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Tribulus terrestris

Description
 

Caltrop (Tribulus terrestris) is a mat-forming annual. Its the stems can be up to 1 m long and spread radially from a central crown with a deep and somewhat woody (and possibly perennial) taproot. The stems are many-branched, green or reddish-brown and in Australia are invariably prostrate. In other countries the stems may become erect where they are competing with other plants or where they occur in shade. Leaves and stems have a silvery appearance because of the presence of hairs, particularly on the lower surface of the leaflets. The leaves are compound (pinnate) and usually opposite on the stem. They are unequal in size with the larger having 4-7 leaflet pairs per leaf plus a single apical leaflet, and the smaller with less leaflet pairs or even missing altogether so that the leaves appear to be alternate on the stem.

The bright yellow flowers (5-15 mm across), open in the morning and have 5 petals, each 4-12 mm long, which are quickly lost. They are borne singly in the junction of the smaller leaf with the stem. The flattened fruits are circular, 8-12 mm wide and up to 8 mm high. At maturity they break into 5, sometimes fewer, wedge-shaped woody nutlets. Each nutlet has short spines all over the exposed surface, a pair of diverging 3-8 mm long spines about the middle of the outer surface and a pair of shorter downward pointing spines on the outer base. These are the dispersal agents for Caltrop and the cause of the problems the species poses (Navie 2004; Barker 2007, pers. comm.)

Extracts from this species have a number of medicinal uses (PFAF 2004).

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

Distribution:

Caltrop is widespread throughout mainland Australia, except perhaps for northern Australia where the very similar Tribulus cistoides (also known as Caltrop) is found in coastal areas. It infests vineyards, irrigated cotton, overgrazed pastures and neglected areas. It is densest in areas of habitation and cereal-growing and easily spread along the road network (Parsons & Cuthbertson 2001; Naughton & Bourke 2005; Barker 2007, pers. comm.).

Habit:Herb
Key points:
  • Caltrop (Tribulus terrestris) is a nuisance weed of wasteland, pastoral land, cropping, vineyards and recreation areas.
  • Strategically placed spines on the fruits ensure that it is spread widely by animals and machinery.
  • It contaminates orchard crops, dried fruit and wool.
  • It is toxic to sheep, causing photosensitisation, staggers and nitrate poisoning.
  • Some biological control success has been achieved.
How it spreads:

Because of the spines on the nutlets of Caltrop, they are picked up by animals and by vehicle tyres and subsequently spread about. Due to the architecture of the nutlets, no matter how they are placed on the ground there is always at least one spine pointing upwards allowing them to become imbedded into feet, hooves, shoes or tyres - hence they effectively function as "trample burrs" (Parsons & Cuthbertson 2001; Naughton & Bourke 2005; Barker 2007, pers. comm.).

Their propensity to occur in bare areas means that roadsides are particularly good habitats for these plants allowing them to be easily spread by roadworks. It is possible that the bare areas are created by the allelopathic (inhibitory) effect of Caltrop which discourages the growth of grass seedlings (Parsons & Cuthbertson 2001).

The nutlets can also become imbedded in wool and so sheep are also effective dispersal agents (Naughton & Bourke 2005).

Where it grows:

Caltrop occurs within crops and in orchards but mostly in open waste places, such as roadsides, overgrazed pastures, stock yards, roadsides and neglected areas particularly in districts with high summer temperatures and dry sandy soils where there is little competition (Parsons & Cuthbertson 1992).

Flower colour:Yellow
Distribution map:

http://www.weeds.org.au/cgi-bin/weedident.cgi?tpl=plant.tpl&state=&s=&ibra=all&card=H64 

Impacts:

A single plant forms a mat-like cover over a relatively large area and may even discourage the growth of other plants through allelopathic (inhibitory) effects (Parsons & Cuthbertson 2001).

Each plant produces a very large number of spiny nutlets which can injure people and animals. The nutlets can injure the feet of horses, sheep, cattle and dogs. They are a particular nuisance to humans on playing fields and caravan parks. If they are growing in orchards, vineyards and market gardens they can both be an annoyance to pickers and can contaminate the harvest, particularly cereal or dried-fruit crops. Nutlets may also become embedded in sheep fleeces, lowering their value (Parsons & Cuthbertson 2001).

Caltrop foliage is toxic to livestock, especially sheep. Grazing of Caltrop has been associated with nitrate poisoning, photosensitisation and sheep staggers (Parsons & Cuthbertson 2001; Naughton & Bourke 2005).

Origin:

The native distribution of Caltrop is obscure and the species is very much in need of a global revision. It is generally accepted that it is native to southern Europe and northern Africa but the plant is now so widely spread that this is not certain. It may have originated in the Saharan region, and spread into the Mediterranean region (Squires 1979, cited in Guertin, 2005). However only global DNA studies are likely to be able to suggest where it might have originally been native (Barker 2007, pers. comm.).

History:

It is not known when Caltrop first appeared in Australia. It was first recorded in New South Wales in the 1890s (Parsons & Cuthbertson 2001) and was also recorded in South Australia at this time (Tate 1890). It was possibly introduced in ballast (Kloot 1986). Support for this time frame is given by the fact that it was apparently accidentally imported from the Mediterranean into the United States on livestock and first reported in California in 1903 (Guertin 2005). It is likely that there have been a number of introductions in different parts of Australia (Barker 2007, pers. comm.).

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This database is designed to provide information, including biological and ecological, on invasive plant species that are on a national weed list, or are legislated against in a state or territory. While reasonable efforts have been made to ensure the accuracy of the information, no guarantee is given, nor responsibility taken, by the Commonwealth for its accuracy, currency or completeness. The Commonwealth does not accept any responsibility for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the information contained in this database. The information contained in this database does not necessarily represent the views of the Commonwealth. This database is not intended to be a complete source of information on the matters it deals with. For further information on the images contained in the database please contact the copyright owner. All images in the weed identification tool are managed by the Australian Plant Image Index (APII). Various copyright conditions apply for these images. For further information on the copyright conditions of images contained in the database please contact the APII at: photo@anbg.gov.au.