Parthenium is a fast-growing annual herb and a Weed of National Significance in Australia. It invades grazing land, summer cropping areas, and disturbed sites, and can outcompete other plants, including through allelopathic effects. It can cause severe dermatitis and respiratory problems in people, and is unpalatable to stock but may cause livestock health problems if eaten when other feed is scarce. Seeds are readily spread by vehicles, machinery, livestock, water, and contaminated feed or seed.
Identify it
Field cues
01Basal rosette of many divided, fern-like leaves in early growth
02Erect, ribbed or grooved, hairy stem that becomes much-branched above
03Small white to creamy-white flower-heads in clusters at branch tips (about 4–5mm across)
04Leaves alternate along the stem, with upper leaves smaller and less divided
05Black seeds (achenes) about 2mm long with two small white scales
Distribution records are indicative. Always confirm current status with your state biosecurity authority.
Found in: pastures, roadsides, railway lines, disturbed areas, croplands.
Manage it
Control Overview
Management focuses on preventing seed spread and stopping seed set. Small infestations can be physically removed when plants are young, while herbicides are used for broader control. Mature plants that have set seed can increase infestations if disturbed. Ongoing vigilance is needed because new seedlings can emerge after control, and treated areas may need repeat control over multiple seasons.
1
Prevent seed movement on machinery and vehicles
Clean down machinery, vehicles and equipment before leaving infested areas and before entering clean paddocks or properties. Pay extra attention after working in disturbed areas and along roadsides.
2
Remove small plants before flowering
Target individual plants at the rosette stage or before flowering and seed set. Avoid bare-hand contact and avoid disturbing mature plants that have already set seed.
3
Treat actively growing plants and recheck sites
Use appropriate herbicide application methods for the site and ensure foliage coverage on actively growing plants. Keep watch on treated areas for at least 2 years and repeat control if new plants emerge.
Use chemicals to the label. Always read the current APVMA-approved label before application.
Check permit conditions for your state, as some uses are limited to council or government staff.
Aquatic situations require products registered for use in or near water.
Why it matters
Impacts
Spreads readily via contaminated movement
Seed can be transported long distances on vehicles, farm machinery, livestock, and in contaminated feed and pasture seed, allowing new infestations to establish in disturbed sites.
Suppresses other plants
It can outcompete other plants for nutrients and moisture and can release chemicals into the soil that inhibit the growth of other plants.
Impacts crops and pasture production
It can compete with crop seedlings and reduce crop yields, contaminate grain, and reduce pasture carrying capacity.
Toxicity note (humans, livestock): Can cause severe dermatitis and respiratory problems in humans and can be toxic to livestock if eaten.
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