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puncturevine (Tribulus terrestris)
ODA rating: B
Oregon puncturevine distribution
Other common names
goat’s head, bullhead, Mexican sandbur, Texas sandbur
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Images courtesy of Weed Science Society of America

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Annual that blooms July to October. grows prostrate to the ground from a taproot and forms dense mats 4 ft across. The plant produces numerous stems some growing up to six feet long.  Leaves are opposite, hairy, divided into 4 to 8 pairs of leaflets, each about 1/2 in long. Flowers are small , yellow, 1/2 inch wide with five petals, borne in the leaf axil. Fruits is woody bur that consists of 5 sections which, at maturity, break into tack-like structures with sharp rigid spines. Each section contains 2 to 4 seeds. A single plant can produce around 400 fruit each containing two or three seeds.  

Puncturevine infests pastures, fields, ditches and roadsides where it forms dense mats. The heavily spined seeds are easily spread by animals, humans, and vehicles. Puncturevine seeds are very painful to step on and easily punture bicycle tires or light summer footware.  Even though not readily grazed it is toxic to animals.  If growing in orchards or vineyards , it is a problem to the fruit packers. This painful weed is common along roadsides. Puncturevine is tolerant of drought conditions and survives well in sandy or gravelly soils.
Native of the Mediterranean region. It was first documented in the Pacific Northwest in 1924 and is suspected to have been introduced as seed in wool from Europe.
Distribution in Oregon

Biological controls
Two biocontrol agents, a seed weevil and a stem weevil, are approved for release and are established in Oregon.

Microlarinus lareynii 
Microlarinus lypriformis