Caltrop (Tribulus terrestris).

Caltrop aka Bindy-eye or Cathead, is a prostrate, warm season annual weed that has spiny burrs. It is common in areas of frequent Spring and Summer rain, and is in all Australian states except Tasmania.

In NSW it is common in the Central West slopes and the North West slopes because:

• They have good Summer rainfall.
• High Summer temperatures and bright light.
• The soils are fertile.

After you read this, you will be able to:

• Identify Caltrop, Cathead or Bindy-eye.
• Know the habitat of Bindy-eye or Cathead.
• Know the best cultural and chemical options for Bindy-eye or Cathead.

Why is Caltrop a Problem?

Caltrop is a problem weed because:

• Each plant produces up to 20,000 seeds.
• Its seeds are viable for up to 5 years.
• It adapts to a wide range of climates.
• It has a very deep taproot that is up to 2.6 m long.
• It has a low need for water. This means it is able to survive for long periods without any water.
• Cathead has allelopathic effects on other plants.
• The spiny burrs of Caltrop damage the feet of livestock and injure humans.
• Caltrop is toxic to stock, and causes nitrate poisoning and staggers. The nitrate poisoning makes the skin of livestock sensitive to light (photosensitive).

How to Identify Caltrop.

Caltrop is an annual prostrate herb that spreads by seeds. It has red to brown, wiry stems with a cover of fine hairs. The stems spread out for up to 2 m from the crown. The stems leaves are made up of several opposite leaflets.

Flower: The flowers of Bindy-eye or Cathead only last a single day. They have five yellow petals, that are 8 to 15 mm in diameter. It flowers from the Spring to the Autumn.

Category: Cathead is a Broadleaf (Dicot) weed.

Photosynthetic Pathway: It is a C4 Weed.

Height: Caltrop is prostrate, and grows low along the ground.

Leaf length: The leaves are 5 to 12 mm long.

Leaf width: Cathead leaves are 3 to 5 mm wide, and have very small stalks.

Caltrop Reproduction.

• Caltrop reproduces from seed that germinate after Summer rain.
• In drought as little as 25 mm of rain is enough for the seeds to germinate. This small amount of rain will maintain a Tribulus community for up to 8 weeks.
• It needs relatively high temperatures to germinate. A daily maximum temperature above 29°C is ideal. When the maximum air temperature is 24° to 27°C you can get several germinations
• In the right conditions weed germination begins within 6 to 8 hours, and does not need light.
• Seedlings germinate from a depth of up to 5 cm in sandy soils, but not this deep in heavy soils.
• Caltrop produces up to 20,000 seeds a season, and these are viable for up to 5 years.
• The fruit of Caltrop comprises a woody burr with sharp, rigid spines, which splits into segments when it is ripe.
• Each of these segments has four hard spines, with two of these being shorter than the other two.
• When Caltrop fruit is on the ground, one of the spines always points upwards, and it is these spines attach to animals, tyres, and shoes.
• Once the seedlings emerge, they flower within 3 weeks.
• They then continue to flower for several months if the plants do not die from drought.
• Bindy-eye also regrows from its deep taproot if shoot damage occurs from cultivation or herbicides.

Comments: The upper sides of the leaves are darker, and arranged in 4 to 8 pairs of opposite, oblong leaflets. The undersides appear silvery due to the presence of hairs.

Habitat: Caltrop is a weed of warm regions and light soils. You find it in crops, lawns, sports grounds, and waste areas.

Caltrop indicates low soil Ca or high P, Mg, Fe, and Al.

More information on weeds is in our weed ID chart.

• Caltrop is a poor competitor. If your turf is healthy, dense, and actively growing, it helps prevent this weed from getting a foothold.
• Cathead does not tolerate shade. A dense turf cover outcompetes this weed.
• Bare areas are its friend, so the best plan is to avoid them. The key to this is a thick turf canopy.
• If it is not too widespread, then you can hand pull Cathead.
• However, if you remove this weed by hand, it is important to remove the tap root as otherwise it will re-grow. The best way to do this is to pull the weed up from the root crown. This ensures that you remove the tap root.
• The burrs easily attach to the feet and coats of pets, shoes, mowers, and any machinery that moves on it.
• This means it is important to clean shoes and machinery when you move out of areas where Caltrop is present.

• There can be multiple germinations of this weeds over the Summer, and its grows quickly.
• This means that you may need more than one spray application to get control.
• You should time spray applications so that you spray the majority of the weed before it seeds.

Thankfully, there are several options for Caltrop control as it has not evolved resistance to herbicides. The problem with Caltrop is not the being able to kill it part.

Instead the problem is its rapid seed germination, and the multiplef germinations that occur when suitable conditions exist.

Chemical control is best from March to May and October to December.

Pre-Emergent Control of Caltrop.

There are several pre-emergent options for Cathead.

• Pendimethalin (ProForce Battalia 435),
• S-Metalochlor.
• BASF Freehand Herbicide suppresses it.

Post Emergent Control of Caltrop.

Several post-emergent herbicides are available for Cathead control, and you will need to make several repeat sprays. This is because the plant only needs a month to produce viable seed.

The best time to spray for Cathead or Bindy-eye is in the Spring and the Autumn. This is when the plants are small, actively growing and not under any stress.

• 2,4-D. This tends to burn the top of Caltrop but it then grows back.
• Warhead Trio. Safe on Buffalo grass.
• Contra M herbicide. Don’t use Contra M on Buffalo grass.
• MCPA.
• Weed Blast MA. Safe on Buffalo grass.
• Casper Turf. Registered in South Africa for Caltrop. Don’t use Casper Turf on Buffalo grass.
• Dicamba. Don’t use Dicamba on Buffalo grass.
• Slinger 240.

Non Selective Control of Caltrop.

Non selective options for Cathead include:

• Glufosinate-ammonium provides control for 4 to 6 weeks. However it will regrow and then recovers due to the limited movement of glufosinate.
• Glyphosate. You can use Glyphosate to control Cathead but if you use this, and water quality is an issue then use ProForce Manta Ray.

The following are non-selective. They also have a long term residual, and stop re-growth of Caltrop.

• Renegade. Renegade stops the germination of Bindy-eye for up to 12 months. This reduces the need for multiple herbicide applications.
• Numchuk Quad. This gives effective post and pre emergent Cathead control for up to 12 months.
• Cortex Duo. Cortex Duo gives a rapid knockdown of Caltrop, and residual control for up to 3 months. It is also safe to use around trees.