Identification & Biology
Alias' : goat's-head, bullhead, caltrop, tackweed
Latin Name : Tribulus terrestris
Category : Terrestrial Plants
Description :

 Summer annual with a shallow taproot
• Green to reddish brown stems (0.3 – 1.5 m long)
• Normally forms dense mats but may grow upright where there is competition for light
• Leaves are 13 mm long, opposite and divided into 4-8 pairs of oval, 13 mm leaflets
• Yellow flowers appear from late spring or early summer until frost, opening in the mornings only

Flowers are solitary on short stalks, each consisting of five petals, 13 mm wide. The fruit is a roughly circular, hard spiny bud with five sections that split when mature. Each bur has two spines and contains 2-5 seeds. The spines hook into humans, wild and domestic animals, tires and other surfaces, allowing for the seeds to be dispersed to new areas.

Puncturevine reproduces only by seed. Without competition a single plant may produce up to 1 million seeds. Germination usually starts during spring (as early as mid-May in Osoyoos) and continues until frost. Three weeks after the plant begins to grow, flowers begin to appear. Fruits occur 1-2 weeks later.
Look-a-likes: Puncturevine may be confused with other small-leaved, mat forming or low growing plants including prostrate knotweed (Polygonum aviculare), prostrate spurge (Euphorbia maculata) and common silverweed (Potentilla anserina).

Look Alikes

In British Columbia, puncturevine occurs only in the Okanagan Valley, as far north as Vernon, and in the lower Similkameen Valley. It prefers sandy or well-drained soils and readily invades disturbed ground. Puncturevine typically infests vacant lots, gravel parking areas, roadsides, unpaved trails and beaches. More recently, this invader has moved onto agricultural lands, particularly where crops border roads, in sandy headlands, between crop rows and in other locations where there is limited vegetation to compete with the puncturevine.

Impact & Risks

Puncturevine is toxic to sheep. Stiff spines on the fruit readily attach to footwear, clothing, animals, tires, machinery and supplies. The sharp spines can injure the feet, hides, mouths, eyes, and digestive tracts of livestock. Those burrs can also injure people. They are especially problematic to fruit pickers working in orchards and vineyards. The spiny pods also puncture bike and small vehicle tires. This ‘hitchhiking’ is the primary mechanism for dispersal of puncturevine. With a deep taproot, puncturevine competes aggressively for water and nutrients in tree and field crops.

Prevention & Mitigation

 Monitor your property regularly during the growing season to watch for puncturevine
• Reduce the amount of bare ground/ minimize soil disturbances
• Re-seed or plant disturbed soils
• Develop clear protocols for cleaning vehicles, machinery and footwear, such as check points or cleaning stations

Placement of bins, equipment and supplies is also an essential part of reducing the spread of puncturevine. Seedpods can easily attach and be unknowingly transported from one location to another.

Clearly mark locations for parking vehicles and machinery. Areas heavily infested with invasive plants should be considered “out of bounds” until measures are taken to remove or otherwise destroy the plants.

Long-term control of puncturevine can be achieved by reducing the amount of seeds in the soil. Remove plants before they produce seeds and continue this practice for several years.

Treatment & Disposal

Hand removal or hoeing is effective for controlling small outbreaks. Shallow tilling (~2.5 cm deep) of young plants can be effective in larger areas. Deeper tilling is not recommended since this practice may bury seeds which will continue to germinate for several years afterwards. Mowing is not an effective method of control.

Mulch applied prior to germination or when plants are small can effectively suppress outbreaks of puncturevine but it needs to be at least 7-8 cm thick. Some hand removal may additionally be required.

Chemical control is also an option. Recent research conducted in the South Okanagan has indicated that pre-emergent herbicides including Chateau, Prism and Sandea provide season-long suppression of puncturevine. Refer to the label for crops that these herbicides can be used on. Post-emergent herbicides including Clearview and Overdrive also showed control of puncturevine throughout the growing season, but can only be applied in non-cropland sites. Contact the Pesticide Specialist with the BC Ministry of Agriculture for further information: kenneth.sapsford@gov.bc.ca

Okanagan Distribution

Priority Level Definitions

Watch For - Poses a significant threat (very high risk) and does NOT presently occur in the region OR is relatively new to the region and is very limited in extent.
High - High risk/impact; limited population with significant potential to spread in the region.
Medium - Medium risk/impact; limited distribution – broader population distribution with potential to spread further in a region.
Low - Low risk/impact; may be widespread or not, may be of concern in specific situations with certain high values – e.g. specific agriculture crops. Some species may be treated primarily with biological control agents.