Named after an ancient military device, the common weed, caltrop (Tribulus terretris) forms spreading green mats of tangled stems; it has simple yellow flowers which are followed by fruits with sharp pointed spines. The seeds penetrate the soles of shoes (and feet) and tyres and is sometimes referred to as puncture weed.

Extracts of Tribulus terrestris have been used since ancient times to improve male physique and virility. It is still a popular supplements, perhaps the association with Roman legionnaires is influential.

For more on caltops or tribulae

see page on Centaurea calcitrapa