Caltrop Vine
Puncture Vine
Tribulus terrestris
Image taken May 8, 2005 Phoenix, Maricopa Co., Arizona.
Close-up of the sharply thorned fruit which are sometimes referred to as Goat's Heads. This Wikimedia image was taken by Forest & Kim Starr. Permission:CC-BY-3.0.
If you ride a bicycle it is wise to learn to recognize this distinctive plant. It grows prostrate on the ground with shiny, dark green, compound leaves and has five-pointed, bright yellow flowers. What is often not obvious from the height of a bicycle are the extremely sharp and sturdy seeds with points arranged in a tetrahedron. There is a similar man-made device known as a caltrop which was made from wood or metal and deployed in wartime as a defense against cavalry or track vehicles. The tetrahedron shape means that a sharp point is oriented upward at all times. Caltrop Vine seeds lie ready and waiting to be carried away in a tire, shoe or bare foot.
This relative of the creosote bush, native to North Africa, was likely dispersed to the Southwest via the hooves of horses carrying early Spanish Conquistadores. A couple of weevil species have been introduced as biological control agents, but my observation is that the weevils are heavily parasitized by wasps and as a result are not so effective in reducing puncture vine populations.