through weed beds.
• Check and remove all plant material from boats, canoes, trailers, vehicles, machinery and fishing gear before leaving waterways.
• Stop nutrient-rich runoff entering infested waterways.
• Never dump unwanted water plants .

Physical removal

By hand

Small infestations of yellow waterlily can be manually removed. All parts of the plant need to be removed because it can regrow from the rhizomes.

By machine

Large infestations can be harvested or excavated remaining fragments will regrow. Follow up removal or other control may be required.

Chemical control

Spraying

Apply approved herbicide twice per year. Herbicides are most effective when water levels are low. Repeat applications might be required.

Herbicide options

See Using herbicides for more information.

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Diquat 20 g/L (Watrol®)
Rate: 4.0 L per 100 L of water
Comments: Apply as an overall spray. Thoroughly wet foliage. Best if clean water is used; higher rate if dense weed or dirty water. Observe withholding period.
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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Diquat 20 g/L (Watrol®)
Rate: 50–100 L/ha
Comments: Apply as an overall spray. Thoroughly wet foliage. Best if clean water is used; higher rate if dense weed or dirty water. Observe withholding period.
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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Diquat 200 g/L (Reglone®)
Rate: 400 mL per 100 L of water
Comments: Add Agral 600 wetter; use clean water for best results. Observe withholding period.
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: L, Inhibitors of photosynthesis at photosystem I (PSI inhibitors)
Resistance risk: Moderate

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Diquat 200 g/L (Reglone®)
Rate: 5.0–10.0 L/ha
Comments: Add Agral 600 wetter; use clean water for best results. Observe withholding period.
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: L, Inhibitors of photosynthesis at photosystem I (PSI inhibitors)
Resistance risk: Moderate

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Glyphosate 360 g/L (Only products registered for aquatic use)
Rate: 1 L to 100 L of water
Comments: Re-treat unaffected plants.
Withholding period: Nil.
Herbicide group: M, Inhibitors of EPSP synthase
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.
Central West	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.
Murray	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.
North West	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.
Northern Tablelands	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.
Riverina	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. Your local biosecurity weeds officer can help to identify, advise on control, and how to remove this weed.
	*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