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Water mimosa - plena (Neptunia plena)

Introduced to northern Australia as a vegetable, water mimosa - plena has been recorded in scattered locations in Queensland and in the Northern Territory. It can form floating rafts that outcompete native aquatic plants and affect waterways.

Key facts

Type: Weed, aquatic.

Native to: Introduced to Australia.

Established in: QLD, NT.

Other names: dead and awake.

Identification

  • A perennial upright herb that grows in water or on land, with stems up to 2 m long.
  • Its olive-green leaves have pairs of small oblong leaflets that close together when touched.
  • Yellow flowers form ball-shaped clusters 1–2 cm wide.
  • Its flat, leathery pods contain 8–20 brown, egg-shaped seeds.
  • Look-alike: Water mimosa - oleracea (Neptunia oleracea) has no upright stems, is not usually branched and has 4–8 seeds per pod, which are slightly larger.
  • Look-alike: Common sensitive plant (Mimosa pudica) has pink flowers and hairy leaves, stems and pods.

Where it grows

In Australia it has been recorded at scattered locations in Queensland from Brisbane to north of Cairns, and in Darwin and northeastern parts of the Northern Territory; it has not been found in NSW.

Why it matters

  • It can outcompete native aquatic plants and reduce food and habitat for fish and other aquatic animals.
  • It can reduce light and oxygen in the water and increase nitrogen levels, which could lead to more algal blooms.
  • It can increase water loss from dams, limit water flows and access to waterways, and create more suitable habitat for mosquitoes.

How it spreads

  • Viable seeds are spread by water and could also travel in mud attached to vehicles and machinery.
  • New plants can grow from stem fragments, which can spread in flowing water, especially during floods.
  • People have deliberately spread it by planting stem fragments as a vegetable crop.

Control options

  • For small infestations, dig out the roots where plants are growing in soil and remove all plant parts; remove the whole plant when it is growing in water.
  • Check treated sites regularly for regrowth.
  • Larger infestations can be treated with registered herbicides labelled for aquatic use.
  • In the North Coast, notify the local control authority if it is found.

Hygiene and washdown

  • Seeds could spread in mud attached to vehicles and machinery, so avoid moving mud between sites.
  • Inspect and clean vehicles, plant and footwear before leaving an infested area, paying attention to tyres, radiators, belly plates, tracks and floor mats.
  • Wash down at a designated bay where runoff is contained and can be monitored for seedlings.
  • Record each washdown so the client has evidence of clean entry and exit.

Classification by state

  • Australia (national): No national listing. Managed under state and territory law.
  • Queensland: Restricted matter, categories 2, 3, 4, 5. Report it to Biosecurity Queensland within 24 hours; do not give it away, sell or release it into the environment; do not move it; do not keep it. Everyone must also take all reasonable and practical steps to minimise its risks (the general biosecurity obligation).
  • New South Wales: Regional priority in 1 of 11 regions. Regional priority weed in North Coast. The general biosecurity duty applies: anyone dealing with it must prevent, eliminate or minimise the risk it poses.
  • Victoria: Not declared. Not declared under the Catchment and Land Protection Act 1994. Other Victorian laws may still apply.
  • South Australia: Not declared. Not declared under the Landscape South Australia Act 2019. Other laws may still apply.
  • Western Australia: Declared pest (C1), prohibited. C1 Exclusion: to be kept out of WA, or out of the parts of WA where it is not established. Report any sighting to DPIRD. It must not be brought into WA (prohibited organism).
  • Tasmania: Not declared. Not a declared weed in Tasmania. Other laws may still apply.
  • Northern Territory: Declared weed, Class A. Class A: to be eradicated. All declared weeds are also Class C, so they must not be brought into the NT.
  • Australian Capital Territory: Not declared. Not declared under the Biosecurity Act 2023 (ACT). A general biosecurity duty still applies.

Legal obligations

Every state and territory places a duty on landholders and people working on land to prevent the spread of invasive species, such as the general biosecurity obligation in Queensland and the general biosecurity duty in New South Wales. Declarations and control requirements differ between states and local government areas, so check the current official list before work starts.

  • Queensland: Biosecurity Act 2014 (Qld), administered by Biosecurity Queensland (13 25 23)
  • Northern Territory: Territory Parks and Wildlife Conservation Act 1976 (NT), administered by NT Department of Lands, Planning and Environment

Sources