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Uruguay primrose-willow (Ludwigia grandiflora)

For Australian land managers, water primrose is important to recognise because dense growth can choke waterways and wetlands if it establishes. Early identification and careful removal help limit the spread of fragments to new sites.

Key facts

Type: Weed, aquatic.

Native to: South and Central America.

Established in: .

Other names: Perennial water primrose, Uruguayan primrose-willow, Large-flowered primrose-willow.

Best time to act: Inspect during active growth and flowering, when the yellow flowers aid identification, and treat before seed set where practicable.

Identification

  • It is a perennial aquatic plant that can form very dense mats on water or mud, with stems growing horizontally and sometimes emerging above the water.
  • Stems are hairless or sparsely hairy, and the alternate leaves vary in shape: young growth often has rounded leaves in rosette-like clusters, while flowering growth has longer, lance-shaped or oval leaves.
  • Bright yellow flowers, 2–5 cm across, have five petals and grow from the leaf axils.
  • The fruit is a narrow cylindrical capsule, 13–25 mm long, containing numerous small seeds.
  • Look-alike: Ludwigia peploides is very similar and can be difficult to distinguish from L. grandiflora when flowers are absent; seek expert identification if the species is uncertain.

Where it grows

It grows in water and on damp ground such as riverbanks and wet meadows, and can occupy gravel, sand, mud and peat around wetlands and waterways.

Why it matters

  • Dense growth can block waterways, irrigation ditches and canals, hindering navigation, fishing, hunting, irrigation and drainage.
  • Large stands can slow water flow, increase sedimentation and contribute to reduced channel capacity and flooding risk.
  • Dense growth can reduce light and oxygen in the water, harming aquatic fauna and submerged plants.
  • It can outcompete native plants, reduce biodiversity and displace wetland grasses used for grazing.

How it spreads

  • The plant spreads readily from vegetative fragments, which can float away from parent plants and regrow.
  • It also produces seeds; the species is insect-pollinated, and germination requires cold stratification.
  • Ornamental planting is a pathway for movement, so do not move plants or fragments between sites.

Control options

  • For small infestations, remove plants carefully by hand or with suitable machinery, collecting all fragments so they cannot float away and establish elsewhere.
  • Mechanical removal or bank reprofiling may be suitable at some sites; plan the work to avoid breaking up and dispersing plants.
  • Use only herbicides registered for the target plant and aquatic situation in your state or territory, and follow the product label and any permit requirements.
  • Revisit treated sites to check for regrowth and seedlings, and report suspected plants to the relevant state or territory biosecurity agency where appropriate.

Hygiene and washdown

  • Before leaving an infested site, remove and contain plant material from boats, machinery, vehicles, boots and other gear; do not carry fragments to another waterway.
  • Avoid transferring water, mud or soil from infested areas, and clean equipment before moving 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): National priority exotic pest. On the National Priority List of Exotic Environmental Pests, Weeds and Diseases, so any new detection should be reported straight away.
  • Queensland: Not prohibited or restricted. Not listed as prohibited or restricted matter in Queensland. Everyone must also take all reasonable and practical steps to minimise its risks (the general biosecurity obligation).
  • New South Wales: Not a priority weed. Not a state or regional priority weed in NSW. 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: Not listed. Not found in the Western Australian Organism List. Check with DPIRD before moving or keeping it.
  • Tasmania: Not declared. Not a declared weed in Tasmania. Other laws may still apply.
  • Northern Territory: Check NT list. There is no NT weed profile for it. Check the NT declared weeds list for its current status.
  • 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.

Sources