Acaena is a genus of about one hundred species of perennial herbs and subshrubs in the Rosaceae, native mainly to the Southern Hemisphere, notably New Zealand, Australia and South America, but with a few species extending into the Northern Hemisphere, north to Hawaiʻi (A. exigua) and California (A. pinnatifida).

The leaves are alternate, 4–15 centimetres (1.6–5.9 in) long, and pinnate or nearly so, with 7-21 leaflets. The flowers are produced in a tight globose [inflorescence] 1–2 centimetres (0.39–0.79 in) in diameter, with no petals. The fruit is also a dense ball of many seeds; in many (but not all) species the seeds bear a barbed arrowhead point, the seedhead forming a burr which attaches itself to animal fur or feathers for dispersal.

Several Acaena species in New Zealand are known by the common name bidibid. The word is written variously biddy-biddy, biddi-biddi, biddi-bid and a number of other variations. These names are the English rendition of the original Māori name of piripiri.^[1]

Selected species

Invasive species

Some species have been introduced accidentally to other areas, attached to sheep's wool, and have become invasive species. A. novae-zelandiae, one of the bidibids from New Zealand, is the most commonly encountered species in the United Kingdom, where it is often abundant on coastal sand dunes, crowding out native vegetation and creating an often painful nuisance with the barbed burrs. In California, A. pallida, from New Zealand and southeast Australia, is similarly a problem species.

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