Summary:An erect, branching, shallow rooted, annual herb to 3.5 m tall (usually 0.5-2 m) with resinous and aromatic glands. It has mainly opposite, deeply lobed, greyish leaves and yellow-green flowers with no 'petals' around autumn. The stems are angled and often reddish.
Leaves:Mainly opposite. Green. Upper ones tend to be alternate.
Stems:Erect. Branching. 500-3500 mm tall, brown/green. Hairless to roughly hairy.
Flower head:Each head has only one sex of flower. Male heads are higher on the plant than female. Male heads have 10-100 flowers in drooping hemispherical clusters with a set of cup-like bracts underneath. Many of these clusters are attached to a bare stalk, up to 200 mm long, at the ends of stems or branches. Females in single saucer shape, wavy edged flowered heads, 2.5-5 mm long, and borne singly or in clusters in the upper leaf axils or on short branches. Often very branched. 5-7 fine and tapering spines surround the head near the top.
Flowers:Cream or pale green. Separate male and female flowers.
Fruit:Head with 4-7 spines or teeth about 1 mm long near the top.
Seeds:Light brown. 3-5 mm long, 2-3 mm diameter. Woody. Top shaped with an awl shaped, 1-2 mm long, beak and surrounded by 4-7, 1 mm long, spines.
Roots:Strong taproot to 500 mm deep. Many branching fibrous roots.
Flowering times:Late summer to May.
Seed Biology and Germination:Produces an average 3000 seeds per year with some producing up to 62,000. Seeds won't germinate for some time after maturity. Exposure to light and alternating temperatures improves germination. Seeds become dormant with high temperatures and may survive for over 40 years.
Allelopathy:It produces toxins that affect the growth of neighbouring plants, especially ryegrass, oats, onions and Amaranthus.
Population Dynamics and Dispersal:The main spread is by the spiny seed attaching to animals, clothing or machinery. It is also spread by water and top soil used for roads and gardens.
Origin and History:North America.
Soil:Prefers wet areas.
Plant Associations:Open areas.
Detrimental:Weed of pastures, stubbles, fallows, arable and disturbed land, water courses, roadsides.
Toxicity:Toxic. The unpleasant odour usually makes it distasteful to stock.
Symptoms:It may cause a sore mouth in stock that graze it.
Treatment:Exclude stock from infested areas.
Legislation:Noxious weed of NT, QLD, SA.
Management and Control:Cultivation, mowing and grazing are generally not effective control techniques because of the high levels of dormant seed. Herbicides provide good short term control for crops. Establishment of perennial pasture provides good long term control of the weed. It is not common as a contaminant of agricultural seed or produce.
Thresholds:It is an aggressive coloniser of waste ground.
Eradication strategies:It is susceptible to many common herbicides, but the seed dormancy and longevity make eradication difficult.
Herbicide resistance:Resistant forms of ragweed have developed where triazine herbicides have been regularly used.
Biological Control:A number of bio control agents have been selected and two have been released in Qld and NSW.
Related plants:Burr Ragweed (Ambrosia confertiflora)
Plants of similar appearance:References: