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Chromolaena squalida (Chromolaena squalida)

Siam weed can grow rapidly, smother crops and native vegetation, and scramble up taller plants to 20 m. In Australia it threatens grazing, horticulture and forestry, and can degrade natural areas.

Key facts

Type: Weed, shrub.

Native to: Central and South America.

Established in: QLD.

Other names: white lantana.

Best time to act: Control before flowers mature and set seed; flowering most commonly occurs from May to July and again from September to October, but a variation in the upper Tully/Murray Rivers catchment flowers in February/March.

Identification

  • In the open, it forms a dense, tangled bush up to 3 m high, with several smooth, round stems growing from the plant’s crown.
  • Its soft, hairy, triangular green leaves are 5–12 cm long and have a distinctive three-vein pattern; crushed leaves have a pungent odour, and new growth is purple.
  • Pale pink-mauve, sometimes white, tubular flowers grow in clusters of 10–35 at branch ends.
  • The brown to black seeds are 4–5 mm long and have a tuft of white hairs and fine barbs.
  • Look-alike: Young plants resemble blue top or Billy goat weed (Ageratum spp.); mature Siam weed has a growth habit similar to lantana (Lantana camara).

Where it grows

In Australia it occurs in Queensland, including the Tully River, Bingil Bay, Townsville, Mossman, Innot Hot Springs and Mount Garnet areas, with an isolated infestation near Rockhampton.

Why it matters

  • It can out-compete and smother crops and native vegetation, including horticultural crops such as bananas, pawpaw and sugarcane, as well as forestry plantations.
  • It can degrade wet tropics, tropical savannah grasslands, subtropical and coastal regions, and conservation areas.
  • It may cause skin complaints and asthma in allergy-prone people; its high nitrate levels have been associated with cattle deaths and abortions overseas when stock ate contaminated fodder.
  • Dense dry-season thickets can contribute to frequent, intense bushfires, with dry stalks burning hotter and flames reaching higher into trees than in a grassfire.

How it spreads

  • Seeds can be carried by wind and water, and can attach to vehicles, machinery, clothing, footwear and animals.
  • The plant produces more than 80,000 seeds per plant per season, within 8–10 weeks after flowering.

Control options

  • Remove the basal/root ball for smaller infestations, and ensure the removed plant does not remain in contact with soil because it can re-shoot.
  • Use registered herbicides or permit-approved options according to their label and permit conditions; control before flowering and repeat as needed.
  • The stem-galling fly Cecidochares connexa was introduced in late 2019; its galls weaken the plant and reduce flowering and seeding.
  • Take reasonable and practical measures to minimise biosecurity risks, and check with local government about local requirements.

Hygiene and washdown

  • Clean vehicles, machinery and equipment, and clean clothing, shoes and camping gear before leaving an infested area.
  • Do not move potentially contaminated sand or soil, and quarantine livestock for at least one week before they leave an infested property.
  • 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: Restricted matter, category 3. Do not give it away, sell or release it into the environment. 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.

  • Queensland: Biosecurity Act 2014 (Qld), administered by Biosecurity Queensland (13 25 23)

Sources