Biosecurity Spotters logo

Giant arrowhead (Sagittaria montevidensis)

Arrowhead is a serious weed of rice fields in the Murrumbidgee Irrigation Area, where it is likely to spread. Dense infestations can block channels and drainage ditches and are an increasing problem in rice fields.

Key facts

Type: Weed, aquatic.

Native to: North and South America.

Established in: NSW.

Other names: Californian Arrowhead, Sagittaria.

Best time to act: Seeds germinate in spring, plants mature during summer and flower from January to March, and seeds mature in autumn.

Identification

  • It is an aquatic annual or short-lived perennial that grows in an erect tuft to about 1 metre tall.
  • Emergent leaves are broadly arrow-shaped, up to 25 cm long and 20 cm wide, with long basal lobes.
  • Submerged leaves are strap-like and translucent.
  • White flowers, sometimes with a purple patch in the centre, have three petals and grow in whorls of three.
  • Its fruit forms a cluster about 2 cm wide, with flattened, winged seeds 2–3 mm long.
  • Look-alike: It may be confused with Sagittaria platyphylla, Vallisneria species, Ottelia ovalifolia and Alisma plantago-aquatica.
  • Look-alike: The submerged strap-like leaves can resemble juvenile leaves of these species, but Arrowhead’s emergent leaves have a distinctive arrowhead shape.

Where it grows

In Australia it is naturalised in the Murrumbidgee Irrigation Area in New South Wales, in shallowly flooded or marshy areas and in drainage ditches, rice fields and irrigation-associated swamps.

Why it matters

  • Dense infestations block channels and drainage ditches.
  • Arrowhead is an increasing problem in rice fields.
  • It is not yet an abundant weed in Australia, but spread through the Murrumbidgee Irrigation Area rice fields is considered likely.

How it spreads

  • In Australia, Arrowhead spreads by seed.
  • Seeds move in water along channels and ditches.
  • Seeds can also move in mud stuck to animals, machinery, clothes and footwear.

Control options

  • Herbicides enable good control; use registered herbicides and consult the Australian Pesticides and Veterinary Medicines Authority for chemical information.
  • For physical control information, consult NSW Weedwise.
  • No biological control agent is reported for Arrowhead.

Hygiene and washdown

  • Clean mud from machinery, clothing and footwear before moving between sites, as seeds can be carried in mud.
  • Avoid moving infested water or mud along channels and 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: 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: Declared plant. Depending on the declaration, it can be an offence to move it on public roads or sell it, and landowners may be required to control it on their land.
  • 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: Declared weed. Property managers are required by law to control declared weeds, and their import and sale are prohibited.
  • 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.

  • New South Wales: Biosecurity Act 2015 (NSW), administered by NSW Department of Primary Industries and Regional Development

Sources