What Does It Look Like?
What is it?
Siam Weed (Chromolaena odorata) is an upright shrub growing up to 5 m tall, forming dense spreading thickets, or sprawling or scrambling plants growing up to 20 m high when climbing up vegetation. Its stems are slender and woody towards the base, with many stems produced from a long-lived root stock. Side branches are usually produced in pairs. The leaves are arrowhead-shaped, 50-120 mm long and 30-70 mm wide, with three characteristic veins in a 'pitchfork' pattern. They grow in opposite pairs along the stems and branches. As the species name 'odorata' suggests, the leaves emit a pungent odour when crushed.
The small flower-heads do not have any 'petals' and are borne in dense clusters of 10–35 tubular flowers, at the ends of the branches. These flower-heads (about 10 mm long and 3 mm wide) are pale pink or pale mauve in colour (sometimes appearing whitish when older) and consist of numerous tiny flowers (tubular florets). The flowers are surrounded by several layers of overlapping slender bracts 8–9 mm long. Flowering occurs from late summer through to early spring, but is most abundant during winter (Weeds of Australia 2016).
The seeds are achenes (fruit) and are dark coloured, 4-5 mm long, narrow and oblong, with a parachute of white hairs which turn brown as the seed dries. The root system is fibrous and generally reaches a depth of 300 mm (Parsons & Cuthbertson 1992; Hill & Ostermeyer 2000; CRC 2003).
For further information and assistance with identification of Siam Weed contact the herbarium in your state or territory.
Flower colour
Purple, White, Pink
Growth form (weed type/habit)
Herb, Shrub
Where it currently grows? Preferred habitat
Siam Weed is an opportunistic species generally confined to forest edges and clearings and forms dense thickets in disturbed situations. It grows best in the tropics and subtropics in areas receiving 1200 mm of rainfall or more, and though not tolerant of frost it can be found at altitudes up to 1000 m. It grows on most soil types but prefers well drained soils in full sun (Hill & Ostermeyer 2000; CRC 2003). It has also been reported as occurring on stream banks, bushland, roadsides, waste areas, neglected pastures, crops and plantations (Navie 2004).
Are there similar species?
Siam Weed may occasionally be confused with Gymnocoronis spilanthoides (Senegal Tea Plant), Ageratina adenophora (Crofton Weed) and Ageratina riparia (Mistflower). Navie (2004) provided the following guide for distinguishing between these species:
Siam Weed is a large upright, terrestrial plant 1.5-5 m tall with stems that are not hollow. Leaves are relatively broad with slightly toothed margins. Its small flower heads are 15-20 mm in diameter, pale pink or mauve in colour and borne in dense clusters at the tips of the branches.
Senegal Tea Plant is an aquatic plant with somewhat hollow stems and narrowly egg shaped (ovate) or lance-shaped (lanceolate) leaves with finely toothed (serrated) margins. Flower heads are 15-20 mm in diameter and are white or purplish white and borne in clusters at the tips of the stems.
Crofton Weed is an erect terrestrial plant 1-2 m tall with stems that are not hollow. Leaves are relatively broad with slightly toothed (crenate or serrate) margins. Flower heads are relatively small 5-7 mm in diameter, and are white in colour and borne in dense clusters at the tips of the branches.
Mistflower is a creeping or scrambling terrestrial plant 40 -60 cm high. Stems are not hollow. Leaves are relatively narrow with toothed margins. Flower heads are relatively small, about 5 mm in diameter, are white in colour and borne in dense clusters at the tips of the branches.
Billygoat Weed (Ageratum conyzoides), Blue Billygoat Weed (Ageratum houstonianum) and Vernonia (Cyanthillium cinereum) have similar flowers to Siam Weed, but these are much smaller annual plants (less than 1 m) and usually have alternately arranged upper leaves.
