bristly star bur

Acanthospermum hispidum
older flower-head with fruit beginning to develop (Photo: Sheldon Navie)
habit of old plants (Photo: Sheldon Navie)
close-up of young flower-head showing its hairy floral bracts and flower buds (Photo: Sheldon Navie)
spiny mature fruit (Photo: Sheldon Navie)
close-up of seeds (Photo: Steve Hurst at USDA PLANTS Database)
a large infestation of old plants (Photo: Sheldon Navie)
dense infestation (Photo: Sheldon Navie)
habit (Photo: Sheldon Navie)
hairy branched stems and paired leaves (Photo: Sheldon Navie)
young plant (Photo: Sheldon Navie)
immature fruit (Photo: Sheldon Navie)
hairy leaves with slightly toothed margins (Photo: Sheldon Navie)
close-up of young flower-head and leaf undersides (Photo: Sheldon Navie)
Herb
Opposite
Simple
Green
Yellow
Green

Starburr (Acanthospermum hispidum) is an upright, rough to touch (due to being covered with stiff white hairs), branched annual (short lived) herb growing up to 1 m high (although usually growing less than 50 cm) and is a member of the daisy family. Its leaves are stalkless, covered in stiff hairs, and borne in pairs along the stems. its inconspicuous yellowish-green flowers are borne in the forks of the leaves near the top of the plant. its fruit develop into 5-10 spiny wedge-shaped 'seeds' which radiate outward and form a distinctive star-shaped 'burr'.

Common names 
Also known as: bristly star-bur, bristly starbur, bristly starburr, goathead, goat's head, goat's-head, hispid starrburr, slingshot weed, Texas cockspur, upright star burr,
Family 
Asteraceae
Deciduous 
No
Flowering time 
Summer to Autumn
Native/Exotic 
Exotic
Origin 
This species is native to Central America (i.e. Costa Rica, El Salvador, Guatemala, Honduras and Nicaragua), parts of the Caribbean and tropical South America (i.e. French Guiana, Guyana, Surinam, Venezuela, Brazil, Bolivia, Colombia, Peru, Paraguay, Urug
Notifiable 
No
Council declaration 
SIL – Special Investigation List
Known distribution 

Starburr (Acanthospermum hispidum) is widely naturalised throughout northern and eastern Australia. It is widespread in Queensland, in the northern parts of the Northern Territory, and in the Kimberley region in northern Western Australia. It has also been recorded in north-western and south-western Western Australia and is present in many parts of northern New South Wales (north from Kendall).

A small number of plants were also reported to be growing in the Maffra area of Gippsland in eastern Victoria in 2004. These plants were thought to have originated from a load of contaminated peanut straw imported from Queensland. Hence, this species is now regarded as being sparingly naturalised in Victoria.

Starburr (Acanthospermum hispidum) is also widely naturalised in other parts of the world including Africa, India, Sri Lanka, Thailand, Indonesia, Papua New Guinea, Hawaii and the southern parts of the USA.

Habitat 

A weed of pastures, crops, disturbed sites, roadsides and waste areas in the warmer (i.e. tropical, sub-tropical and semi-arid) climatic regions of northern Australia and prefers summer rainfall. It is also often very common in alluvial soils along waterways and on floodplains in rangelands and natural areas in these regions. This species has also occasionally been found growing in warmer temperate regions in southern Australia.

Habit 

An upright (i.e. erect), or occasionally low-growing (i.e. decumbent), short-lived (i.e. annual) herbaceous plant growing 15-100 cm tall, but usually less than 50 cm in height.

Impact and control methods 

Though this species is largely seen as a pest of agicultural ecosystems (i.e. summer cropping systems and rangelands) in northern and eastern Australia, it is also regarded as an environmental weed in the northern parts of the country (i.e. in northern Queensland, the Northern Territory and northern Western Australia). In these areas it is known to invade native rangeland pastures and out-compete more desirable native species, particularly along waterways and on nearby floodplains. This not only has a negative impact on the productivity of these pastures but also effects their biodiversity. Starrburr (Acanthospermum hispidum ) has also invaded national parks and aboriginal lands, where it may cause even more significant environmental damage.

