Caltrop – the puncture vine

Control of Caltrop “Puncture Vine” in urban areas

(Mike Norman, Feb 2009)

Caltrop (Tribulus terrestris) is a southern European plant that has become naturalized within Australia and many other parts of the world.  Although not considered a weed of agricultural significance in Western Australia (and therefore not covered by the Biosecurity and Agricultural Management Act) it is considered as a weed that threatens the amenity values of recreational areas in many municipalities, especially impacting on cyclists, children playing with bare feet, and animals.

The term “caltrop” comes from a medieval device originally used by knights in battle and thrown into the path of oncoming horses to cripple them.

In the United States it is commonly called “Puncture Vine” which indicates the thorny nature of this weed.  In the SW of Western Australia, it is a summer growing annual plant with drought tolerance.  It typically starts to germinate in mid December and there can be repeated germinations right through to the end of March.  Soon after germination, Caltrop plants grow rapidly, flowering (small yellow flowers) and producing large numbers of spiny woody burrs that split up into very sharp woody thorns.  Wiry stems radiate out on the ground to a metre or more from a central tap root, with each stem holding the numerous woody burrs.  The thorns are slow to break down in the soil and can remain dormant for at least 5 years (another very good survival mechanism displayed by this weed). 

Although Caltrop has been found adjacent to many cycle paths in the Perth metropolitan area, it is considered not to be as wide spread in suburbia as many other weed species at present, but there is some concern that it is a “sleeper weed” that could, under the right conditions, “take off” in the future and become far more widespread, making the control of it significantly more difficult and expensive.   For example, in the SW, if Climate Change leads to dryer winters and wetter summers as predicted, this will create better conditions for the spread of Caltrop.

State and local government authorities have spent hundreds of millions of dollars on paths for the use of cyclists and pedestrians, which is a vital encouragement for greater participation in passive recreation in an age of disease due to lack of exercise and a need to promote alternatives to car use.  So are we to undo the value of all that investment by allowing Caltrop to get out of control in urban areas?

Caltrop is reputedly more effective in puncturing bicycle tyres than broken glass (although broken glass is still a serious hazard to cyclists and more widespread than Caltrop at present).  There are two reasons for this.  Firstly, the woody thorns are very hard and sharp, and can penetrate “puncture proof” tyres in my experience.  Secondly, unlike glass, Caltrop thorns carried onto a path cannot be easily seen while cycling, and therefore cannot be avoided.

Another weed with a similar impact on cyclists is “Doublegee” (Emex australis) which is a declared weed originating from South Africa.  Unlike Caltrop, it grows actively in winter/ spring (in the SW of WA) but like Caltrop, it produces large numbers of sharp thorns.  It is a serious problem in various agricultural areas, but does not seem to a major problem near cycle paths (in the Perth metropolitan area at least).  However, if an infestation of Doublegee is found, it can be removed using the same methods described in this document for Caltrop.

Biological control is not seen as practical at present, given the expense and trials that would be required to develop it, with no guarantee that biological control would be possible anyway.  The only solution is persistent eradication by mapping the location of known infestations, then by monitoring those locations at least three times per summer (commencing mid to late December in the SW) for up to 5 years, manually removing or spraying germinating Caltrop plants.  The intent of monitoring and removal is to significantly reduce or even eliminate the “seed bank” of thorns stored in the soil.  So once such a program of visits has commenced, just missing one season and allowing one or more mature Caltrop plants to shed their thorns means any gains made in reducing the seed bank in previous years will be lost. 

This approach has proven to work on the northern suburbs PSP between the Perth CBD and Warwick station.  During the summer of 2003, seventeen infestations of varying area were noted on this route (with an associated anecdotal increase in the number of punctures).  Since then, Caltrop plants have been consistently removed, and Caltrop punctures are now virtually non-existent, benefitting thousands of cyclists who now use this route.  But monitoring is still required because some mature Caltrop plants were pulled up or sprayed but the thorns already shed were left in the soil.

Main Roads Western Australia have commenced a project to map all reported Caltrop infestations (reported via the Hazard Reporting system or otherwise) using GPS, enter them in a database and schedule regular inspections.  Step-by-step instructions to describe the characteristics of this weed and methods of control (for the use of their PSP maintenance staff) are also being prepared.  The WACC should commend this initiative.  Main Roads needs to respond within a week or so, due to the fast rate at which Caltrop plants grow in size and produce their thorns.

Local Government (at least those within the bounds of the MRS and other regional towns) needs to be involved for areas under their control.  The best means of doing this is to declare Caltrop a “Pest Plant” (for those local government areas where they have not already done so) which gives the local authority to control it on both public land and order its removal from private land (such as vacant blocks, industrial land etc).  Local Government will also need to include step-by-step instructions in their own training for the external work staff.

Finally, cyclists and other path users also need to play a role in the control of Caltrop by learning how to recognize this weed and then either reporting it or (for small infestations) removing the Caltrop themselves.  If Caltrop can be recognized soon after it has germinated (ie with no thorns, or immature burrs), it can be easily pulled up, turned over and left on site.  Cyclists who regularly use a particular route are best equipped to notice the Caltrop and then to look closely at that location in subsequent months and over the next summer, to see if there has been any subsequent germination.  Persistent action over several years by individual cyclist in reporting and removing Caltrop has been proven to be effective in removing and reducing infestations.

CONTROL METHODS

A careful, methodical approach is needed if Caltrop is to be controlled/ eliminated from areas were it has been established.  Control is best achieved if Caltrop is manually removed or sprayed prior to seed set.

Three approaches to control are possible:

  1. Diesel can be applied to kill the plant and seeds and penetrate and kill seed in the soil.  Not a recommended method for control in public places, but can be potentially used on private land without other useful vegetation and no public access.
  2. Apply herbicide – Glyphosate 1% is effective on seedlings, but does not kill the thorns, so best applied BEFORE seed set.  Other herbicides can also be used – see the Farmnote at: http://www.agric.wa.gov.au/content/PW/WEED/WC/FN042_2003.PDF
  3. Manual removal – most effective for small infestations, or after re-germination of seed after initial spraying.  Best done BEFORE seed shed.

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