Quick Facts

Quick facts

  • Spinyhead Sida (Sida acuta) is one of the most serious weeds of crops and pastures in the Top End of the Northern Territory.
  • Plants are grazed occasionally but infested areas are usually ignored by stock.
  • It is a prolific seeder.
  • A biological control agent, a chrysomelid beetle called Calligrapha pantherina, provides substantial control in coastal and near coastal areas of the Northern Territory.

What Does It Look Like?

What is it?

Spinyhead Sida (Sida acuta) is an erect annual or perennial shrub, usually growing to a height of about 1 metre. The stems are woody, branching several times, and there is a well developed tap root. The leaves are lance-shaped (tapered at both ends) with serrated margins.

The flowers are yellow, usually solitary or growing in pairs in the leaf axils.

The seed capsules (called schizocarps) divide into five to eight portions called mericarps, each of which has two sharp points (awns) approximately 1.5 mm long at one end (Pitt 2002).

Also see Wilson et al. (1995), Smith (2002) and Navie (2004).

For further information and assistance with identification of Spinyhead Sida contact the herbarium in your state or territory.

Flower colour


Growth form (weed type/habit)


Where it currently grows? Preferred habitat

Spinyhead Sida tends to prefer open scrublands of the wet/dry tropics where it grows in an array of soil types in disturbed areas, such as roadside verges, campsites, degraded pastures, tree plantations and areas under cultivation (Navie 2004).

Are there similar species?

Spinyhead Sida is most likely to be confused with other species of Sida, especially Flannel Weed (S. cordifolia) and S. rhombifolia. See Smith (2002) for illustrations comparing leaves and mericarps of each of these species. Spinyhead Sida may be confused with S. spinosa, S. hackettiana [as S. subspicata], Malvastrum americanum and M. coromandelianum (Navie 2004).

Malvastrum spp. can be distinguished from Sida because flowers have what is known as an epicalyx, in this case three free segments situated immediately below the calyx, which all species of Sida lack.

Sida Hackettiana [as Sida subspicata] is readily distinguished from the others because its mericarps lack awns. Of those that have awns S. cordifolia is readily distinguished from the rest as its awns have retrorsely (bent and pointing downwards) barbed spines, while in S. acuta, S. rhombifolia, and S. spinosa the awns lack spines (but may be hairy). Sida rhombifolia is distinguished from S. acuta and S. spinosa by having 7–11 mericarps (not 5–7) in each schizocarp and by its flowers having pedicels which are more than 7 mm long.

Sida acuta has 5–7 glabrous mericarps and plants quickly lose their cover of hairs as they mature while in S. spinosa adult plants are generally hairy, as too are the mericarps (Navie 2004). Very occasionally the pedicels in specimens which are otherwise probably of S. rhombifolia (flannel weed) are shorter than 7 mm, a fact which can make it difficult to differentiate it from S. acuta. However, in flannel weed the leaf surfaces are discolorous, not concolorous as in S. acuta (Barker 2007, pers. comm.).

Why Is It A Weed?

What are its impacts?

Spinyhead Sida was included in the list of 71 species that were nominated by state and territory governments for assessment as Weeds of National Significance (WONS). Following an assessment process, Spinyhead Sida was not included as one of the 20 WONS. However, it remains a weed of potential national signficance.

Agriculture: Overseas, Spinyhead Sida is a serious weed of several crops – including onions, beans and coffee – while in Australia it is a weed of pastures and sugarcane. Together with a related species, S. cordifolia (Flannel Weed), it has been described as one of the most serious weeds of crops and pastures in the Top End of the Northern Territory. Although it is grazed occasionally, infested areas are usually ignored by stock. In consequence, the animals eat more of the remaining species, reducing their competitiveness and favouring the spread and persistence of the weed (Parsons & Cuthbertson 2001).

Native ecosystems: Spinyhead Sida also invades open woodlands (Navie 2004). Once plants become established they are very competitive, holding and denying sites to other plants, in part due to their tough woody stems and deep taproots (PIER 2006).

How does it spread?

Spinyhead Sida is dispersed by seed which is enclosed in the 2-awned mericarps which readily adhere to fibrous material, e.g., clothing and animal fur. It is also spread as impurities in hay and pasture seed, in mud sticking to footwear, machinery and other vehicles, and the hooves and manure of animals (Parsons & Cuthbertson 2001).