Isolated plants and small populations can be physically controlled by removing the plants by hand or digging and then burning or deeply burying the material. Monitoring of the area and follow-up controls may be required to remove any new seedlings. Starburr is an annual, so preventing it from seeding will eventually eliminate it. Cultivation or slashing is effective if undertaken prior to seed set, but if left till after seed set these actions could spread the plant instead. Where cultivation is practicable, the field needs to be ploughed to bury any existing seeds at least 10 cm below the surface and then sown with a suitable perennial pasture, preferably a species that provides a dense surface cover and shading. Any weed seedlings sprouting from the newly sown pasture must be removed as soon as practicable. Plants can also be effectively controlled by chemicals (Kleinschmidt & Johnson 1977; Miller & Schultz 1997; Parsons & Cuthbertson 2001).

For example, starrburr (Acanthospermum hispidum) is seen as a threat to the Flora River Nature Park in the northern parts of the Northern Territory. It is one of several species of particular concern in this reserve, because it is common in surrounding areas and could easily invade the park through flood waters, feral animals, wandering stock, vehicles and visitor activities. Starrburr (Acanthospermum hispidum) is also ranked as a high priority weed in aboriginal lands of the Northern Land Council area, and listed as an environmental weed in the Townsville City Council region. It was also recently listed as a priority environmental weed in two of Australia's Natural Resource Management regions.

Stem and leaves 

The slender stems are generally round in cross-section and densely covered with stiff, sometimes sticky, hairs. They regularly divide in two (i.e. they are dichotomously branched) near the top of the plant. These stems are dull green when young but turn brown as the plant matures.

The leaves (2-12.5 cm long and 1-3 cm wide) are borne in pairs oppositely opposed along the stems and are either stalkless or shortly-stalked with a winged base up to 10 mm long. They are egg-shaped in outline or oval in shape with pointed tips. These leaves can have entire, wavy, shallowly toothed, or slightly lobed margins and both leaf surfaces (and their margins) are covered in stiff hairs.

Flowers and fruits 

The inconspicuous greenish-yellow coloured flower-heads are borne singly on short stalks ranging from 1-15 mm long. These flower-heads (4-5 mm across) are found in the forks or axils of the leaves, and sometimes also in the forks of the branches, near the top of the plant. They have an outer row of 4-6 shorter greenish bracts (3-5 mm long) and an inner row of 5-10 longer bracts that become spiny as the fruit develops. Each flower-head also has several (5-8) inconspicuous yellow 'petals', that are shorter than the bracts, and at its centre there are several (about 7) tiny yellow tubular flowers that are less than 2 mm long. Flowering occurs mostly during summer and autumn (i.e. from January to May).

The flower-heads quickly develop into five to ten spiny wedge-shaped 'seeds' (i.e. cuneate achenes) which radiate outward and form a yellowish-brown to dark brown star-shaped 'burr' (10-18 mm across). Each of these 'seeds' (i.e. achenes) is about 4-7 mm long when mature and covered in short hooked prickles, with two much larger spreading spines (3-4 mm long) at its tip.

Reproduction and dispersal 

This plant reproduces only by seed. Its 'burrs' are very commonly dispersed after becoming attached to animals, clothing and vehicles. They are also dispersed by water, particularly during floods, and as a contaminant of agricultural produce (e.g. in fodder and grain).

Similar species 

Starburr (Acanthospermum hispidum) is usually easily distinguished by its burrs and is therefore rarely confused with other species. However, it is somewhat similar to the closely related Paraguay burr (Acanthospermum australe). These two species can be distinguished by the following differences:

starburr (Acanthospermum hispidum) is a semi-upright to upright (i.e. decumbent to erect) plant growing up to 1 m tall. Its wedge-shaped 'seeds' (i.e. cuneate achenes) are covered with small hooked prickles, and also have two larger spines near their tips.

Paraguay burr (Acanthospermum australe) is a creeping (i.e. prostrate) plant usually less than 10 cm tall. Its oval-shaped 'seeds' (i.e. ellipsoid achenes) are covered with small hooked spines, that are all about the same size.