Caltrop – Know your enemy

Cat-heads, goat-heads, bindi-eye, three corner jacks, lawn-lego. All names to strike fear into the hearts of cyclists and barefoot walkers. These names all refer to a specific type of small hard seed. Each seed has 3 or 4 thorns that are about 5mm long – see pic below.

Through evolution these seeds learned how to arrange their thorns to assist propagation and spread. Some thorns provide a firm support base. While the longest thorn sticks up vertically, waiting to stick into something to carry the seed to a distant new germination site. Modern humans with their cars, rubber soled shoes and bike tyres are the perfect helpers to spread these thorny seeds.

Although many cyclists know about the damaging thorns, it is more important to know about the plants that they come from.

If you can recognise the source plant, you can avoid it and detour around it. The seeds are difficult to see on the ground, but they don’t move far from the source plants on their own. By keeping well away from the plants you are also keeping well away from the thorny seeds.

Two different plants generate these thorny seeds.

Caltrop – Tribulus terrestris
Three Corner Jack – Rumex hypogaeus, also known as Emex australis

Neither is native to Australia. Both are recognised as unwelcome visitors and designated as controlled plants. Although regulations differ between States, land owners are required to take reasonable steps to remove these two plants from their properties.

Both seeds can last up to 4 years in the ground before germinating – so they are difficult plants to erradicate. But the plants can be controlled and restricted to their current locations if new plants are removed as soon as they are seen.

I will concentrate on Caltrop because that is more common in this part of Victoria. Three Corner Jack is rare here, but is more common further west in the Mallee and in South Australia. See the distribution maps below.

Caltrop distribution – often seen around towns in north central Victoria

Caltrop – Tribulus terrestris

The name Caltrop might sound tame compared to the more colorful colloquial names noted above. However, the terms caltrop and tribulus have a long history of also referring to a range of military devices with a similar spiky pyramid shape to our thorny seeds. Military caltrop is used to deny enemies access to an area. Land mines without explosives is a good comparison for how they are used.

The Caltrop plant is an annual and grows in the warmer months of the year. The branching stems radiate from the crown to a diameter ranging from 10 cm to 100 cm. The plants are usually prostrate, forming flat patches. The leaves are small and dull grey-green (see pic at top of post). It has small yellow flowers.

The thing that makes the plants easiest to identify for me is the leaves have no sheen at all. Even in hot, dry Australia most grasses and plants have a little shine. But Caltrop leaves don’t reflect light which is consistent with them being the devil’s work. Their very dull green colour and low sheen makes these small leaves stand out among other grasses and plants.

Caltrop wants to spread along the ground, but it cannot compete with lots of grass growing vertically. So it is often seen spreading by itself on a patch of bare ground. It often sprouts and grows very quickly after heavy rain in an area. A single plant can create up to 1,000 thorny seeds in 2 months.

The next question is where is Caltrop likely to be? In this area of north central Victoria Caltrop is mostly close to towns and spreads along the grass and dirt strips beside human travel paths and roads. Fortunately, Caltrop is currently very rare in local National Parks and remote backroads as these get a lot less human activity.

If you see a mat of Caltrop plant it is tempting to ride across it if you don’t know what it is. DON’T DO THIS. I also suggest not riding over nature strips in towns – you never know what is lurking in them. Instead stick to the roads, paths and footpaths near towns where it is easier to see what you are riding over.

What should you do if you see a Caltrop plant or the seeds?

Give it a wide berth. Probably get off and walk circling at least 3 m around the plant.
Report it. Take a photo of it. Take other photos of surroundings, mail box numbers or street signs so you know your location. If you are tracking yourself walk around in a big circle so that will mark your location on your gpx trail. Call the property owner when you get a chance. Most likely that will be the local Council. Most local property owners know they are required to remove Caltrop and will respond promptly. Nobody wants another 1,000 thorny seeds to appear.
Pull it up. If it is a small plant you can pull it up yourself for bonus hero points. Only do this if you have a plastic bag or similar container to put the whole plant into immediately. If you have gloves put them on. Find the centre of the plant. Put fingers either side of the centre of the plant, under the spreading branches, and lift the plant straight up out of the ground. Put the plant and all the attached seeds into the plastic bag and dispose of it in a red bin.
Don’t spread the seeds. Before leaving the area, check your shoes and your tyres. If you see any seeds pull them out. Put them in a container, and then into a red bin. Do not just throw the seeds onto the ground. If you throw the seeds onto the ground you are helping the plant to spread.

For more Information see the following links –

Three Corner Jack – Rumex hypogaeus, also known as Spiny Emex and Emex australis

I know a lot less about Three Corner Jack. I have probably have seen some of these plants in my travels, but did not recognise what it was at the time.

Three Corner Jack is also an annual plant. It flowers and produces thorny seeds all through its life. From the pictures it reminds my of a spinach plant with its rich green colour and broad floppy leaves. Because it is taller (up to 60cm high) it will grow with other plants and grasses.

You can read more about it and see more pictures on these sources –

Although I am no expert in this plant the main ways for cyclists to deal with Three Corner Jack apear to be:

Avoid it – Know how to recognise the plants and give them a wide berth.
Report it – to the landowner who is required to take reasonable efforts to remove the plants.
Don’t Spread It – If thorny seeds hitch a ride with you or your bike put them in a sealed container and escort them to a red bin. Don’t just throw them on the ground.

Nix-Tracks