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Long-leaved willow primrose (Ludwigia longifolia)

Long-leaved Willow Primrose forms dense colonies that can dominate wetlands and riparian vegetation, replacing similar native plants. Its growth can obstruct water flow and reduce biodiversity, and it is considered a potential pest in tropical, subtropical and warmer temperate parts of Australia.

Key facts

Type: Weed, aquatic.

Native to: South America, including Brazil, Paraguay, Uruguay and north-eastern Argentina.

Established in: QLD, NSW.

Other names: Longleaf Willow Primrose, Longleaf Primrose Willow, Longleaf Ludwigia, Willow Primrose.

Best time to act: It flowers commonly in summer and autumn; look for and treat actively growing plants in spring or summer, preferably before seed maturity and seed set.

Identification

  • It is an erect annual shrub that can grow to 3 m tall, with stems that branch heavily near the top.
  • The hairless, four-angled stems are green or reddish when young and become rounded and woody with age.
  • Its glossy, hairless, stalkless leaves alternate along the stems and are lance-shaped, 5–35 cm long and 4–25 mm wide.
  • Single yellow flowers grow in the axils of the upper leaves; they have four green or red pointed sepals and four or five veined petals.
  • The hairless, sharply four-angled fruit is 10–42 mm long and contains numerous tiny seeds; mature fruit turns light brown and splits open or disintegrates.
  • Look-alike: The native Ludwigia octovalvis has hairy stems, leaves and fruit, almost round stems, and cylindrical or only slightly ribbed fruit.
  • Look-alike: The native Ludwigia peploides subsp. montevidensis has a creeping habit and usually five petals and sepals, while the introduced Peruvian Water Primrose (Ludwigia peruviana) has hairy stems, leaves and fruit and shorter, broader leaves.

Where it grows

In Australia it occurs in waterways, wetlands, swamps, marshes, drains, channels and riparian areas, with records in New South Wales and Queensland including coastal areas, Sydney and Brisbane.

Why it matters

  • Dense colonies can dominate wetlands and riparian vegetation and replace similar native species such as Ludwigia octovalvis.
  • In Port Stephens it has formed dominant, highly invasive colonies with the potential to reduce biodiversity and threaten native species.
  • Dense growth can obstruct water flow and reduce the use and enjoyment of aquatic habitats.
  • It can reduce stream flow, increase sedimentation and decrease oxygen levels.

How it spreads

  • It reproduces by seed and by branches that detach during floods, take root and grow into new plants.
  • Its small, numerous seeds can be spread by water, wind and animals, as well as by human activities.
  • Contaminated soil and dumped garden waste can spread seeds; a mature plant can produce up to 2.5 million seeds per year.

Control options

  • Prioritise seedlings and the soil seed bank, and carefully remove seed heads first, securing them in plastic bags.
  • Hand-pull seedlings; larger plants can regrow unless most of the long root system is removed.
  • Plant segments can resprout, so incinerate or seal all plant parts in plastic bags, using approved methods.
  • For mature plants, scrape or cut and paint the stems with registered herbicides; treat while actively growing in spring or summer, preferably before seed set, and recheck for regrowth and new seedlings.

Hygiene and washdown

  • Avoid spreading seed on clothing, and do not move contaminated soil or plant material to new sites.
  • Bag seed heads securely; seeds may also be dispersed by water, so avoid transferring seed-bearing plant material between waterways.
  • 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: 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: Regional priority in 3 of 11 regions. Regional priority weed in Hunter, North Coast, South East. 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: Not declared. Not on the NT A to Z weed list, and Weeds Australia does not list it as declared in the NT. Check the NT declared weeds list before moving or planting it.
  • 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)
  • New South Wales: Biosecurity Act 2015 (NSW), administered by NSW Department of Primary Industries and Regional Development

Sources