What is its history in Australia?

Spinyhead Sida was probably introduced into Australia in the late 1800s by Chinese prospectors who made brooms from its tough fibrous stems (Flanagan et al. 2000; Pitt 2002).


How To Manage It?

Best practice management

Non-chemical control: Manual control: Small outbreaks of Spinyhead Sida can be grubbed out before flowering, with care being taken to cut the root well below the surface to prevent or minimise regrowth. Larger areas can be controlled through repeated cultivation but this may not be practical. The sowing of well-fertilised "improved" pastures results in a more dense pasture which inhibits the growth of the Sida, with such practices in use in the Northern Territory. However, care should be taken when deciding what to plant as pasture species such as Buffel Grass are also serious environmental weeds (Parsons & Cuthbertson 2001).

Generally, slashing of plants before flowering helps reduce seed production in infected areas. The spread of seed from infected paddocks to uninfected paddocks can be reduced if stock are yarded for four to five days beforehand, this allowing time for digested seed to have been excreted and seed attached to their hides to have been brushed off (Parsons & Cuthbertson 2001).

Biological control: Spinyhead Sida has been recognised as a target for biological control through a cross-jurisdictional government process. This allows activities to be undertaken to develop effective biological controls. A biological control agent, the Mexican Leaf-feeding chrysomelid beetle (Calligrapha pantherina), which was released at 80 sites in the Northern Territory between 1989 and 1992, readily established near the coast and provided substantial control (Flanagan et al. 2000). Two stem-boring weevils, species of the genus Eutinobothrus, were also released in Australia in 1994 and 1996 in the hope that they will control the weed in drier areas (Forno et al. 1997-2004).

Please see the Australian Pesticides and Veterinary Medicines Authority for chemical information http://www.apvma.gov.au .

Does it have a biological control agent?


When does it grow? (lifecycle/growth calendar)

In northern Australia Spinyhead Sida seed mostly germinate in the first month or two of the wet season (October–November) but will continue to grow throughout the summer months. In the absence of competition plants, it will rapidly establish and mostly flower in mid to late summer. Individual flowers only open for a day, opening in the morning and withering in the afternoon. During the dry season the leaves and stems of some plants will be lost, with plants developing new growth from their rootstock during the next wet season. However, if plants are not unduly stressed they may produce flowers for most of the year.

Spinyhead Sida is a prolific seeder, with most seed being dormant when shed. In the field, dormancy is broken when soil acids, bacterial action, and contraction and expansion of the seed-coat, brought about by extremes of temperatures at the soil surface, eventually crack the seed coat and admit moisture. Even so, it has been shown experimentally that about 30% of the seed produced in the Northern Territory during one wet season is still dormant at the beginning of the next wet season. The seed also require high alternating temperatures to stimulate germination (Parsons & Cuthbertson 2001).

Where Is It Found?

Which states and territories is it found?


What areas within states and territories is it found?

Spinyhead Sida is mostly found, and widespread, in tropical and subtropical regions of northern Australia, being found in Western Australia, Northern Territory and Queensland. There are also isolated occurrences in arid areas of the Northern Territory, south-eastern Queensland and in New South Wales (Navie 2004).

Where does it originate?

Spinyhead Sida is native Central America and has spread throughout the tropics and sub-tropics in the Pacific, Asia, Africa and Australia. It is now a pantropical weed of cultivated and disturbed areas (Flanagan et al. 2000; Pitt 2002; Navie 2004).

National And State Weed Listings

Is it a Weed of National Significance (WONS)?


Where is it a declared weed?


Government weed strategies and lists – Weeds Australia

Is it on the National Alert List for Environmental Weeds?


Government weed strategies and lists – Weeds Australia

Is it on the Agricultural Sleeper List?


Government weed strategies and lists – Weeds Australia

Names And Taxonomy

Main scientific name

Sida acuta

Other scientific names (synonyms)?

  • Sida acuta subsp. carpinifolia (L.f.) Borss.Waalk. (misapplied by Wheeler, J.R. 1992, Malvaceae. Flora of the Kimberley Region. 226.)
  • Sida carpinifolia L.f. (misapplied by Beard, J.S. 1965, Descriptive Catalogue of West Australian Plants. 63.)

Does it have other known common name(s)?

Spiny-head Sida, Spiny-headed Sida, Sida

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