Caltrop (Tribulus terrestris)

Introduction:

Tribulus terrestris is a flowering plant in the family Zygophyllaceae, native to warm temperate and tropical regions of the Old World in southern Europe, southern Asia, throughout Africa, and Australia. It can thrive even in desert climates and poor soil. Like many weedy species, this plant has many common names, including bindii,bullhead, burra gokharu, caltrop, cat’s head, devil’s eyelashes, devil’s thorn, devil’s weed, goathead, puncturevine, and tackweed.

Growth:

It is a taprooted herbaceous perennial plant that grows as a summer annual in colder climates. The stems radiate from the crown to a diameter of about 10 cm to over 1 m, often branching. They are usually prostrate, forming flat patches, though they may grow more upwards in shade or among taller plants. The leaves are pinnately compound with leaflets less than 6 mm (a quarter-inch) long. The flowers are 4–10 mm wide, with five lemon-yellow petals. A week after each flower blooms, it is followed by a fruit that easily falls apart into four or five single-seeded nutlets. The nutlets or “seeds” are hard and bear two to three sharp spines, 10 mm long and 4–6 mm broad point-to-point. These nutlets strikingly resemble goats’ or bulls’ heads; the “horns” are sharp enough to puncture bicycle tires and to cause painful injury to bare feet.

Eradication:

Where this is a non-indigenous species, eradication methods are often sought after. There are both biological and herbicidal solutions to the problem, but neither of them provide a solution which is both quick and long-lasting, because T. terrestris seeds remain viable for up to 3–7 years on average.

Physical:

In smaller areas, puncture vine is best controlled with manual removal using a hoe to cut the plant off at its taproot. While this is effective, removing the entire plant by gripping the taproot, stem or trunk and pulling upward to remove the taproot is far more effective. This requires monitoring the area and removing the weed throughout the preseeding time (late spring and early summer in many temperate areas). This will greatly reduce the prevalence of the weed the following year. Mowing is not an effective method of eradication, because the plant grows flat against the ground.

Another avenue of physical eradication is to crowd out the opportunistic weed by providing good competition from favorable plants. Aerating compacted sites and planting competitive desirable plants including broad-leaved grasses such as St Augustine can reduce the impact of puncture vine by reducing resources available to the weed.

Chemical:

Chemical control is generally recommended for home control of puncture vine. There are few pre-emergent herbicides that are effective. Products containing oryzalin, benefin, or trifluralin will provide partial control of germinating seeds. These must be applied prior to germination (late winter to midspring).

After plants have emerged from the soil (postemergent), products containing 2,4-dichlorophenoxyacetic acid (“2,4-D”), glyphosate, and dicamba are effective on puncture vine. Like most postemergents they are more effectively maintained when caught small and young. Dicamba and 2,4-D will cause harm to most broad-leaved plants so the user should take care to avoid over-application. They can be applied to lawns without injuring the desired grass. Glyphosate will kill or injure most plants so it should only be used as spot treatments or on solid stands of the weed.

Another product from DuPont called Pastora is highly effective, but expensive and not for lawn use.

Biological:

Two weevils, Microlarinus lareynii and M. lypriformis, native to India, France, and Italy, were introduced into the United States as biocontrol agents in 1961. Both species of weevils are available for purchase from biological suppliers, but purchase and release is not often recommended because weevils collected from other areas may not survive at the purchaser’s location.Microlarinus lareynii is a seed weevil that deposits its eggs in the young burr or flower bud. The larvae feed on and destroy the seeds before they pupate, emerge, disperse, and start the cycle over again. Its life cycle time is 19 to 24 days. Microlarinus lypriformis is a stem weevil that has a similar life cycle, excepting the location of the eggs, which includes the undersides of stems, branches, and the root crown. The larvae tunnel in the pith where they feed and pupate. Adults of both species overwinter in plant debris. Although the stem weevil is slightly more effective than the seed weevil when each is used alone, the weevils are most effective if used together and the puncture vine is moisture-stressed.