By Jeanette Alosi, UC Master Gardener of Butte County, August 10, 2018
Puncturevine (Tribulus terrestris) is an attractive green plant with small yellow flowers commonly seen growing prostrate along the side of the road. A native to Southern Europe, it's also referred to as “goathead.” However, underneath its foliage lies danger: spiky seedpods with needle-point spikes. If puncturevine is stepped on, it is painful to bare feet and dogs' paws; it will pierce and flatten bicycle tires. Because of its spiky seedpods puncturevine is also referred to as “caltrop,” after the spiked metal devices thrown on the roadway to stop vehicles. Caltrops have four projecting spikes with one spike always pointed up; just in the right position to puncture a tire or flip-flop, hence the similarity to puncturevine seedpods.
Puncturevine seedpods, J. Alosi
Puncturevine is a hardy summer annual that thrives in our hot, dry Mediterranean climate, even in areas of highly compacted soil. The plants are prostrate in sunny areas but can grow upright in shade. Their small yellow flowers tend to be open in mornings from April to October. A single plant can form a dense mat up to five feet in diameter, spreading outward from a center tap root. The spiky seedpods are hidden underneath the plant. Puncturevine plants are prolific; a single plant can produce 200 to 500 seeds during the growing season. And these seeds may be viable for up to five years, leading to increased spreading over time.
Puncturevine raditing from taproot, J. Alosi
Puncturevine is listed as a “C-rated” noxious weed in California. This means it is widespread and a “known economic or environment detriment.” Besides being a danger to the public, it causes major problems along roadways and agricultural lands in several ways. Ingesting the hard spiky seedpods injures the mouths and digestive systems of livestock. The leaves contain compounds called saponins which are toxic, especially to sheep. And the puncturevine's deep taproot competes for water and nutrients in croplands.
Puncturevine flowers, J. Alosi
Unfortunately, there are is no easy way to control this noxious weed. For most homeowners, the mechanical control methods of hand removal or cutting the plant off at the taproot are most effective. Any seeds left on the ground must be removed by raking or sweeping. Use heavy gloves to protect hands from the spiky seedpods. Of course, as with any weed, it's best to remove it before it flowers and sets seeds. This is especially important for puncturevine, as seeds are viable for years, and can be spread by shoes or the wheels of lawn mowers or carts.
If the plants are still small and have not produced seeds, shallow tilling (about one inch) is recommended for larger areas. Deep tilling is not recommended as any buried seeds are still viable, and can lead to future infestations. Mowing is usually not effective because the plants grow so close to the ground.
Puncturevine along road, J. Alosi
Biological control using several species of weevils have been tried but are not always effective. Chemical control of puncturevine in the home garden is often unnecessary. However, in heavily infested areas, or when hand removal is difficult, herbicide may be an option.
For more information on puncturevine see the IPM Pestnote No. 74128 and the IPM Weed Gallery.
For more information on gardening in our area, visit the Butte County Master Gardener web page at: http://ucanr.edu/sites/bcmg/. If you have a gardening question or problem, call our Hotline at (530) 538-720l.