Burrs: The trials and Tribulus

Tribulus terrestris is a terror of a burr. Large, strong spines that can stick into tyres, shoes and of course feet, spreading in harsh dry environments where other plants are struggling, resistant to council spraying efforts.

Locals here usually call it three-corner jack (a reference to the shape of the burrs). Caltrop is another name, taken from a weapon of ancient warfare, a steel spike that was left on the ground to pierce the feet of enemies like a precursor to landmines.

It goes by various common names. One of these, bindi-eye, is more familiar to me as the common name of Soliva sessilis, a small lawn daisy with a nasty burr that my bare feet met as a kid in Queensland.

The weed is well adapted to sandy dry soils and dry weather. It competes for moisture well with other plants, and can survive drought. An annual, it spreads rapidly along the ground in all directions, setting thousands of seeds, which can persist for several years in the soil before germinating.

Tribulus terrestris, three-corner jack
Three-corner jack, caltrop, and bindi-eye are some of the printable things it’s been called

It’s a hazard because it could cause injury to pets (or people who walk barefoot, like me). It is spread rapidly on shoes, car tyres, and I suspect council mowing machinery.

The council sends contractors to spray outbreaks, but this has little effect because the plants have already set seed by the time they are noticed, and while they may be killed, the cycle continues. Further, the spraying creates a dead zone, where no plants grow and the soil compacts and dries out – which seems to be ideal conditions for this weed’s growth!

I’ve removed about a dozen shopping bags’ worth of plants and their fallen seed from the nature strip and riverside reserve in the last few weeks. Since spraying is ineffective, the best way to remove them at any small scale is by hand. I use a small, sturdy knife to dig down next to the plant’s stem, sever the taproot a couple of centimeters below the soil surface, and bag the whole plant plus any fallen seed I can see lying around.

2017-03-02 09.53.54
Tribulus terrestris burr, with my digging knife for scale. I got the knife as a “boot knife” whatever that is, it has a sturdy blade with a sharp chisel-like end, pretty handy for weeding with.

This method requires coming back every few weeks in summer to check for more plants around where you pulled them. They will germinate again after rain. You also need to keep an eye out for them in subsequent years (perhaps up to five).

Tribulus terrestris lifecycle calendar
Lifecycle and treatment calendar for Tribulus terrestris. Souce: http://agriculture.vic.gov.au/agriculture/pests-diseases-and-weeds/weeds/a-z-of-weeds/caltrop

Identification of the plant is reasonably easy. From the photos above, you can see it has pinnate leaves; that is, each leaf consists of many pairs of leaflets, either side of a central leaf stem. Small yellow five-petal flowers turn into the rounded, spiny burrs. These in turn split into five wedge-shaped fruit, which carry a couple of uneven pairs of spines. Each fruit may have up to four seed, and if left alone may produce a thousand burrs in its annual lifecycle, for a total of maybe 20,000 seeds! Agriculture Victoria has a decent information sheet as part of their weeds guide which can help to identify.

The pinnate leaves may remind you of pea family plants, but Tribulus is actually in the family Zygophyllaceae. It has a native relative, Tribulus cistoides which appears similar from the photos but I expect it is unlikely to be found in southern  Australia.

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