Acanthospermum australe
(spiny-bur)

Acanthospermum australe is a creeping annual or short-lived perennial plant, which originates from the tropics and sub-tropics of Central and South America. A. australe has been introduced to China, Australia, Africa, and the USA, where it is classed as invasive in the states of Hawaii and Oregon. It spreads to form dense mats that can smother other low-growing vegetation. In Australia, A. australe is seen as a threat to native mat-forming species in coastal sand dunes and in hind-dune vegetation. It is also regarded as an invasive species within its native range in Brazil, where it occurs in conservation areas.

A. australe is native to South and Central America, but has been introduced widely to Mexico and USA, including Hawaii. Distribution is generally localized in China and Australia, and mostly sporadic in a number of countries in Africa. GBIF (2015) has single records from many African countries, perhaps suggesting some mis-identification. GBIF (2015) records for A. australe in South Africa and Swaziland include multiple specimens. Contrastingly, there are no occurrences of A. australe listed in these countries in Flora Zambesiaca (2015).

A. australe, a plant of disturbed ground, commonly occurs on roadsides and in crops in its native South America. In Hawai’i it is naturalized along roadsides and other relatively dry, open, disturbed areas (PIER, 2015). In Australia it occurs on sand dunes and sandy soils along footpaths and roadsides in the near-coastal areas (Technigro, 2010). 

Genetics

The chromosome number of A. australe has been found as 2n = 20 and 22 (Missouri Botanical Garden, 2015). Other sources usually indicate 2n = 22.    

Reproductive Biology

In the cogeneric A. hispidum, self-pollination and cross-pollination occur by wind. A. australe is assumed to have similar pollination behaviour, since it is also monoecious, having male flowers in the centre and female flowers on the outside of the inflorescence. However, it is also known to be pollinated by bees (Isahara and Miamoni-Rodell, 2011).

Very little information is available on the germination of A. australe seeds, but B & T World Seeds (2015) suggest that it may be stimulated by smoke.

Vegetative propagation can occur from the prostrate stems (Technigro, 2010).

Physiology and Phenology

As a member of the Heliantheae (tribe Asteraceae), A. australe is known to have C₄ physiology.

Longevity

Longevity of the plant is not normally more than one year. However, although A. australe is generally referred to as an annual, Technigro (2010) describes it as ‘long-lived’ and refers to the dangers of vegetative propagation, presumably from the prostrate stems.

Associations

The following endophytic fungi have been found to have associations with A. australe in Argentina: Alternaria alternata, Aureobasidium pullulans, Fusarium oxysporum, F. solani, Myrothecium roridum, Phoma sp. and Sordaria fimicola. It is suggested that some of these may have significance in medicinal terms (Giusiano et al., 2010).

Environmental Requirements

A. australe thrives in warm, relatively dry conditions.

A. australe is an alternate host of Thanatephorus cucumeris, the causal agent of web blight on dry beans, and the plants are quite severely affected (Costa et al., 2007). No other natural enemies are commonly reported.

A. australe is similar to A. hispidum, but the latter can be distinguished by its upright habit and the fact that its fruits have two larger spines at their tips. A short petiole in A. hispidum is another diagnostic trait that distinguishes it from A. australe. Types of glandular trichomes can also be used to distinguish these two species (Araújo et al., 2013).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Cultural Control and Sanitary Measures

It is especially important to avoid the spread of A. australe by the sticky burs on animal coats or human clothing.

Physical/Mechanical Control

A. australe can be controlled mechanically in the early stages of growth, but this becomes more difficult as the plant matures and becomes woody at the base.

Movement Control

Avoid transfer by grazing animals and in human clothing.

Chemical Control

A. australe is controlled by glyphosate in non-crop situations and in glyphosate-tolerant soybeans. Other herbicides of use in soybean include: a mixture of diclosulam and flumetsulam (Fortuna and Dutra,1999); chlorimuron-ethyl plus metribuzin (with or without cyanazine) (Guimarães, 1988); and sethoxydim plus bentazone (sethoxydim is not effective on its own).

Herbicides for other crops include: benthiocarb (thiobencarb) + propanil, propanil + 2,4-D, propanil + oxadiazon (for rice); diuron +/- sethoxydim, paraquat (for cotton); trifluralin (for okra).

A. australe apparently has an unusual susceptibility to fluazifop-butyl (Zhang and Luo, 2010), a characteristic which could be of value in a range of broad-leaved crops.

There is a scarcity of information on many aspects of this weed, particularly its environmental requirements, germination requirements, and seed longevity.

17/04/15 Original text by:

Chris Parker, Consultant, Bristol, UK