The species has also become a naturalised weed in tropical and subtropical regions in the Americas and East Asia.

The plants develop characteristic dark seeds with a single long awn at one end and a sharp spike at the other.

The awn becomes twisted when dry and straightens when moistened, and in combination with the spike is capable of drilling the seed into the soil.

The species is known by many common names, including black speargrass, tanglehead, steekgras (in Afrikaans) and pili (in Hawaiian, ultimately from Proto-Austronesian *pilit₁ "to adhere/stick").

H. contortus seeds are also responsible for similar injuries in dogs with thick undercoats, or becoming embedded in the socks and skin of hikers.