Tribulus terrestris aka Goathead

Tribulus terrestris is an annual weed in the Zygophyllaceae family. Its common names include puncturevine, goathead, and caltrop. It comes from Eurasia and Africa, including their southern regions. It has spread through much of the Americas. It also grows in Australia and New Zealand. It often occupies fields, roadsides, yards, and other disturbed ground. Sandy shores and dry, open sites also suit it.

This fast-growing plant forms low mats from a deep taproot. Plants reach about 0.5–1 foot [0.15–0.3 metres] tall. Their spread ranges from 3–6 feet [0.9–1.8 metres]. Branches fan out from the crown along the ground. The round, hairy stems turn from green to brown. Opposite compound leaves give the plant a fine, fern-like look. Each leaf has three to seven pairs of small leaflets. These oblong leaflets have smooth edges and dense hairs. The stems do not root where they touch the ground.

Small yellow flowers open in summer at the leaf axils. Each cup-shaped bloom has five petals and grows alone. Bees visit the flowers. The fruit starts green, then turns brown as it ripens. Its surface has bumps, ridges, and sharp spines. Each fruit breaks into five hard burs that hold seeds. Each bur bears two to five spines. The burs fall from the plant and create a hazard. Their spines can pierce feet, thin shoes, and bicycle tyres. They can also injure the mouths of grazing animals.

Puncturevine grows in full sun and tolerates some shade. It prefers loose, sandy soil with good drainage. It also grows in loam, rocky ground, and heavier soils. It withstands drought and can grow on compacted ground. Sources disagree about its cold tolerance. PFAF suggests growing it as a frost-tender annual in Britain. It recommends spring sowing under glass, then planting after frost. Useful Tropical Plants also reports tolerance of coastal exposure.

Its dense mats can crowd out grasses and other plants. North Carolina lists it as invasive, limiting its landscape value. NCSU reports no major pest or disease problems. The plant is toxic to sheep, despite other sources listing no known hazards. People eat cooked leaves and young shoots in some regions. PFAF also records cooked fruit and powdered young capsules as famine foods. Sources describe herbal uses and use as a dietary supplement. These records do not establish safety or proven medical benefit.