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Grass-leaved arrowhead (Sagittaria graminea)

In Australia, grass-leaved arrowhead can form dense infestations in shallow waterways, blocking irrigation channels and drainage ditches. It can restrict water flow, increase sedimentation and aggravate flooding, and it is a nuisance in rice fields.

Key facts

Type: Weed, aquatic.

Native to: Eastern North America, from Labrador to Cuba and west to Wyoming.

Established in: NSW, VIC.

Other names: Slender arrowhead.

Best time to act: Inspect during the warm growing season when flowering plants are easier to identify, and aim to control infestations before seed matures.

Identification

  • An emergent or submerged perennial aquatic plant that grows in shallow, still or slow-moving fresh water.
  • Leaves above the water are upright, dark green and spear-shaped, with conspicuous radiating veins; submerged leaves are long, narrow and strap-like.
  • The white, sometimes pink, flowers have three petals and a bright yellow centre, and are carried in whorls on an upright stalk.
  • Flowers are about 3 cm across; the plant produces ball-shaped clusters of fruit containing numerous beaked seeds.
  • A short, brown, fleshy rootstock bears fibrous roots, fleshy rhizomes and tubers.
  • Look-alike: Native Sagittaria species may look similar, but typically have arrow-shaped leaves; flowers are needed for reliable identification to species.

Where it grows

In Australia it grows in shallow fresh water, including lakes, streams, pond margins, irrigation channels, drainage ditches and rice fields.

Why it matters

  • Dense growth can block irrigation channels and drainage ditches.
  • Infestations can restrict water flow and increase sedimentation, aggravating flooding.
  • It can become a nuisance in rice fields, including in the Murrumbidgee Irrigation Area of New South Wales.
  • Dense stands can inhibit the growth of native aquatic species.

How it spreads

  • Fleshy rhizomes and tubers help the plant spread and increase in density locally; fragments can establish elsewhere.
  • Seeds can be carried by water and wildlife, and can also be moved by machinery.
  • Floating mats can break apart and take root elsewhere in a waterway.

Control options

  • For small infestations, remove plants carefully, including the rootstock, rhizomes and tubers; contain all material so fragments cannot escape into the water.
  • Where physical removal is unsuitable, use only herbicides registered for aquatic use and approved methods for the site, following the product label and relevant state requirements.
  • Arrange treatment by suitably qualified operators where required, and avoid control work that spreads fragments or seed.
  • Revisit treated sites and adjoining waterways for follow-up control, and report suspected infestations to the relevant state or territory biosecurity agency.

Hygiene and washdown

  • Clean boats, machinery, footwear and other gear after working in infested water, and contain and dispose of plant fragments so they cannot enter another waterway.
  • Check and clean equipment before moving between waterways, including any soil or mud that could carry seeds or plant fragments.
  • 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: 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: 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
  • Victoria: Catchment and Land Protection Act 1994 (Vic), administered by Agriculture Victoria

Sources