What Does It Look Like?
What is it?
Burr Ragweed (Ambrosia confertiflora) is an erect perennial herb to 2 m high, forming large colonies from creeping runner-like roots. It has grey-green, twice-divided, fern-like leaves, 12–16 cm long and 10–15 cm wide.
Flowers are yellow-green and of two kinds. The male flower consists of shortly stalked hemispherical heads (made up of many tiny male florets), 1 cm in diameter, which are grouped into branching spikes at the ends of the stems. The female florets, without petals, form 1-flowered heads, 5 mm long and 4 mm wide, clustered between the base of the upper leaves and the stems, and surrounded by a ring of spiny bracts (modified leaves). In the male flowers, the bracts are not spiny and they are fused together.
The fruit is about 4 mm long and covered with 10–20 short, hooked spines (Parsons & Cuthbertson 2001). The seeds are brown, woody, 3–4 mm in diameter a
For further information and assistance with identification of Burr Ragweed, contact the herbarium in your state or territory.
Growth form (weed type/habit)
Where it currently grows? Preferred habitat
Burr Ragweed occurs in the dry plains of temperate regions, on a variety of soils especially in run-down pastures, orchards, roadsides, wastelands and other disturbed areas (Parsons & Cuthbertson 2001).
Are there similar species?
Burr Ragweed is reminiscent of Chrysanthemum, in the height and leaf form, but the flowers are inconspicuous. Several other weed species are similar, such as Perennial Ragweed (Ambrosia psilostachya) and Chinese Mugwort (Artemisia verlotiorum) (Southern Tablelands and South Coast Noxious Plants Committee undated) but Artemisia species have heads that contain clusters of flowers of both sexes. The seeds of Burr Ragweed have 10–20 short hooked spines, whereas the seeds of Perennial Ragweed have a short pointed beak surrounded by 4–5 blunt projections (Parson & Cuthbertson 2001).