Acaena

Acaena is a genus comprising of about 100 species of perennial herbs, shrubs, or subshrubs in the Rosaceae family. Members of this genus are found mainly in the Southern Hemisphere, in places such as New Zealand, Australia, and South America. Some species, however, are native to the Northern Hemisphere, in places such as Hawaii and California.

Leaves are alternate, 1.6-5.9 inches long, and pinnate or nearly pinnate, with 21 leaflets. The flowers are created in an inflorescense 0.39-0.79 inches in diameter with no petals. The fruits are a dense ball of seeds that usually bear a barbed arrow-head point. The seedhead will form a bur, which will attach to animal fur or feathers for transportation.

In New Zealand, several species of this genus are commonly called bidibid, or also biddy-biddy, biddi-biddi, biddi-bid, and several other variations. These names are English rendition of the original Māori name, piripiri.

Invasive species
Some Acaena species have accidentally been introduced to new areas, due to burs being attached to sheep's wool. These plants are invasive. Acaena novae-zelandiae originally lived in New Zealand, has been commonly encountered in the United Kingdom of Europe, found on coastal sand dunes, crowding out local vegetation, and being a nuisance with their burs. In California, Acaena pallida from New Zealand and Australia is similarly a problem species.