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Mistflower (Ageratina riparia)

Mistflower is an aggressive introduced weed that invades pastures and bushland in New South Wales and Queensland. It can reduce pasture carrying capacity and displace native plants and the animals that depend on them.

Key facts

Type: Weed, herb.

Native to: Central America, including Mexico, and South America.

Established in: QLD, NSW.

Other names: Mist Flower, Creeping Croftonweed, River Eupatorium, Spreading Mistflower.

Best time to act: Look for mistflower during flowering from August to October, with flowering most abundant in mid-spring.

Identification

  • A low-growing, scrambling perennial herb or subshrub, usually 40 to 60 cm tall but sometimes reaching 1 m.
  • Its purplish, cylindrical stems have a sparse covering of fine hairs, and it has a short, thick rootstock.
  • Leaves grow in opposite pairs and are green, narrow-oval to lance-shaped, 3 to 13 cm long, with toothed margins.
  • Small white flowers, about 5 mm long, form clusters at the ends of long branches; flowering occurs from late winter to late spring.
  • Its dry, slightly curved seeds are dark brown to black, 1 to 2 mm long, and topped with a ring of rough hairs.
  • Look-alike: Crofton weed (Ageratina adenophora) is erect and often 1 to 2 m tall, unlike the creeping or scrambling mistflower.
  • Look-alike: Young Crofton weed stems have dense sticky glandular hairs, and its toothed leaves are broader and trowel-shaped.

Where it grows

In Australia, mistflower occurs mainly in coastal and near-coastal areas of New South Wales and southern Queensland, especially along shaded riverbanks and in humid subtropical and tropical rainforest.

Why it matters

  • It spreads into pastures in south-eastern Queensland and can significantly reduce their carrying capacity.
  • It can restrict livestock and farm machinery movement, and it has no feed value for stock.
  • In gullies, riparian areas and other bushland, it can form dense groundcover that excludes native plants and animals reliant on them.
  • It is reported to threaten rare and endangered plants, including the ravine orchid, Hartman's sarcochilus, Border Ranges daisy and giant spear lily.

How it spreads

  • Spread is mainly by seed, which is adapted to move in wind and water.
  • Seeds can also be carried as impurities in agricultural produce or in roadwork sand and gravel, and in mud on animals, machinery, vehicles, clothing and footwear.
  • Existing colonies can expand when stems layer and form an interwoven mat.

Control options

  • Avoid overgrazing pasture, and establish thick pasture to compete with mistflower.
  • Use registered herbicides where appropriate; spot-spray regrowth, and be prepared to repeat treatment because mistflower is resistant to some herbicides.
  • Hoeing, grubbing and burning can be used, but sow replacement pasture or native plants in treated areas to limit re-establishment and erosion.
  • The leaf smut fungus Entyloma ageratinae is reported as an established biological control agent impacting mistflower in Australia.

Hygiene and washdown

  • Check and remove seed-bearing mud from machinery, vehicles, clothing and footwear before moving between sites.
  • Avoid moving contaminated agricultural produce, sand or gravel, as these can carry seeds.
  • 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 1 of 11 regions. Regional priority weed in Hunter. 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: Permitted, not declared. Listed as a permitted organism in WA, so it can be brought in, but it is not a declared pest.
  • Tasmania: Not declared. Not a declared weed in Tasmania. Other laws may still apply.
  • Northern Territory: Declared weed. Weeds Australia lists it as a declared weed in the NT under the Weeds Management Act 2001. Check the NT declared weeds list for its class and what it requires.
  • 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