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Arrowhead (Sagittaria calycina)

Arrowhead grows quickly and forms dense infestations in wetlands, waterways and irrigation channels in Australia. It can choke watercourses, disrupt irrigation and make waterways harder to use.

Key facts

Type: Weed, aquatic.

Native to: North and South America.

Established in: NSW.

Best time to act: Arrowhead is most visible in summer; control plants in spring and summer before seeds are produced, and apply herbicides when water levels are low and plant growth is high.

Identification

  • Arrowhead grows as a submerged rosette or as an emergent plant up to 1 m tall, and is most visible in summer.
  • Leaves above water are green, prominently veined and strongly arrow-shaped, up to 25 cm long and 20 cm wide.
  • Underwater leaves are narrow, strap-like and green, measuring 2.5–45 cm long and 0.5–2 cm wide.
  • Small flowers have three white petals, sometimes with a purple base, and grow in whorls along a leafless stem.
  • Flattened seeds have a beak at the tip and wings on the sides, and are 1.5–3 mm long.
  • Look-alike: Sagittaria (Sagittaria platyphylla) has oval leaves and triangular leaf stalks, unlike arrowhead’s arrow-shaped leaves and round stalks.
  • Look-alike: Alisma (Alisma lanceolatum) has spear-shaped leaves with up to seven prominent veins and smaller flowers; native water plantain (Alisma plantago-aquatica) and star fruit (Damasonium minus) have oval leaves and smaller flowers.

Where it grows

In NSW, arrowhead is most common in the Riverina, where it grows in rice crops and irrigation channels, as well as in shallow, warm, slow-moving or still waters such as wetlands, swamps, marshes, drainage gullies and flooded paddocks.

Why it matters

  • Dense infestations can reduce water flow, outcompete native water plants and restrict movement of fish and other aquatic animals.
  • Arrowhead can reduce food and habitat for aquatic animals and make swimming, boating and fishing difficult.
  • In irrigation channels it traps silt, reduces water capacity and efficiency, and can be expensive to remove.

How it spreads

  • Each plant can produce hundreds of thousands of seeds, which can float for up to three weeks before sinking.
  • Water, birds and other animals spread the floating seeds; seeds may sprout straight away or remain dormant until conditions are suitable.
  • New plants can also grow from stem or root fragments.

Control options

  • Dig or excavate plants, removing all fragments, including roots and rhizomes, and prevent fragments from floating downstream.
  • Physical removal can suit new or isolated infestations, but excavation can be labour-intensive and costly and may interfere with irrigation-channel structures.
  • Only use herbicide products approved for aquatic use; foliar herbicides may not kill underwater parts, especially in deep water.
  • Check treated areas and remove regrowth and new seedlings, as follow-up is needed for successful control.

Hygiene and washdown

  • Remove all plant material and soil from machinery and equipment before leaving the site.
  • Use bunding, fences or screens to contain arrowhead and prevent plant fragments moving downstream.
  • 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: 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.

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

Sources