caltrop >> /ˈkæltrəp/
Definition: [noun] A spiked metal device thrown on the ground to impede wheeled vehicles or (formerly) cavalry horses.
Example: militant demonstrators threw a box of caltrops across the highway
Definition: [noun] A creeping plant with woody carpels that typically have hard spines and resemble military caltrops.
Example: Caltrop is quite rare in Cottesloe, as far as I know, and it looks like this infestation may have been introduced during works associated with the path construction, perhaps.
Definition: [noun] another term for water chestnut (sense 3)